MODELING A CONFORMATIONALLY SENSITIVE REGION OF THE MEMBRANE SECTOR OF THE FUNGAL PLASMA-MEMBRANE PROTON PUMP

A molecular model for transmembrane segments 1 and 2 from the fungal p roton pumping ATPase has been developed, and this structure is predict ed to form a helical hairpin loop structure in the membrane. This regi on was selected because it is highly conformationally active and is be lieved to be an important site of action for clinically important ther apeutics in related animal cell enzymes. The hairpin loop is predicted to form an asymmetric tightly packed structure that is stabilized by an N-cap between D140 and V142, by hydrogen bonding between residues i n the turn region and the helices, and by pi-pi interactions between c losely apposed aromatic residues. A short four-residue S-shaped turn i s stabilized by hydrogen bonding but is predicted to be conformational ly heterogeneous. The principal effect of mutations within the hairpin head region is to destabilize the local close packing of side groups which disrupts the pattern of hydrogen bonding in and around the turn region. Depending on the mutation, this causes either a localized or a more global distortion of the primary structure in the hairpin region . These altered structures may explain the effects of mutations in tra nsmembrane segments 1 and 2 on ATP hydrolysis, sensitivity to vanadate , and electrogenic proton transport. The conformational sensitivity of the hairpin structure around the S-turn may also account for the effe cts of SCH28080 and possibly ouabain in blocking ATPase function in re lated animal cell enzymes. Finally, the model of transmembrane segment s 1 and 2 serves as a template to position transmembrane segments 3 an d 8. This model provides a new view of the H+-ATPase that promotes nov el structure/function experimentation and could serve as the basis for a more detailed model of the membrane sector of this enzyme.