The DExH protein NPH-II is a processive and directional motor for unwinding RNA

Abstract

All aspects of cellular RNA metabolism and processing involve DExH/D proteins, which are a family of enzymes that unwind or manipulate RNA in an ATP-dependent fashion¹. DExH/D proteins are also essential for the replication of many viruses, and therefore provide targets for the development of therapeutics². All DExH/D proteins characterized to date hydrolyse nucleoside triphosphates and, in most cases, this activity is stimulated by the addition of RNA or DNA¹. Several members of the family unwind RNA duplexes in an NTP-dependent fashion in vitro^1,3; therefore it has been proposed that DExH/D proteins couple NTP hydrolysis to RNA conformational change in complex macromolecular assemblies⁴. Despite the central role of DExH/D proteins, their mechanism of RNA helicase activity remains unknown. Here we show that the DExH protein NPH-II unwinds RNA duplexes in a processive, unidirectional fashion with a step size of roughly one-half helix turn. We show that there is a quantitative connection between ATP utilization and helicase processivity, thereby providing direct evidence that DExH/D proteins can function as molecular motors on RNA.