Component of the serine palmitoyltransferase multisubunit enzyme (SPT) that catalyzes the initial and rate-limiting step in sphingolipid biosynthesis by condensing L-serine and activated acyl-CoA (most commonly palmitoyl-CoA) to form long-chain bases (PubMed:19416851). The SPT complex is composed of SPTLC1, SPTLC2 or SPTLC3 and SPTSSA or SPTSSB. Within this complex, the heterodimer consisting of SPTLC1 and SPTLC2/SPTLC3 forms the catalytic core (PubMed:19416851). Within the SPT complex, SPTSSA stimulates the catalytic activity and plays a role in substrate specificity, which depends upon the overall complex composition (PubMed:19416851, PubMed:33558761). 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 (PubMed:19416851). Independently of its action as a SPT component, may be involved in MBOAT7 localization to mitochondria-associated membranes, a membrane bridge between the endoplasmic reticulum and mitochondria, may hence affect MBOAT7- catalyzed incorporation of arachidonic acid into phosphatidylinositol (PubMed:23510452). {ECO:0000269|PubMed:19416851, ECO:0000269|PubMed:23510452, ECO:0000269|PubMed:33558761}. This is the function of SPTSSA (serine palmitoyltransferase small subunit A, Ensembl gene identifier ENSG00000165389).