佐藤 成樹

The effects of mechanical stress on cultured muscle cells were examined with particular interest in myofibril assembly by using a cell-stretching system. We observed that formation and maintenance of cross-striated myofibrils in chick muscle cell cultures was suppressed in the media containing higher concentration of KCl, tetrodotoxin, or ML-9 (an inhibitor of myosin light chain kinase), but periodic stretching of myotubes for several days enabled formation of striated myofibrils just as in standard muscle cultures. However, ryanodine (a blocker of the Ca2 + channel in sarcoplasmic reticulum) and BDM (an inhibitor of myosin-actin interaction) suppressed the stretch-induced myofibrillogenesis. We further found that stretching of myotubes causes quick and transient elevation of the intracellular Ca2 + concentration and this elevation is disturbed by inhibition of Ca2 + channels of sarcoplasmic reticulum and suppression of Ca2 + influx from culture medium. These observations indicate that periodic stretching induces elevation of intracellular Ca2 + concentration and that this elevation may be due to release of Ca2 + from sarcoplasmic reticulum and Ca2 + influx from outside of the cells. The increased Ca2 + may activate actin-myosin interaction by interacting with troponin that is located along actin filaments and/or inducing phosphorylation of myosin light chains and thereby promote myofibril assembly.

There are two classes of myosin, XI and VIII, in higher plants. Myosin XI moves actin filaments at high speed and its enzyme activity is also very high. In contrast, myosin VIII moves actin filaments very slowly with very low enzyme activity. Because most of these enzymatic and motile activities were measured using animal skeletal muscle α-actin, but not plant actin, they would not accurately reflect the actual activities in plant cells. We thus measured enzymatic and motile activities of the motor domains of two Arabidopsis myosin XI isoforms (MYA2, XI-B), and one Arabidopsis myosin VIII isoform (ATM1), by using three Arabidopsis actin isoforms (ACT1, ACT2, and ACT7). The measured activities were different from those measured by using muscle actin. Moreover, Arabidopsis myosins showed different enzymatic and motile activities when using different Arabidopsis actin isoforms. Our results suggest that plant actin should be used for measuring enzymatic and motile activities of plant myosins and that different actin isoforms in plant cells might function as different tracks along which affinities and velocities of each myosin isoform are modulated.

The functions of type II diacylglycerol kinase (DGK) and - in the brain are still unclear. As a first step, we investigated the spatial and temporal expression of DGK and - in the brains of mice. DGK2, but not DGK1, was highly expressed in layers II-VI of the cerebral cortex; CA-CA3 regions and dentate gyrus of hippocampus; mitral cell, glomerular and granule cell layers of the olfactory bulb; and the granule cell layer in the cerebellum in 1- to 32-week-old mice. DGK2 was expressed just after birth, and its expression levels dramatically increased from weeks 1 to 4. A substantial amount of DGK (1/2) was detected in layers II-VI of the cerebral cortex, CA1 and CA2 regions and dentate gyrus of the hippocampus, mitral cell and glomerular layers of the olfactory bulb, and Purkinje cells in the cerebellum of 1- to 32-week-old mice. DGK2 expression reached maximum levels at P5 and decreased by 4 weeks, whereas DGK1 increased over the same time frame. These results indicate that the expression patterns of DGK isozymes differ from each other and also from other isozymes, and this suggests that DGK and - play distinct and specific roles in the brain.

Cofilin is an actin regulatory protein that plays a critical role in actin filament dynamics in a variety of cells. We have previously demonstrated that excess cofilin in skeletal muscle cells leads to disruption of actin filaments, followed by actin-cofilin rod formation in the cytoplasm. In this study, to further clarify the role of cofilin in actin assembly during myofibrillogenesis, cofilin expression was suppressed in cultured chicken skeletal muscle cells. First, we confirmed that turnover of cofilin in myotubes was much higher than that of actin, and that the cofilin level could be decreased drastically within 2 days when cofilin de novo synthesis was suppressed. Next, cofilin expression in individual myotubes was suppressed by introducing antisense morpholino oligonucleotides into the cells by microinjection. Cofilin depletion at the early phase of myofibrillogenesis caused abnormal actin aggregates in myotubes and impaired actin organization into cross-striated myofibril structures. However, when cofilin expression was suppressed in developed myotubes, actin localization in striated myofibrils was scarcely affected. These results indicate that cofilin plays a critical role in the regulation of actin assembly at the early process of myofibrillogenesis. (C) 2012 Wiley Periodicals, Inc

