Examination of eukaryotic chaperonin-mediated nascent chain folding in the cytosol: a photocrosslinking approach

TRiC (TCP-1 ring complex), a type II chaperonin, facilitates protein folding, and we previously showed that TRiC crosslinks to ribosome-bound actin and luciferase nascent chains. Here, it was found that actin and luciferase nascent chains were adjacent to more than one TRiC subunit at different stages of translation. Six and seven out of the eight TRiC subunits were photocrosslinked to the luciferase and actin nascent chains, respectively. Actin nascent chains with widely-spaced, site-specific probe locations were adjacent to the same three TRiC subunits (a, b and e) at different stages of translation. The exposure of other TRiC subunits to nascent chains varied with the length and identity of the nascent chain. In addition, the presence or absence of ATP influences the photocrosslinking yields. This suggests that ATP alters the conformation of the subunits and/or their affinity for the nascent chain. Photocrosslinking also revealed that TRiC is in close proximity to the exit site of the ribosomal tunnel, presumably to create a protected folding environment for the nascent chain. Immunoprecipitations under native conditions revealed that prefoldin photocrosslinks to the actin nascent chain and that these prefoldin-containing photoadducts are coimmunoprecipitated with antibodies specific for the TRiC a subunit. This result suggests that prefoldin and TRiC bind simultaneously to the same actin nascent chain. Photocrosslinking studies with probes at position 68 in the actin nascent chain revealed that prefoldin binds to the nascent chain subsequently to TRiC binding. An unknown protein with an apparent molecular mass of 105 kDa was shown to photocrosslink to the luciferase nascent chain in a length-dependent manner at specific probe locations close to the N-terminus of the nascent chain. Thus, the nascent chain sees a variety of proteins in its immediate environment as it emerges from the ribosomal tunnel and undergoes its chaperonin-assisted folding.