Structural evidence for a sequential release mechanism for activation of heterotrimeric G proteins

J Mol Biol. 2009 Nov 6;393(4):882-97. doi: 10.1016/j.jmb.2009.08.043. Epub 2009 Aug 22.

Abstract

Heptahelical G-protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptors couple to heterotrimeric G proteins to relay extracellular signals to intracellular signaling networks, but the molecular mechanism underlying guanosine 5'-diphosphate (GDP) release by the G protein alpha-subunit is not well understood. Amino acid substitutions in the conserved alpha5 helix of G(i), which extends from the C-terminal region to the nucleotide-binding pocket, cause dramatic increases in basal (receptor-independent) GDP release rates. For example, mutant Galpha(i1)-T329A shows an 18-fold increase in basal GDP release rate and, when expressed in culture, it causes a significant decrease in forskolin-stimulated cAMP accumulation. The crystal structure of Galpha(i1)-T329A.GDP shows substantial conformational rearrangement of the switch I region and additional striking alterations of side chains lining the catalytic pocket that disrupt the Mg(+2) coordination sphere and dislodge bound Mg(+2). We propose a "sequential release" mechanism whereby a transient conformational change in the alpha5 helix alters switch I to induce GDP release. Interestingly, this mechanistic model for heterotrimeric G protein activation is similar to that suggested for the activation of the plant small G protein Rop4 by RopGEF8.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Amino Acid Substitution
  • Animals
  • Catalytic Domain
  • Cell Line
  • Crystallography, X-Ray
  • Enzyme Activation
  • GTP-Binding Protein alpha Subunits, Gi-Go / chemistry
  • GTP-Binding Protein alpha Subunits, Gi-Go / genetics
  • GTP-Binding Protein alpha Subunits, Gi-Go / metabolism
  • Guanosine Diphosphate / chemistry
  • Guanosine Diphosphate / metabolism*
  • Heterotrimeric GTP-Binding Proteins / chemistry*
  • Heterotrimeric GTP-Binding Proteins / genetics
  • Heterotrimeric GTP-Binding Proteins / metabolism*
  • Humans
  • Magnesium / chemistry
  • Models, Molecular
  • Protein Structure, Tertiary*
  • Rats
  • Signal Transduction / physiology*
  • Thermodynamics

Substances

  • Guanosine Diphosphate
  • GTP-Binding Protein alpha Subunits, Gi-Go
  • Heterotrimeric GTP-Binding Proteins
  • Magnesium

Associated data

  • PDB/3FFA
  • PDB/3FFB