Stimulates the activity of serine palmitoyltransferase (SPT). The composition of the serine palmitoyltransferase (SPT) complex determines the substrate preference. The SPTLC1-SPTLC2-SPTSSA complex shows a strong preference for C16-CoA substrate, while the SPTLC1-SPTLC3-SPTSSA isozyme uses both C14-CoA and C16-CoA as substrates, with a slight preference for C14-CoA. Plays a role in MBOAT7 location to mitochondria-associated membranes (MAMs), may me involved in fatty acid remodeling phosphatidylinositol (PI). Lipid metabolism; sphingolipid metabolism. Interacts with SPTLC1; the interaction is direct. Component of the serine palmitoyltransferase (SPT) complex, composed of SPTLC1, either SPTLC2 or SPTLC3, and either SPTSSA or SPTSSB. Interacts with MBOAT7; the interaction facilitates MBOAT7 location to mitochondria-associated membranes (MAMs). Belongs to the SPTSS family. SPTSSA subfamily. It is uncertain whether Met-1 or Met-4 is the initiator. Truncated N-terminus.