A fundamental question in biology is how proteins, which are synthesized by the ribosome as a linear sequence of amino acids, fold into their native functional state. Many proteins require the assistance of molecular chaperones to maneuver through the folding process to protect them from aggregation and to help them reach their native state in the very concentrated protein environment of the cell. This study focuses on the roles of Phosducin-like Protein 1 (PhLP1) and Programmed Cell Death Protein 5 (PDCD5) as molecular co-chaperones of the Cytosolic Chaperonin Complex (CCT).Signaling in retinal photoreceptors is mediated by canonical G protein pathways. Previous in vitro studies have demonstrated that Gβ subunits rely on CCT and its co-chaperone PhLP1 to fold and assemble into Gβγ and RGS-Gβ5 heterodimers. The importance of PhLP1 in the assembly process was first demonstrated in vivo in a retinal rod photoreceptor-specific deletion of PhLP1. To test whether this mechanism applied to other cell types, we prepared a second mouse line that specifically disrupts the PhLP1 gene in cone photoreceptor cells and measured the effects on G-protein expression and cone visual signal transduction. In PhLP1 depleted cones, Gt2 and RGS9-Gβ5 levels were dramatically reduced, resulting a 60-fold decrease in cone sensitivity and a 50-fold increase in cone photoresponse recovery time. These results demonstrate a common mechanism of Gβγ and RGS9-Gβ5 assembly in rods and cones, underlining the significance of PhLP1/CCT-mediated folding in G protein signaling.PDCD5 has been proposed to act as a pro-apoptotic factor and tumor suppressor. However, the mechanisms underlying its apoptotic function are largely unknown. A proteomics search for PhLP1 binding partners revealed a robust interaction between PDCD5 and CCT. PDCD5 formed a complex with CCT and β-tubulin, a key CCT folding substrate, and specifically inhibited β-tubulin folding. Cryo-electron microscopy studies of the PDCD5-CCT complex suggested a possible mechanism of inhibition of β-tubulin folding. PDCD5 binds the apical domain of the CCTβ subunit, projecting above the folding cavity without entering it. Like PDCD5, β-tubulin also interacts with the CCTβ apical domain, but a second site is found at the sensor loop deep within the folding cavity. These orientations of PDCD5 and β-tubulin suggest that PDCD5 sterically interferes with β-tubulin binding to the CCTβ apical domain and inhibits β-tubulin folding. Given the importance of tubulins in cell division and proliferation, PDCD5 might exert its apoptotic function at least in part through inhibition of β-tubulin folding.



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Physical and Mathematical Sciences; Chemistry and Biochemistry



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Chemistry Commons