Interactome of NOP58 WT vs KA_AR mutant

The PAQosome is composed of the HSP90/R2TP chaperone and a prefoldin-like module. It promotes the biogenesis of cellular machineries but it is unclear how it discriminates closely related client proteins. Among the main PAQosome clients are C/D snoRNPs and in particular their core protein NOP58. Using NOP58 mutants and proteomic experiments, we identify different assembly intermediates and show that C12ORF45, which we rename NOPCHAP1, acts as a bridge between NOP58 and PAQosome. NOPCHAP1 makes direct physical interactions with the CC-NOP domain of NOP58 and the RUVBL1/RUVBL2 AAA+ ATPases. Interestingly, NOPCHAP1 binds several RUVBL1/2 mutants except those unable to hydrolyze ATP. Moreover, while it robustly binds both yeast and human NOP58, it makes only weak interactions with NOP56 and PRPF31, two other closely related CC-NOP proteins. Transient expression of NOP58, but not NOP56 or PRPF31, is decreased in NOPCHAP1 KO cells. We propose that NOPCHAP1 is a client-loading PAQosome cofactor that select NOP58 to promote box C/D snoRNP assembly.