%0 Journal Article
%T Unwinding and Rewinding: Double Faces of Helicase?
%A Yuliang Wu
%J Journal of Nucleic Acids
%D 2012
%I Hindawi Publishing Corporation
%R 10.1155/2012/140601
%X Helicases are enzymes that use ATP-driven motor force to unwind double-stranded DNA or RNA. Recently, increasing evidence demonstrates that some helicases also possess rewinding activity¡ªin other words, they can anneal two complementary single-stranded nucleic acids. All five members of the human RecQ helicase family, helicase PIF1, mitochondrial helicase TWINKLE, and helicase/nuclease Dna2 have been shown to possess strand-annealing activity. Moreover, two recently identified helicases¡ªHARP and AH2 have only ATP-dependent rewinding activity. These findings not only enhance our understanding of helicase enzymes but also establish the presence of a new type of protein: annealing helicases. This paper discusses what is known about these helicases, focusing on their biochemical activity to zip and unzip double-stranded DNA and/or RNA, their possible regulation mechanisms, and biological functions. 1. Introduction Helicases are molecular motors that couple the energy of nucleoside triphosphate hydrolysis to the unwinding and remodeling of structured DNA or RNA [1¨C3]. The number of helicases expressed in higher organisms is strikingly high, with approximately 1% of the genes in many eukaryotic genomes apparently encoding RNA or DNA helicases. Helicases are involved in virtually all aspects of nucleic acid metabolism, including replication, repair, recombination, transcription, chromosome segregation, and telomere maintenance [4¨C7]. Based on their substrates, helicases can be classified as DNA or RNA helicases, although some can function on both DNA and RNA molecules [8]. DNA helicases have been reported to function in a variety of DNA metabolic processes, including unwinding duplex or alternative DNA structures (triplex, G-quadruplex), stripping protein bound to DNA, and chromatin remodeling [5, 6, 9, 10]. Traditionally, helicases are known to unwind double-stranded DNA or RNA in an ATP-dependent manner. However, increasing evidence suggests that some helicases can rewind, or anneal, complementary strands of polynucleic acids in the presence or absence of nucleoside triphosphate (Figure 1). Moreover, two so-called human helicases that were identified recently appear to only have ATP-dependent rewinding activity [11¨C15]. These discoveries not only enrich the definition of helicases but also establish the presence of a new type of protein: annealing helicase. The mechanism of this novel strand annealing activity and its biological consequences remain largely unknown. In this paper, I will provide a brief overview of strand annealing activity found in various
%U http://www.hindawi.com/journals/jna/2012/140601/