Troponin is well known as a Ca2+-dependent regulator of striated muscle contraction and it has been generally accepted that troponin functions as an inhibitor of muscle contraction or actin-myosin interaction at low Ca2+ concentrations, and Ca2+ at higher concentrations removes the inhibitory action of troponin. Recently, however, troponin became detectable in non-striated muscles of several invertebrates and in addition, unique troponin that functions as a Ca2+-dependent activator of muscle contraction has been detected in protochordate animals, although troponin in vertebrate striated muscle is known as an inhibitor of the contraction in the absence of a Ca2+. Further studies on troponin in invertebrate muscle, especially in non-striated muscle, would provide new insight into the evolution of regulatory systems for muscle contraction and diverse function of troponin and related proteins. The methodology used for preparation and characterization of functional properties of protochordate striated and smooth muscles will be helpful for further studies of troponin in other invertebrate animals. (C) 2011 Elsevier Inc. All rights reserved.

Troponin, a Ca2+-dependent regulator of muscle contraction, acts as an inhibitor of the actin-myosin interaction in the absence of Ca 2+ during contraction in vertebrate striated muscle. However, variation has been observed in the mode of troponin-dependent regulation among the animals belonging to Protochordata, the taxon most closely related to Vertebrata. Although troponin in striated muscle of a cephalochordate amphioxus functions as an inhibitor in the absence of Ca2+ as in vertebrates [Dennisson, J. G., et al. (2010) Zool. Sci. 27, 461-469], troponin in the smooth muscle of a urochordate ascidian (Halocynthia roretzi) regulates actin-myosin interaction as an activator in the presence of Ca2+ and not an inhibitor in the absence of Ca2+ as in vertebrates [Endo, T., and Obinata, T. (1981) J. Biochem. 89, 1599-1608]. In this study, to further clarify the functional diversity of troponin, we examined the role of troponin in Ca2+-dependent regulation of the actin-myosin interaction in striated and smooth muscles in another member of Ascidiacea (Ciona inetestinalis) using three recombinant troponin components, TnT, TnI, and TnC, produced using an Escherichia coli expression system. On the basis of actomyosin ATPase assays, we show here that troponins in both smooth and striated muscles of ascidian function as a Ca2+-dependent activator of the actin-myosin interaction and TnT is the component responsible for this activation. These results indicate that troponin of ascidian has evolved in a manner different from that of amphioxus and vertebrates in terms of function. © 2010 American Chemical Society.

Troponin regulates contraction of vertebrate striated muscle in a Ca(2+)-dependent manner. More specifically, it acts as an inhibitor of actin-myosin interaction in the absence of Ca(2+) during contraction. In vertebrates, this regulatory mechanism is unlike that in some less highly derived taxa. Troponin in the smooth muscle of the protochordate ascidian species Halocynthia roretzi regulates actin-myosin contraction as an activator in the presence of Ca(2+), not as an inhibitor in the absence of Ca(2+) as is the case in vertebrates. In this study, contractile regulation of striated muscle from another protochordate, the amphioxus Branchiostoma belcheri, was analyzed using recombinant troponin components TnT, TnI, and TnC that were produced in an Escherichia coli expression system to further elucidate their roles in Ca(2+)-dependent regulation of the actin-myosin interaction. Combination of these troponin components in an actin-myosin ATPase activity assay showed that troponin in amphioxus striated muscle functions in a similar manner to troponin in vertebrate striated muscle, and differently from ascidian smooth muscle troponin. Thus, troponin function appears to have evolved differently in different protochordate muscles.

