HELICASES - AMINO-ACID-SEQUENCE COMPARISONS AND STRUCTURE-FUNCTION-RELATIONSHIPS

DNA and RNA helicases are ubiquitous enzymes that mediate the nucleoside-triphosphate-dependent unwinding of nucleic acid duplexes, a necessary step in genome replication, expression, recombination and repair. All proteins with demonstrated helicase activity contain the purine nucleoside-triphosphate-binding pattern; subsets of helicases possess additional conserved motifs. Three large superfamilies and two smaller families of helicases are described. Experimental results support the value of the conserved motifs for prediction of structure and function of the helicases. Some of these motifs can be used as reliable identifiers of the respective groups of helicases in database searches. The two largest helicase superfamilies share similar patterns of seven conserved sequence motifs, some of which are separated by long poorly conserved spacers. Helicase motifs appear to be organized in a core domain which provides the catalytic function, whereas optional inserts and amino- and carboxy-terminal sequences may comprise distinct domains with diverse accessory roles.