Small Subunits of Serine Palmitoyltransferase (ssSPTs) and Their Physiological Roles

Abstract

Abstract : Sphingolipids are essential components of cells, having diverse functions in cell signaling, survival and other dynamic processes such as trafficking and maintenance of membrane integrity. Serine palmitoyl transferase (SPT) catalyzes the first committed step in sphingolipid synthesis, the condensation of palmitoyl-CoA with serine, to produce 3-ketosphinganine, a precursor of the sphingoid bases that are the backbone of all complex sphingolipids. SPT is a multi-subunit enzyme found in almost all eukaryotes, and even some species of bacteria. Bacterial SPT is a homodimer of two identical subunits. Yeast SPT is a heterodimer, composed of the Lcb1p and Lcb2p subunits. These SPT subunits are highly conserved throughout evolution. The Lcb1-Lcb2p heterodimers have low basal catalytic activity. In fact, when the LCB1 and LCB2 subunits from higher eukaryotes are expressed in a yeast mutant lacking endogenous SPT, although they are highly expressed and localize to the ER membrane, microsomal SPT activity is much lower than when measured in the host organism. This observation along with the discovery that yeast SPT has an additional small subunit, Tsc3p, that increases SPT activity more than 50 fold, suggested that the higher eukaryotic enzymes may also require additional subunits for full activity, but blast homology searches of Tsc3p against the human genome showed no candidate homologs. However, recently two functional orthologs of Tsc3p, the small subunits of SPT (ssSPTa and ssSPTb), were identified from humans. These proteins activate the human heterodimer over 100 fold and strongly influence the heterodimer with regard to acyl-CoA substrate preference. Exactly how these small subunits exert their influence on the catalytic activation and substrate selection and whether or not they have other physiological roles is not known. In this study it was hypothesized that such stimulatory subunits of SPT heterodimers are present throughout eukaryotes.