The signaling interface of the yeast multidrug transporter Pdr5 adopts a cis conformation, and there are functional overlap and equivalence of the deviant and canonical Q-loop residues.

ABC transporters are polytopic proteins. ATP hydrolysis and substrate transport take place in separate domains, and these activities must be coordinated through a signal interface. We previously characterized a mutation (S558Y) in the yeast multidrug transporter Pdr5 that uncouples ATP hydrolysis and drug transport. To characterize the transmission interface, we used a genetic screen to isolate second-site mutations of S558Y that restore drug transport. We recovered suppressors that restore drug resistance; their locations provide functional evidence for an interface in the cis rather than the trans configuration indicated by structural and cross-linking studies of bacterial and eukaryotic efflux transporters. One mutation, E244G, defines the Q-loop of the deviant portion of NBD1, which is the hallmark of this group of fungal transporters. When moved to an otherwise wild-type background, this mutation and its counterpart in the canonical ATP-binding site Q951G show a similar reduction in drug resistance and in the very high basal-level ATP hydrolysis characteristic of Pdr5. A double E244G, Q951G mutant is considerably more drug sensitive than either of the single mutations. Surprisingly, then, the deviant and canonical Q-loop residues are functionally overlapping and equivalent in a strikingly asymmetric ABC transporter.