The process of G protein heterotrimer assembly begins with the association of the Gβ with the G protein γ subunit (Gγ) into the Gβγ dimer. Gβγ is an obligate dimer, meaning that neither subunit is stable in the cell without the other. As a result, Gβ and Gγ must be brought together by chaperones. At some point during or after translation, the nascent Gβ subunit binds CCT and is folded into its β-propeller structure. However, the β-propeller is not stable in the absence of the Gγ subunit, and Gβ cannot associate with Gγ until it is released from CCT. This conundrum is resolved by the CCT co-chaperone, phosducin-like protein 1 (PhLP1). PhLP1 binds Gβ in the CCT folding cavity and initiates the release of Gβ from CCT. Once released, Gγ is able to bind Gβ in the PhLP1-Gβ complex and form the stable Gβγ dimer. The G protein α subunit then associates with Gβγ, forming the active Gαβγ heterotrimer and simultaneously releasing PhLP1. All four of the typical Gβ subunits are assembled with their 12 associated Gγ subunits by this same mechanism involving CCT and PhLP1.
The atypical Gβ5 subunit forms a dimer with regulators of G protein signaling (RGS) proteins of the RGS7 subfamily. These dimers have a different function than Gβγ dimers. They turn off G protein signaling in neurons by accelerating the rate of GTP hydrolysis on the Gα subunit. We have found that CCT and PhLP1 also assist in the assembly of these Gβ5-RGS complexes. In fact, the conditional deletion of the PhLP1 gene in the rod photoreceptor cells of mice results in the loss of the Gβ5-RGS9 dimer from these cells in addition to the loss of Gβγ dimers. Consequently, G protein-dependent responses to light by rod photoreceptors were diminished and their recovery was slow. These findings have confirmed the importance of PhLP1 in Gβγ and Gβ5-RGS dimer formation in vivo.