Abstract
The absence in the Protein Data Bank of full-length structures of bitopic membrane proteins with one transmembrane helix, probably because of difficulties with ordered crystallization, has limited understanding of how single-transmembrane helices orient enzymes and sensors at the bilayer surface. X-ray crystal structures of full-length yeast lanosterol 14α-demethylase, a cytochrome P450, show how a helix spanning a single transmembrane may lead to constraints on the orientation of the putative substrate entry portal from within the bilayer. The crystal structures also locate the substrate lanosterol, identify putative substrate and product channels, and reveal constrained interactions with triazole antifungal drugs that are important for drug design and understanding the drug resistance associated with orthologs of the enzyme found in fungal pathogens.
Bitopic integral membrane proteins with a single transmembrane helix play diverse roles in catalysis, cell signaling, and morphogenesis. Complete monospanning protein structures are needed to show how interaction between the transmembrane helix and catalytic domain might influence association with the membrane and function. We report crystal structures of full-length
Saccharomyces cerevisiae
lanosterol 14α-demethylase, a membrane monospanning cytochrome P450 of the CYP51 family that catalyzes the first postcyclization step in ergosterol biosynthesis and is inhibited by triazole drugs. The structures reveal a well-ordered N-terminal amphipathic helix preceding a putative transmembrane helix that would constrain the catalytic domain orientation to lie partly in the lipid bilayer. The structures locate the substrate lanosterol, identify putative substrate and product channels, and reveal constrained interactions with triazole antifungal drugs that are important for drug design and understanding drug resistance.