A common mechanism for ATP hydrolysis in ABC transporter and helicase superfamilies

ABC (ATP-binding cassette) transporters and helicases are large superfamili es of seemingly unrelated proteins, whose functions depend on the energy pr ovided by ATP hydrolysis. Comparison of the 3D structures of their nucleoti de-binding domains reveals that, besides two well-characterized ATP-binding signatures, the folds of their nucleotide-binding sites are similar. Furth ermore, there are striking similarities in the positioning of residues thou ght to be important for ATP binding or hydrolysis. Interestingly, structure s have recently been obtained for two ABC proteins that are not involved in transport activities, but that have a function related to DNA modification . These ABC proteins, which contain a nucleotide-binding site akin to those of typical ABC transporters, might constitute the missing link between the two superfamilies.