Fig. 7: Conservation on phosducin-like protein family and the proposed mechanism of chaperone-in-chaperonin mediated folding cycle.

a Phylogenetic tree of the PhLP family. The kingdom of each branch and each human subtype (PhLP1, PhLP2A, PhLP2B, PhLP3, PDC) are indicated in the phylogenetic tree. b (i) The domain-wise conservation score of PhLPs. Each domain is divided according to human PhLP2A domain features. The dotted line indicates the average value of the conservation score for each domain. (ii) The secondary structures of 5 human PhLPs based on AlphaFold predictions. c Conservation scores of human PhLPs and TRiC. (i) Residue conservation of human PhLPs. The yellow circle indicates the conserved surface in H3 helix. (ii) Residue conservation among CCT subunits, and CCT3, CCT4 is shown. d Proposed mechanism of PhLP2A-mediated substrate folding. (i) PhLP2A contributes to TRiC-mediated substrate folding. When ATP hydrolysis occurs, the positively charged surface inside the TRiC chamber is formed through CCT3/6. The negatively charged NTD of PhLP2A acts as a pivot point to induce 90 degrees relocation of PhLP2A in the cis-ring. The substrate migrates to the trans-ring. Once the trans chamber is occupied by the substrate, H1 and H2 of PhLP2A traverse the two chambers. H2 penetrates the middle of the two rings, and H1 makes direct contact with the substrate. A hydrophobic network between the PhLP2A H1 helix and the substrate might assist the substrate folding. (ii) PhLP2A binding displaces PFD from open TRiC. When PhLP2A binds to TRiC, its NTD displaces PFD by competing with the same binding sites. PhLP2A also inhibits further PFD interactions and possibly prevents substrate overloading.