CONSERVATION BETWEEN HUMAN AND FUNGAL SQUALENE SYNTHETASES - SIMILARITIES IN STRUCTURE, FUNCTION, AND REGULATION

Squalene synthetase (farnesyl diphosphate:farnesyl diphosphate farnesy ltransferase; EC 2.5.1.21) is thought to represent a major control poi nt of isoprene and sterol biosynthesis in eukaryotes. We demonstrate s tructural and functional conservation between the enzymes from humans, a budding yeast (Saccharomyces cerevisiae), and a fission yeast (Schi zosaccharomyces pombe). The amino acid sequences of the human and S. p ombe proteins deduced from cloned cDNAs were compared to those of the known S. cerevisiae protein. All are predicted to encode C-terminal me mbrane-spanning proteins of approximately 50 kDa with similar hydropat hy profiles. Extensive sequence conservation exists in regions of the enzyme proposed to interact with its prenyl substrates (i.e., two farn esyl diphosphate molecules). Many of the highly conserved regions are also present in phytoene and prephytoene diphosphate synthetases, enzy mes which catalyze prenyl substrate condensation reactions analogous t o that of squalene synthetase. Expression of cDNA clones encoding S. p ombe or hybrid human-S. cerevisiae squalene synthetases reversed the e rgosterol requirement of S. cerevisiae cells bearing ERG9 gene disrupt ions, showing that these enzymes can functionally replace the S. cerev isiae enzyme. Inhibition of sterol synthesis in S. cerevisiae and S. p ombe cells or in cultured human fibroblasts by treatment with the 3-hy droxy-3-methylglutaryl coenzyme A reductase inhibitor lovastatin resul ted in elevated levels of squalene synthetase mRNA in all three cell t ypes.