hClp1: a novel kinase revitalizes RNA metabolism.

The RNA-induced silencing complex (RISC) is the effector complex in the RNA interference (RNAi) pathway. In order to become assembled into RISC, synthetic small interfering RNAs (siRNAs) are phosphorylated at the 5' end upon transfection into cells. The enzymatic activity responsible for this phosphorylation event has so far remained elusive. Using a classical chromatographic approach, we recently identified and characterized hClp1 as the "siRNA-kinase" in HeLa cells. hClp1 is in fact a general RNA-kinase, and a component of the tRNA splicing endonuclease and the mRNA 3' end formation machinery. We discuss the relevance of this finding, and provide further views and perspectives for the analysis of hClp1 in tRNA splicing, mRNA cleavage and polyadenylation and other RNA metabolic processes in which hClp1 might play a role.