Myosin-binding protein-C (MyBP-C), also known as C-protein, is a major myosin-binding protein characteristic of striated muscle, and plays a critical role in myofibril organization, especially in registration of thick filaments in the sarcomeres during myofibrillogenesis. We previously demonstrated that cardiac-type MyBP-C is involved early in the process of myofibrillogenesis in both cardiac and skeletal muscle during chicken muscle development. Two variants (type I and type II) have been detected in chicken cardiac MyBP-C; they differ only in the presence or absence of a sequence of 15 amino acid residues (termed P-seq) that includes a phosphorylation site for cyclic AMP-dependent kinase in the cardiac MyBP-C motif (Yasuda et al, 1995). Therefore, types I and II are regarded as phosphorylatable and non-phosphorylatable isoforms, respectively. In this study, an antibody specific for P-seq was prepared. With this and other monoclonal antibodies to cardiac MyBP-C (C-315), expression and localization of the two MyBP-C isoforms in developing chicken cardiac and skeletal muscle were examined by immunocytochemistry and immunoblotting. The results showed that type I is predominantly expressed in the heart and is localized in myofibrils of both atrial and ventricular muscles through development. In contrast, type II is mainly expressed in embryonic skeletal muscle, although type I is faintly expressed in cultured skeletal muscle. These observations were confirmed by RT-PCR.

Two myosin light chain (MLC) kinase (MLCK) proteins, smooth muscle (encoded by mylk1 gene) and skeletal (encoded by mylk2 gene) MLCK, have been shown to be expressed in mammals. Even though phosphorylation of its putative substrate, MLC2, is recognized as a key regulator of cardiac contraction, a MLCK that is preferentially expressed in cardiac muscle has not yet been identified. In this study, we characterized a new kinase encoded by a gene homologous to mylk1 and -2, named cardiac MLCK, which is specifically expressed in the heart in both atrium and ventricle. In fact, expression of cardiac MLCK is highly regulated by the cardiac homeobox protein Nkx2-5 in neonatal cardiomyocytes. The overall structure of cardiac MLCK protein is conserved with skeletal and smooth muscle MLCK; however, the amino terminus is quite unique, without significant homology to other known proteins, and its catalytic activity does not appear to be regulated by Ca2+/calmodulin in vitro. Cardiac MLCK is phosphorylated and the level of phosphorylation is increased by phenylephrine stimulation accompanied by increased level of MLC2v phosphorylation. Both overexpression and knockdown of cardiac MLCK in cultured cardiomyocytes revealed that cardiac MLCK is likely a new regulator of MLC2 phosphorylation, sarcomere organization, and cardiomyocyte contraction.

Despite fast protein degradation in muscles, protein concentrations remain constant during differentiation and maintenance of muscle tissues. Myogenin, a basic helix-loop-helix-type myogenic transcription factor, plays a critical role through transcriptional activation in myogenesis as well as muscle maintenance. TBP-interacting protein 120/cullin-associated neddylation-dissociated (TIP 120/CAND) is known to bind to cullin and negatively regulate SCF (Skp1-Cullin1-F-box protein) ubiquitin ligase, although its physiological role has not been elucidated. We have identified a muscle-specific isoform of TIP120, named TIP120B/CAND2. In this study, we found that TIP120B is not only induced in association with myogenic differentiation but also actively accelerates the myogenic differentiation of C2C12 cells. Although myogenin is a short lived protein and is degraded by a ubiquitin-proteasome system, TIP120B suppressed its ubiquitination and subsequent degradation of myogenin. TIP120B bound to cullin family proteins, especially Cullin 1 (CULL), and was associated with SCF complex in cells. It was demonstrated that myogenin was also associated with SCF and that CULL small interference RNA treatment inhibited ubiquitination of myogenin and stabilized it. TIP120B was found to break down the SCF-myogenin complex. Consequently suppression of SCF-dependent ubiquitination of myogenin by TIP 120B, which leads to stabilization of myogenin, can account for the TIP1208-directed accelerated differentiation of C2C12 cells. TIP120B is proposed to be a novel regulator for myogenesis.

