Serine palmitoyltransferase catalyses the committed step in sphingolipid synthesis, the condensation of serine with palmitoyl-CoA to form 3-ketosphinganine. Two proteins, Lcb1p and Lcb2p, are essential for enzyme activity and a third protein, the 80-amino acid Tsc3p, stimulates the activity of serine palmitoyltransferase several-fold. Tsc3p physically associates with a complex of Lcb1p-Lcb2p and stimulates enzyme activity posttranslationally, but its precise function is not known. Tsc3p is essential for cell viability only at elevated temperatures, although serine palmitoyltransferase activity is reduced in the tsc3 delta mutant, even at permissive growth temperatures. Tsc3p is apparently not required for any essential process besides stimulation of serine palmitoyltransferase at 37 degrees C, since providing sphingoid bases to the growth medium reverses the temperature-sensitive growth phenotype of the tsc3 delta mutant. To gain further insight into the function of Tsc3p, suppressor mutants that eliminate the Tsc3p requirement for growth at 37 degrees C were isolated and characterized. These studies show that dominant mutations in the Lcb2p subunit of serine palmitoyltransferase suppress the temperature-sensitive growth phenotype of the tsc3 delta null mutant by increasing the Tsc3p-independent serine palmitoyltransferase activity.