Stimulates the activity of serine palmitoyltransferase (SPT). The composition of the serine palmitoyltransferase (SPT) complex determines the substrate preference (By similarity). Lipid metabolism; sphingolipid metabolism. Interacts with sptlc1; the interaction is direct. Component of the serine palmitoyltransferase (SPT) complex (By similarity). Belongs to the SPTSS family. SPTSSB subfamily.