Abstract
MicroRNAs silence mRNAs by guiding the RISC complex. RISC assembly requires cleavage of pre-miRNAs by Dicer, assisted by TRBP or PACT, and the transfer of miRNAs to AGO proteins. The R2TP complex is an HSP90 cochaperone involved in the assembly of ribonucleoprotein particles. Here, we show that the R2TP component RPAP3 binds TRBP but not PACT. Specifically, the RPAP3-TPR1 domain interacts with the TRBP-dsRBD3 and the 1.5 Å resolution crystal structure of this complex is presented. We identify key residues involved in the interaction and show that binding of TRBP to RPAP3 or Dicer is mutually exclusive. In contrast, RPAP3 can simultaneously bind TRBP and HSP90. Interestingly, AGOs and Dicer are sensitive to HSP90 inhibition and TRBP becomes sensitive in absence of RPAP3. These data indicate that the HSP90/R2TP chaperone is an important cofactor of proteins involved in dsRNA pathways.