Trbp Sliding Promotes Dicer's Cleavage Activity

Abstract

Transactivation response (TAR) RNA binding protein (TRBP) is an essential cofactor of Dicer in the RNA interference pathway. TRBP enhances Dicer's processing of both microRNA (miRNA) and small interfering RNA (siRNA) precursors, yet the mechanistic basis of this effect has not been elucidated. Here we report a robust sliding activity of TRBP exclusively on double stranded RNA (dsRNA). TRBP exhibits ATP-independent motion selectively on double stranded RNA in a length dependent manner. We find that the first two double stranded RNA binding domains (dsRBDs) of TRBP, which stimulate the dicing activity, are also directly responsible for the sliding motion, whereas the third dsRBD is dispensable. The same sliding motion was also observed in two other dsRBD-possessing proteins, PACT, and R3D1, implying a universality of sliding activity in this family of proteins. When in complex, Dicer-TRBP displayed two modes of binding to dsRNA: static interaction and dynamic sliding. Upon stimulation of RNA cleavage, the sliding molecules, not the static molecules, disappeared selectively, strongly suggesting that the sliding motion of Dicer-TRBP promotes dicing activity. Our study demonstrates a novel mode of motion, sliding on dsRNA exhibited by dsRBD containing proteins, and suggests that such activity of TRBP leads to an enhanced catalytic cleavage of Dicer.