Cofilin plays a critical role in actin filament dynamics in a variety of eukaryotic cells. Its activity is regulated by phosphorylation/dephosphorylation of a Ser3 residue on the N-terminal side and/or its binding to a phosphoinositide, PIP2. To clarify how cofilin activity is regulated in muscle cells, we generated analogues of the unphosphorylated form (A3-cofilin) and phosphorylated form (D3-cofilin) by converting the phosphorylation site (Ser3) of cofilin to Ala and Asp, respectively. These mutated proteins, as well as the cofilin having Ser3 residue (S3-cofilin), were produced in an E. coli expression system and conjugated with fluorescent dyes. In an in vitro functional assay, A3-cofilin retained the ability to bind to F-actin. Upon injection into cultured muscle cells, A3-cofilin and S3-cofilin promptly disrupted actin filaments in the cytoplasm, and many cytoplasmic rods containing both the exogenous cofilin and actin were generated, while D3-cofilin was simply diffused in the cytoplasm without affecting actin filaments. Several hours after the injection, however, the activity of A3-cofilin and S3-cofilin was suppressed: the actin-A3-cofilin (or S3-cofilin) rods disappeared, the cofilin diffused in the cytoplasm like D3-cofilin, and actin filaments reformed. Both GFP-fused A3-cofilin and S3-cofilin that were produced by cDNA transfection were also suppressed in the cytoplasm of muscle cells in culture. Thus, some mechanism(s) other than phosphorylation can suppress A3-cofilin activity. We observed that PIP2 can bind to A3-cofilin just as to S3-cofilin and inhibits the interaction of A3-cofilin with actin. Our results suggest that the activity of A3-cofilin and also S3-cofilin can be regulated by PIP2 in the cytoplasm of muscle cells.

Certain types of cell both in vivo and in vitro contain invaginated or convoluted nuclei. However, the mechanisms and functional significance of the deformation of the nuclear shape remain enigmatic. Recent studies have suggested that three types of cytoskeleton, microfilaments, microtubules and intermediate filaments, are involved in the formation of nuclear invaginations, depending upon cell type or conditions. Here, we show that undifferentiated mouse C2C12 skeletal muscle myoblasts had smooth-surfaced spherical or ellipsoidal nuclei, whereas prominent nuclear grooves and invaginations were formed in multinucleated myotubes during terminal differentiation. Conversion of mouse fibroblasts to myocytes by the transfection of MyoD also resulted in the formation of nuclear invaginations after differentiation. C2C12 cells prevented from differentiation did not have nuclear invaginations, but biochemically differentiated cells without cell fusion exhibited nuclear invaginations. Thus, biochemical differentiation is sufficient for the nuclear deformation. Although vimentin markedly decreased both in the biochemically and in the terminally differentiated cells, exogenous expression of vimentin in myotubes did not rescue nuclei from the deformation. On the other hand, non-striated premyofibrils consisting of sarcomeric actin-myosin filament bundles and cross-striated myofibrils traversed the grooves and invaginations. Time-lapse microscopy showed that the preformed myofibrillar structures cut horizontally into the nuclei. Prevention of myofibril formation retarded the generation of nuclear invaginations. These results indicate that the myofibrillar structures are, at least in part, responsible for the formation of nuclear grooves and invaginations in these myocytes. mRNA of sarcomeric proteins including myosin heavy chain and alpha-actin were frequently associated with the myofibrillar structures running along the nuclear grooves and invaginations. Consequently, the grooves and invaginations might function in efficient sarcomeric protein mRNA transport from the nucleus along the traversing myofibrillar structures for active myofibril formation.

Cardiac myosin-binding protein-C (MyBP-C), also known as C-protein, is one of the major myosin-binding proteins localizing at A-bands. MyBP-C has three isoforms encoded by three distinct genes: fast-skeletal, slow-skeletal, and cardiac type. Herein, we are reporting a novel alternative spliced form of cardiac MyBP-C, MyBP-C(+), which includes an extra 30 nucleotides, encoding 10 amino acids in the carboxyl-terminal connectin/titin binding region. This alternative spliced form of MyBP-C(+) has a markedly decreased binding affinity to myosin filaments and connectin/titin in vitro and does not localize to A-bands in cardiac myocytes. When MyBP-C(+) was expressed in chicken cardiac myocytes, sarcomere structure was markedly disorganized, suggesting it has possible dominant negative effects on sarcomere organization. Expression of MyBP-C(+) is hardly detected in ventricles through cardiac development, but its expression gradually increases in atria and becomes the dominant form after 6 mo of age. The present study demonstrates an age-induced new isoform of cardiac MyBP-C harboring possible dominant negative effects on sarcomere assembly.

