The RecQ-family helicases

Abstract

Helicases participate in virtually all processes involving nucleic acid metabolism. Based on sequence analysis of a large number of known and presumptive helicases, this vast group of proteins has been divided into several superfamilies; the sole common sequence motif shared among all superfamilies is a nucleoside binding domain, termed the Walker box (1), a sequence also found in many proteins that do not function as helicases. However, helicases in the two largest families of helicases, termed SF1 and SF2, share seven distinct helicase motifs, and therefore newly discovered members of these two families can typically be identified on the basis of sequence homology. Some proteins that possess helicase motifs nevertheless do not appear to act as helicases - Snf2p in yeast, for example (2, 3). The appropriate substrate for a helicase - DNA duplex, DNA-RNA hybrid, or RNA duplex - must be determined through direct experimentation for each helicase; however such data by itself cannot definitively demonstrate the actual in vivo substrate for a given helicase. Helicases are thought to act as multimers, typically either dimers or hexamers (4).