坂根 郁夫

Sphingomyelin (SM) synthase 1 (SMS1), which is involved in lipodystrophy, deafness, and thrombasthenia, generates diacylglycerol (DG) and SM using phosphatidylcholine (PC) and ceramide as substrates. Here, we found that SMS1 possesses DG‐generating activities via hydrolysis of PC and phosphatidylethanolamine (PE) in the absence of ceramide and ceramide phosphoethanolamine synthase (CPES) activity. In the presence of the same concentration (4.7 mol%) of PC and ceramide, the amounts of DG produced by SMS and PC‐phospholipase C (PLC) activities of SMS1 were approximately 65% and 35% of total DG production, respectively. PC‐PLC activity showed substrate selectivity for saturated and/or monounsaturated fatty acid‐containing PC species. A PC‐PLC/SMS inhibitor, D609, inhibited only SMS activity. Mn²⁺ inhibited only PC‐PLC activity. Intriguingly, DG attenuated SMS/CPES activities. Our study indicates that SMS1 is a unique enzyme with PC‐PLC/PE‐PLC/SMS/CPES activities.

We previously reported that diacylglycerol (DG) kinase (DGK) δ interacts with DG-generating sphingomyelin synthase (SMS)-related protein (SMSr), but not SMS1 or SMS2, via their sterile α motif domains (SAMDs). However, it remains unclear whether other DGK isozymes interact with SMSs. Here, we found that DGKζ, which does not contain SAMD, interacts with SMSr and SMS1, but not SMS2. Deletion mutant analyses demonstrated that SAMD in the N-terminal cytosolic region of SMSr binds to the N-terminal half catalytic domain of DGKζ. However, the C-terminal cytosolic region of SMS1 interacts with the catalytic domain of DGKζ. Taken together, these results indicate that DGKζ associates with SMSr and SMS1 in different manners and suggest that they compose new DG signaling pathways.