Cell lines derived from rat aorta and frog kidney were cultured on elastic membrane, and mechanical stress was given to the cells by stretching the membrane periodically. Cell reorientation oblique to the direction of stretching occurred as a result of the rapid withdrawal of cell periphery located along the direction of stretching and gradual extension of the cell membrane toward the direction oblique to the direction of stretching. Dynamic reorganization of stress fibers in living cells was visualized by labeling stress fibers with TRITC3-actin or EGFP-tagged moesin fragments with actin-binding ability. Stress fibers aligned in the direction of stretching disappeared soon after the start of stretching and then obliquely reoriented stress fibers appeared. The stretch-induced reorientation of cultured cells was suppressed by an inhibitor of stretch-activated (SA) cation channels and by a Ca2+ chelator. However, the rearrangement of stress fibers was not affected by these agents. From these results, we suggest that Ca2+ influx via SA channels is involved in stretch-induced cell reorientation but stress fiber rearrangement is independent of SA channels. Therefore, cell reorientation does not simply depend on the arrangement of stress fibers but may be controlled by some additional mechanism(s) which is regulated by calcium signaling. (C) 2001 Academic Press.

With the aim of clarifying the roles of C-protein isoforms in developing mammalian skeletal muscle, we cloned the complementary DNA (cDNAs) encoding mouse fast (F) and slow (S) skeletal muscle C-proteins and determined their entire sequences. Northern blotting with these cDNAs together with mouse cardiac (C) C-protein cDNA was performed. It revealed that in adult mice, C, F, and S isoforms are expressed in a tissue-specific fashion, although the messages for both F and S isoforms are transcribed in extensor digitorum longus muscle, which has been categorized as a fast muscle. In addition, although C isoform is expressed first and transiently during development of chicken skeletal muscles, C isoform is not expressed in mouse skeletal muscles at all through the developmental stages S isoform is first expressed, followed by the appearance of F isoform. Finally, in dystrophic mouse skeletal muscles, the expression of S isoform is increased as it is in dystrophic chicken muscle. These observations suggest that mutations in C isoform (MyBP-C) do not lead to any disturbance in skeletal muscle, although they may lead to familial hypertrophic cardiomyopathy. We also suggest that the expression of S isoform may be stimulated in degenerating human dystrophic muscles.

The ERM proteins, ezrin, radixin, and moesin, are involved in the actin filament/plasma membrane interaction as cross-linkers. CD44 has been identified as one of the major membrane binding partners for ERM proteins. To examine the CD44/ERM protein interaction in vitro, we produced mouse ezrin, radixin, moesin, and the glutathione-S-transferase (GST)/CD44 cytoplasmic domain fusion protein (GST-CD44cyt) by means of recombinant baculovirus infection, and constructed an in vitro assay for the binding between ERM proteins and the cytoplasmic domain of CD44. In this system, ERM proteins bound to GST-CD44cyt with high affinity (K-d of moesin was 9.3 +/- 1.6nM) at a low ionic strength, but with low affinity at a physiological ionic strength. However, in the presence of phosphoinositides (phosphatidylinositol [PI], phosphatidylinositol 4-monophosphate [4-PIP], and phosphatidylinosilol 4,5-bisphosphate [4,5-PIP2]), ERM proteins bound with a relatively high affinity to GST-CD44cyt even at a physiological ionic strength:4,5-PIP2 showed a marked effect (K-d of moesin in the presence of 4,5-PIP2 was 9.3 +/- 4.8 nM). Next, to examine the regulation mechanism of CD44/ERM interaction in vivo, we reexamined the immunoprecipitated CD44/ERM complex from BHK cells and found that it contains Rho-GDP dissociation inhibitor (GDI), a regulator of Rho GTPase. We then evaluated the involvement of Rho in the regulation of the CD44/ERM complex formation. When recombinant ERM proteins were added and incubated with lysates of cultured BHK cells followed by centrifugation, a portion of the recombinant ERM proteins was recovered in the insoluble fraction. This binding was enhanced by GTP gamma S and markedly suppressed by C3 toxin, a specific inhibitor of Rho, indicating that the GTP form of Rho in the lysate is required for this binding. A mAb specific for the cytoplasmic domain of CD44 also markedly suppressed this binding, identifying most of the binding partners for exogenous ERM proteins in the insoluble fraction as CD44. Consistent with this binding analysis, in living BHK cells treated with C3 toxin, most insoluble ERM proteins moved to soluble compartments in the cytoplasm, leaving CD44 free from ERM. These findings indicate that Rho regulates the CD44/ERM complex formation in vivo and that the phosphatidylinositol turnover may be involved in this regulation mechanism.

