Molecular cloning and expression of rat squalene epoxidase.

Squalene epoxidase (SE) (EC 1.14.99.7) catalyzes the first oxygenation step in sterol biosynthesis and is suggested to be one of the rate-limiting enzymes in this pathway. Rat SE cDNA was isolated by selecting yeast transformants expressing rat cDNA in the presence of transformants expressing rat cDNA in the presence of terbinafine, an inhibitor specific for fungal SE. The expression of rat SE in the isolated terbinafine-resistant clone was confirmed by its survival in the presence of either terbinafine or an inhibitor specific for mammalian SE, NB-598, but not in the presence of both terbinafine and NB-598. Rat SE polypeptide deduced from the nucleotide sequence contains 573 amino acids, and its molecular weight is 63,950 Da. The amino acid sequence reveals one potential transmembrane domain, a hydrophobic segment (Leu27 to Tyr43) in the NH2-terminal region. This region also contains a beta 1-alpha A-beta 2 motif, which is the consensus sequence for an FAD binding domain, suggesting that SE is a flavoenzyme. This deduced rat SE sequence is 30.2% identical to the ERG 1 gene, which encodes SE from an allylamine-resistant Saccharomyces cerevisiae mutant. Expression of a full-length rat SE protein in Escherichia coli confirms this polypeptide as a functional SE. This is the first report of the molecular cloning of mammalian SE.