Molecular probing of the Saccharomyces cerevisiae sterol 24-C methyltransferase reveals multiple amino acid residues involved with C-2-transfer activity

Two families of sterol C24-methyltransferase (SMT) are responsible for the formation of the ergostane (C₁-transfer activity; SMT1) and stigmastane (C₂-transfer activity: SMT2) sterol side chains, respectively. The fungal Saccharomyces cerevisiae SMT1 (Erg6p) operates the first C₁-transfer in concerted fashion to form a single product whereas the protozoan and plant SMTs are bifunctional capable of catalyzing two sequential, mechanistically distinct C-methylation activities in the conversion of a Δ²⁴-sterol acceptor to diverse doubly alkylated products. Previous mutation of the amino acids of Erg6p at D79, Y81 and E82 afforded C₁ or C₂-transfer activities typical of the protozoan and plant SMT. In this study, scanning mutagenesis experiments involving a leucine replacement of 52 amino acids in Erg6p followed by substitution of key residues with functionally or structurally similar amino acids indicated that 5 new residues at positions Y192, G217, G218, T219 and Y223 can switch the course of C₁-transfer activity to include plant-like C₂-transfer activity. The data support a model in which several conserved and non-conserved amino acids located in distinct regions of the Erg6p regulate the course of the C-methylation reaction toward product differences.