Human and murine serine-palmitoyl-CoA transferase--cloning, expression and characterization of the key enzyme in sphingolipid synthesis

Abstract

Serine palmitoyltransferase (SPT, EC 2.3.1.50) is the key enzyme in sphingolipid biosynthesis. It catalyzes the pyridoxal-5'-phosphate-dependent condensation of L-serine and palmitoyl-CoA to 3-oxosphinganine. Human expressed-sequence-tag (EST) clones are similar to the two yeast genes for synthesis of long-chain bases, LCB1 and LCB2, which are believed to encode two subunits of SPT [Buede, R., Pinto, W. J., Lester, R. L. & Dickson, R. C. (1991) J. Bacteriol. 173, 4325-5332; Nagiec, M. M., Baltisberger, J. A., Wells, G. B., Lester, R. L. & Dickson, R. C. (1994) Proc. Natl Acad. Sci. USA 91, 7899-7902]. We have cloned and characterized two complete human and murine cDNA sequences named hLCB1 & mLCB1 and hLCB2 & mLCB2, respectively, similar to the yeast LCB1 and LCB2 genes. Human embryonic kidney cells (HEK 293) transfected with murine sequences of LCB1 (mLCB1) and LCB2 (mLCB2) independently and in coexpression showed an overexpression of the transcripts on the mRNA and protein level. The enzymatic activity of cells expressing mLCB2 alone or coexpressed with mLCB1 was three times higher than the activity of untransfected HEK cells. mLCB1 expression was not required for the synthesis of 3-oxo-sphinganine in mammalian cells. Transcription/translation in vitro yielded mLCB1 (53 kDa) and mLCB2 (63 kDa). The two proteins do not contain a signal peptide nor are they glycosylated. The endogenous and overexpressed SPT activity were both sensitive to common SPT inhibitors. Labeling studies with [1-(14)C]palmitic acid indicated that cell lines transfected with mLCB2 preferentially use the excess sphingoid bases for glucocerebroside and galactocerebroside synthesis. Our results provide conclusive genetic and biochemical evidence that the human and murine LCB2 genes described here encode serine palmitoyltransferase. Further studies will be required to unravel the function of the LCB1 gene in mammalian cells.