Cholesterol interaction sites on the transmembrane domain of the hedgehog signal transducer and Class F G protein-coupled receptor Smoothened

Abstract

Transduction of hedgehog signals across the plasma membrane is a key process during animal development. This is facilitated by the Class F G-protein-coupled-receptor (GPCR) Smoothened (SMO), a major drug target in the treatment of basal cell carcinomas. Recent studies have suggested that SMO is modulated via interactions of its transmembrane (TM) domain with cholesterol. Long time scale (>0.35 ms of simulation time) molecular dynamics simulations of SMO embedded in two different cholesterol containing lipid bilayers reveal direct interactions of cholesterol with the transmembrane domain at regions distinct from those observed in Class A GPCRs. In particular the extracellular tips of helices TM2 and TM3 form a well-defined cholesterol interaction site, robust to changes in membrane composition and in force field parameters. Potential of mean force calculations for cholesterol interactions yield a free energy landscape for cholesterol binding. Combined with analysis of equilibrium cholesterol occupancy these results reveal the existence of a dynamic greasy patch interaction with the TM domain of SMO, which may be compared to previously identified lipid interaction sites on other membrane proteins. These predictions provide molecular level insights into cholesterol interactions with a biomedically relevant Class F GPCR, suggesting potential druggable sites.