Molecular Modeling and Simulations of the Transmembrane Domain of Human Growth Hormone Receptor

Abstract

How transmembrane (TM) domains of membrane proteins transmit the signal across the cell membrane has long been a subject of keen interest in biology. There is a recent paradigm shift in the mechanism of activation for the cytokine receptor superfamily. The role of cytokine hormone binding to the extracellular domain is now recognized as an “inducer” of the conformational change of pre-dimerized TM domains that triggers subsequent intracellular responses. This is drastically different from its traditional role as an “organizer” whose sole function was to initiate the receptor TM dimer formation. Toward quantitative understanding of the mechanisms and accompanying energetics of TM-induced signaling of various single-pass TM receptors, we have generated TM homodimer models of human growth hormone receptor (hGHR) from primary sequence information using the GBSW implicit membrane model and replica-exchange molecular dynamics (REX-MD) simulations. The conformational clustering shows that hGHR forms right-handed TM dimers with two different interfacial motifs, i.e., LFFQ and GxxG. To test such prediction, we first carried out TOXCAT experiments of two hGHR TM mutants: Gly256Ile and Gly259Ile. Mutation of either position to isoleucine disrupts dimer formation. These results suggest the involvement of the glycine residues in the TM helix interaction through the GxxG motif, although we need more extensive experiments to examine the involvement of other residues in the TM dimer interface, or the existence of an alternate dimerization point. In addition, we have performed MD simulations of various hGHR dimer models extracted from GBSW REX-MD in explicit POPC membranes. The stability and orientational changes of hGHR TM dimers as well as various helix-lipid interactions will be also presented and discussed.