Chaperone-Mediated Assembly of G Protein Complexes

Abstract

G protein signaling depends on the ability of the individual subunits of the G protein heterotrimer to assemble into functional complexes. Formation of the G protein βγ (Gβγ) dimer is particularly challenging because it is an obligate dimer in which the individual subunits are unstable on their own. Recent studies have revealed an intricate chaperone system that brings the Gβ and Gγ subunits together. This system includes the cytosolic chaperonin containing TCP-1 (CCT) and its co-chaperone phosducin-like protein 1 (PhLP1). CCT assists Gβ in achieving its β-propeller structure, while PhLP1 releases Gβ from CCT and facilitates its interaction with Gγ. Once Gβγ is formed, PhLP1 remains bound until it is displaced by the Gα subunit and the G protein heterotrimer is brought together. Another obligate dimer is the complex between the G protein β(5) subunit and a regulator of G protein signaling protein (Gβ(5)-RGS). Gβ(5)-RGS also requires CCT for Gβ(5) folding, but PhLP1 plays a different role. It stabilizes the interaction between Gβ(5) and CCT, perhaps to increase folding efficiency. After Gβ(5) folding PhLP1 must subsequently release, allowing the RGS protein to bind and form the Gβ(5)-RGS dimer directly on CCT. Gβ(5)-RGS is then freed from CCT to interact with its membrane anchoring protein and form a stable complex that turns off the G protein signal by catalyzing GTP hydrolysis on Gα.

Mutations in G protein β subunits promote transformation and kinase inhibitor resistance

“…Tandem affinity purification (TAP)/MS analysis using stable isotope-labeled amino acids in culture (SILAC) 17 further demonstrated reduced binding of GNB1 K89E, GNB1 I80T and GNB1 K57E to almost all detected Gα subunits, but not to Gγ subunits or to the G protein chaperone PDCL ( Fig. 2c and Supplementary Table 3 ) 18 . This was confirmed by immunoblotting ( Fig.…”

Structures of the Gβ-CCT and PhLP1–Gβ-CCT complexes reveal a mechanism for G-protein β-subunit folding and Gβγ dimer assembly

“…To perform this role, the G-protein heterotrimer must be assembled from its nascent polypeptides. A critical step in this process is the formation of the Gβγ dimer (2). Gβγ is an obligate dimer in which the individual subunits cannot fold into a stable structure on their own, but require the molecular chaperone cytosolic chaperonin containing tailless complex polypeptide 1 (CCT; also called TRiC) and its cochaperone phosducin-like protein 1 (PhLP1) (3,4).…”

The Transduction Cascade in Retinal ON-Bipolar Cells: Signal Processing and Disease

“…As Gα hydrolyses GTP, Gα-GDP and Gβγ reform the inactive heterotrimer, simultaneously antagonizing each’s actions on their effectors (Gilman 1987). Furthermore, Gβγ and Gα co-assemble together during biogenesis and thus mutually promote stability and trafficking (Krumins & Gilman 2006, Willardson & Tracy 2012). This makes the interpretation of loss-of function studies difficult as elimination of Gβγ would necessarily affect its cognate Gα, and vice versa .…”

“…Indeed, for many ON-BC signaling molecules that show interdependence there is evidence for protein-protein interactions. For example, many Gβ and Gγ subunits are well documented to exist in constitutive complexes and bind to Gα in its inactive GDP bound form as well as with GPCRs for activation (Dupre et al 2009, Herve 2011, Willardson & Tracy 2012). Furthermore, interactions among the subunits is important for setting the expression levels of G protein heterotrimers and their localization to the plasma membrane (Krumins & Gilman 2006).…”