To examine the function of C-protein, a thick filament-associated protein of vertebrate striated muscles, during myofibrillogenesis, the cDNA encoding chicken cardiac C-protein and the truncated cDNA were subcloned into a expression vector and introduced into mouse C2 myogenic cells. The expression and assembly of the C-protein was investigated by immunofluorescence methods. When the cDNA containing the entire open reading frame was introduced, in C2 myoblasts, the transiently expressed exogenous cardiac C-protein existed only diffusely in the cytoplasm, but it became localized in striated structures together with sarcomeric myosin heavy chains (MHC) in myotubes. To clarify the functional domains of C-protein, the cDNA constructs that lack the regions encoding the C-terminal immunoglobulin (Ig) C2 motif or the N-terminal Ig C2 motif were introduced into C2 cells to produce mutant proteins. The truncated chicken cardiac C-protein, which lacked the C-terminal Ig C2 motif, apparently lost the ability to bind to myosin filaments the protein was not assembled into myofibrils but diffused in the cytoplasm even in the myotubes. The protein without N-terminal Ig C2 motif, however, was assembled into sarcomeric structures just as complete protein molecules. From these results, we conclude that 1) the assembly of sarcomeric MHC into myofibrils in myotubes is accompanied with that of cardiac C-protein, and 2) the C-terminal Ig C2 motif is necessary for assembly of cardiac C-protein in sarcomeric structures in the cytoplasm. © 1995, Japan Society for Cell Biology. All rights reserved.

C-protein (MyBP-C) is a myosin binding protein of about 140 kDa which is known to modulate myosin asembly in striated uscles. A cardiac-type isoform of C-protein appears not only in cardiac muscle but also in skeletal muscle before skeletal muscle-type isoforms become detectable during myogenesis, suggesting that the cardiac isoform is involved in the early phase of nyofibrillogenesis (Bähler et al., 1985 Kawashima et al., 1986). In this study, in order to understand the structure and functional domains of the cardiac-type C-protein, we cloned and sequenced full-length cDNAs encoding chicken cardiac C-protein from γgt11 cDNA libraries which were prepared with poly (A)+ RNA from embryonic chicken cardiac muscle as well as embryonic chicken skeletal muscle by using antibodies specific for cardiac C-protein. Two cDNA variants, probably generated by alternative RNA splicing and encoding different C-protein isoforms, were detected. As judged by the cDNA sequences determined, overall homology of the peptide sequence between cardiac and skeletal muscle C-proteins (Einheber et al., 1990 Fürst et al., 1992, Weber et al., 1994) was about 50-55%. Like other myosin binding proteins, skeletal C-proteins, 86 kDa protein and M-protein, cardiac C-protein contains several copies of fibronectin type III motifs and immunoglobulin c2 motifs in the molecule, but their number and arrangements differed somewhat from those in the other proteins. Northern blot analysis with the cloned cDNA as a probe demonstrated that mRNA of 5.0 kb is transcribed in both cardiac and embryonic skeletal muscle, and that it is specifically expressed in cardiac muscle among adult tissues. © 1995 Academic Press Limited.

