The role of PACT in the RNA silencing pathway

Small RNA-mediated gene silencing (RNA silencing) has emerged as a major regulatory pathway in eukaryotes. Identification of the key factors involved in this pathway has been a subject of rigorous investigation in recent years. In humans, small RNAs are generated by Dicer and assembled into the effector complex known as RNA-induced silencing complex (RISC) by multiple factors including hAgo2, the mRNA-targeting endonuclease, and TRBP (HIV-1 TAR RNA-binding protein), a dsRNA-binding protein that interacts with both Dicer and hAgo2. Here we describe an additional dsRNA-binding protein known as PACT, which is significant in RNA silencing. PACT is associated with an approximately 500 kDa complex that contains Dicer, hAgo2, and TRBP. The interaction with Dicer involves the third dsRNA-binding domain (dsRBD) of PACT and the N-terminal region of Dicer containing the helicase motif. Like TRBP, PACT is not required for the pre-microRNA (miRNA) cleavage reaction step. However, the depletion of PACT strongly affects the accumulation of mature miRNA in vivo and moderately reduces the efficiency of small interfering RNA-induced RNA interference. Our study indicates that, unlike other RNase III type proteins, human Dicer may employ two different dsRBD-containing proteins that facilitate RISC assembly.