Conserved helicase motifs in the PIF protein.

Recently, Hodgman (1) and Gorbalenya et al. (2) observed the presence of several conserved motifs in a subset of cellular and viral proteins. Seraphin et al. subsequently identified a second set of motifs (3), related to the first one, which is present in a novel series of proteins (3—5). Based on the known activity of several of these proteins, the polypeptides containing these motifs are believed to be endowed with helicase activity. The PIF protein, a protein implicated in yeast mitochondrial DNA recombination, has been suggested to be a distant member of this helicase super-family (1, 6). However, through one of the proposed motifs (motif V), the PIF protein departs from the helicase consensus by several amino-acid insertions and substitutions. Because motif V is one of the most highly conserved motifs (3), I searched the PIF protein sequence for a better match to the consensus motif V. I found an amino-acid stretch which differs by only one conservative substitution from the consensus (see Fig.). Furthermore, this new motif is localized in a conserved position relative to the other motifs of the helicase consensus (1-3) . These data indicate that the PIF protein is a full member of the helicase super-family. Interestingly, both the primary sequence and the location of the new motif V, suggest a stronger similarity between the PIF protein and putative helicases from herpesviruses.