Putative Role of Cholesterol in Shaping the Structural and Functional Dynamics of Smoothened (SMO)

The smoothened (SMO) receptor belongs to the superfamily
of class
F G protein-coupled receptors (GPCRs) and is a potential drug target
in several types of cancer. It has two ligand binding sites, respectively,
in the cysteine-rich domain (CRD) and the transmembrane domain (TMD).
It has been shown that cholesterol is important for its activation
and function. However, the molecular-level understanding of SMO dynamics
in the presence of cholesterol has not been explored in sufficient
detail. In this work, we have carried out atomistic molecular dynamics
simulations totaling 3.6 μs to analyze the effect of cholesterol
binding to TMD and/or CRD on the structure and dynamics of the SMO
receptor. Our results show that the presence of cholesterol in the
CRD and TMD, respectively, alters the conformational dynamics of SMO
differently. We reported that the reorganization of the D-R-E network
at the extracellular end of the TMD is important for the high activity
of SMO. In general, the transmembrane helices 5, 6, and 7 and helix
8 are most affected, which, in turn, leads to changes in the CRD and
intracellular cytoplasmic domain (ICD) dynamics patterns depending
on the presence or absence of cholesterol in the CRD and/or the TMD.
We have also reported that the interaction of membrane lipids with
SMO is different in different SMO states. These results agree with
the experimental structural observations and data of cholesterol-bound
and unbound structures of SMO and add to our molecular understanding
of the SMO–cholesterol interaction.