(57d) Structural Flexibility of the α-Helical Domain in a G-Protein-Coupled-Receptor-Gs Protein Complex

G-protein-coupled receptors (GPCRs) are membrane proteins mediating
cellular responses to hormones and neurotransmitters, which couple
binding of small extracellular diffusible ligands to conformational
changes and activation of cognate heterotrimeric
G-proteins. Prototypical GPCRs have a conserved structural topology of
seven transmembrane helices, which undergo conformational
reorganization on binding to small molecules, and allow for the
docking of G-proteins. Recent crystal structure (PDB 3SN6) of a
G-protein-bound neurotransmitter GPCR,
β₂-adrenergic-receptor, has provided structural bases for
the active receptor-G-protein complex. This structure has revealed
that the α-helical domain of the G_α-subunit of the
hetereotrimeric G-protein is significantly displaced in comparison to
its nucleotide-bound form. Furthermore, single-particle
electron-microscopy analysis (EM) of this complex has shown that the
α-helical domain is relatively flexible in the absence of
nucleotide. However, the exact mechanistic details of this large-scale
conformational change remain unclear. We first study this
conformational change in the G_α-subunit of the G-protein
using temperature-accelerated molecular dynamics (TAMD) simulations,
and molecular dynamics flexible-fitting (MDFF). We find that the
TAMD-generated closed-like conformations of the G_α-subunit
could be relatively easily fitted into simulated low-resolution
EM-maps of its nucleotide-bound form, and the placement of entire
complex is consistent with experimental EM maps. Additionally, we
attempt to understand the thermodynamic basis of this conformational
change in the active complex. We therefore further refine the
''activation-pathway'' generated by TAMD to a minimum-free energy path
(MFEP) using string method in collective variables (CVs) with and
without nucleotide. Preliminary results of this thermodynamic
characterization will also be presented.