Structural Determination of the CCT:PhLP1:Gβ5 Folding Complex

Abstract

Before the Gβ subunit of the G‐protein heterotrimer can be assembled into signaling complexes, it must first be folded by the cytosolic chaperonin CCT and by the co‐chaperone Phosducin‐like Protein 1 (PhLP1). PhLP1 has been observed to release the Gβ1 isoform from CCT after folding. In contrast, PhLP1 increases the affinity of the Gβ5 isoform for CCT. Here we present a structure of CCT in complex with PhLP1 and Gβ5 at less than 4 angstrom resolution. Using a combination of cryo‐electron microscopy and crosslinking coupled with mass spectrometry, we found this structure differs markedly from the previously published complex of CCT with PhLP1 and Gβ1. In the case of Gβ5, the substrate sits deep in the CCT folding chamber between the CCT rings, whereas Gβ1 sits high in the folding chamber associated with the apical domain of the CCT3 subunit. The position of PhLP1 is also distinct, presumably to accommodate the differences in substrate position. These structural observations provide insight into the different roles PhLP1 plays in folding Gβ1 and Gβ5 and their subsequent dimerization with Gγ subunits or RGS proteins, respectively.Support or Funding InformationR01‐EY012287