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J Biol Chem. 2017 Jan 20;292(3):1122-1141. doi: 10.1074/jbc.M116.746602. Epub 2016 Dec 7.

Carboxyl-terminal Tail-mediated Homodimerizations of Sphingomyelin Synthases Are Responsible for Efficient Export from the Endoplasmic Reticulum.

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From the Faculty of Pharma Sciences, Teikyo University, Kaga 2-11-1, Itabashi-ku, Tokyo 173-8605, Japan and.
the Department of Cell Science, Institute of Biomedical Sciences, Fukushima Medical University School of Medicine, Hikarigaoka-1, Fukushima City, Fukushima 960-1295, Japan.
From the Faculty of Pharma Sciences, Teikyo University, Kaga 2-11-1, Itabashi-ku, Tokyo 173-8605, Japan and


Sphingomyelin synthase (SMS) is the key enzyme for cross-talk between bioactive sphingolipids and glycerolipids. In mammals, SMS consists of two isoforms: SMS1 is localized in the Golgi apparatus, whereas SMS2 is localized in both the Golgi and plasma membranes. SMS2 seems to exert cellular functions through protein-protein interactions; however, the existence and functions of quaternary structures of SMS1 and SMS2 remain unclear. Here we demonstrate that both SMS1 and SMS2 form homodimers. The SMSs have six membrane-spanning domains, and the N and C termini of both proteins face the cytosolic side of the Golgi apparatus. Chemical cross-linking and bimolecular fluorescence complementation revealed that the N- and/or C-terminal tails of the SMSs were in close proximity to those of the other SMS in the homodimer. Homodimer formation was significantly decreased by C-terminal truncations, SMS1-ΔC22 and SMS2-ΔC30, indicating that the C-terminal tails of the SMSs are primarily responsible for homodimer formation. Moreover, immunoprecipitation using deletion mutants revealed that the C-terminal tail of SMS2 mainly interacted with the C-terminal tail of its homodimer partner, whereas the C-terminal tail of SMS1 mainly interacted with a site other than the C-terminal tail of its homodimer partner. Interestingly, homodimer formation occurred in the endoplasmic reticulum (ER) membrane before trafficking to the Golgi apparatus. Reduced homodimerization caused by C-terminal truncations of SMSs significantly reduced ER-to-Golgi transport. Our findings suggest that the C-terminal tails of SMSs are involved in homodimer formation, which is required for efficient transport from the ER.


dimerization; glycerophospholipid; protein cross-linking; protein-protein interaction; sphingolipid; sphingomyelin synthase (SMS)

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