Complex formation of sphingomyelin synthase 1 with glucosylceramide synthase increases sphingomyelin and decreases glucosylceramide levels.

Sphingolipids,includingsphingomyelin(SM)andglucosylceramide (GlcCer), are generated by the addition of a polar head group to ceramide (Cer). Sphingomyelin synthase 1 (SMS1) and glucosylceramide synthase (GCS) are key enzymes that catalyze the conversion of Cer to SM and GlcCer, respectively. GlcCer synthesis has been postulated to occur mainly in cis-Golgi, and SM synthesis is thought to occur in medial/trans-Golgi; however, SMS1 and GCS are known to partially co-localize in cisternae, especially in medial/trans-Golgi. Here, we report that SMS1 and GCS can form a heteromeric complex, in which theN terminus of SMS1 and the C terminus of GCS are in close proximity. Deletion of the N-terminal sterile α-motif of SMS1 reduced the stability of the SMS1--GCS complex, resulting in a significant reduction in SM synthesis in vivo. In contrast, chemical- induced heterodimerization augmented SMS1 activity, depending on an increase in the amount and stability of the complex. Fusion of the SMS1Nterminus to the GCS C terminus via linkers of different lengths increased SM synthesis and decreased GlcCer synthesis in vivo. These results suggest that formation of the SMS1--GCS heteromeric complex increases SM synthesis and decreases GlcCer synthesis. Importantly, this regulation of relative Cer levels by the SMS1--GCS complex was confirmed by CRISPR/Cas9--mediated knockout of SMS1 or GCS combined with pharmacological inhibition of Cer transport protein in HEK293T cells. Our findings suggest that complex formation between SMS1 and GCS is part of a critical mechanism controlling the metabolic fate of Cer in the Golgi. [ABSTRACT FROM AUTHOR]