The ERM family members, ezrin, radixin, and moesin, localizing just beneath the plasma membranes, are thought to be involved in the actin filament/plasma membrane association. To identify the integral membrane protein directly associated with ERM family members, we performed immunoprecipitation studies using antimoesin mAb and cultured baby hamster kidney (BHK) cells metabolically labeled with [S-35]methionine or surface-labeled with biotin. The results indicated that moesin is directly associated with a 140-kD integral membrane protein. Using BHK cells as antigens, we obtained a mAb that recognized the 140-kD membrane protein. We next cloned a cDNA encoding the 140-kD membrane protein and identified it as CD44, a broadly distributed cell surface glycoprotein. Immunoprecipitation with various anti-CD44 mAbs showed that ezrin and radixin, as well as moesin, are associated with CD44, not only in BHK cells, but also in mouse L fibroblasts. Furthermore, immunofluorescence microscopy revealed that in both BHK and L cells, the Triton X-100-insoluble CD44 is precisely colocalized with ERM family members. We concluded that ERM family members work as molecular linkers between the cytoplasmic domain of CD44 and actin-based cytoskeletons.

To examine the functions of ERM family members (ezrin, radixin, and moesin), mouse epithelial cells (MTD-1A cells) and thymoma cells (L5178Y), which coexpress all of them, were cultured in the presence of antisense phosphorothioate oligonucleotides (PONs) complementary to ERM sequences. Immunoblotting revealed that the antisense PONs selectively suppressed the expression of each member. Immunofluorescence microscopy of these ezrin, radixin, or moesin ''single-suppressed'' MTD-1A cells revealed that the ERM family members are colocalized at cell-cell adhesion sites, microvilli, and cleavage furrows, where actin filaments are densely associated with plasma membranes. The ezrin/radixin/moesin antisense PONs mixture induced the destruction of both cell-cell and cell-substrate adhesion, as well as the disappearance of microvilli. Ezrin or radixin antisense PONs individually affected the initial step of the formation of both cell-cell and cell-substrate adhesion, but did not affect the microvilli structures. In sharp contrast, moesin antisense PONs did not singly affect cell-cell and cell-substrate adhesion, whereas it partly affected the microvilli structures. These data indicate that ezrin and radixin can be functionally substituted, that moesin has some synergetic functional interaction with ezrin and radixin, and that these ERM family members are involved in cell-cell and cell-substrate adhesion, as well as microvilli formation.

Radixin is an actin barbed-end capping protein which is highly concentrated in the undercoat of the cell-to-cell adherens junction and the cleavage furrow in the interphase and mitotic phase, respectively (Tsukita, Sa., Y. Hieda, and Sh. Tsukita. 1989a. J. Cell Biol. 108:2369-2382 Sato, N., S. Yonemura, T. Obinata, Sa. Tsukita, and Sh. Tsukita. 1991. J. Cell Biol. 113:321-330). To further understand the structure and functions of the radixin molecule, we isolated and sequenced the cDNA clones encoding mouse radixin. Direct peptide sequencing of radixin and immunological analyses with antiserum to a fusion protein were performed to confirm that the protein encoded by these clones is identical to radixin. The composite cDNA is 4,241 nucleotides long and codes for a 583-amino acid polypeptide with a calculated molecular mass of 68.5 kD. Sequence analysis has demonstrated that mouse radixin shares 75.3% identity with human ezrin, which was reported to be a member of the band 4.1 family. We then isolated the cDNA encoding mouse ezrin. Sequence analysis and Northern blot analysis revealed that radixin and ezrin are similar but distinct (74.9%) identity, leading us to conclude that radixin is a novel member of the band 4.1 family. In erythrocytes the band 4.1 protein acts as a key protein in the association of short actin filaments with a plasma membrane protein (glycophorin), together with spectrin. Therefore, the sequence similarity between radixin and band 4.1 protein described in this study favors the idea that radixin plays a crucial role in the association of the barbed ends of actin filaments with the plasma membrane in the cell-to-cell adherens junction and the cleavage furrow.