Differential exposure of surface epitopes in the β-strand region of LOOP1 of the yeast H⁺-ATPase during catalysis

The plasma membrane H⁺-ATPase of yeast assumes distinct conformational states during its catalytic cycle. To better understand structural changes in the LOOP1 domain, a catalytically important cytoplasmic loop segment linking transmembrane segments 2 and 3, surface epitopes were examined at different stages of catalysis. A polyclonal rabbit antibody was prepared to a fusion protein consisting of LOOP1 and the maltose binding protein. This antibody was affinity-purified to produce a LOOP1-specific fraction that could be used in competition enzyme-linked immunosorbent assays to assess surface exposure of the LOOP1 epitopes. It was found that in an E₁ conformation stabilized with either adenosine 5'-(β,γ-imino)triphosphate (AMP-PNP) or ADP, less than 10% of the LOOP1 epitopes were accessible on native enzyme. However, when the enzyme was stabilized in an E₂-state with ATP plus vanadate, approximately 40% of the surface epitopes on LOOP1 became accessible to antibody. The remaining 60% of the LOOP1 epitopes were fully occluded in the native enzyme and never showed surface exposure. Enzyme-linked immunosorbent assays utilizing fusion proteins consisting of LOOP1 subdomains demonstrated that all of the available epitopes were contained in the β-strand region (Glu-195-Val-267) of LOOP1. The epitopes that were differentially exposed during catalysis were included in regions upstream and downstream of the highly conserved TGES sequence. Our results suggest that during catalysis either the β-strand region of LOOP1 or an interacting domain undergoes substantial structural rearrangement that facilitates epitope exposure.