The three-dimensional structure of the Manduca sexta midgut V-1 ATPase has
been determined at 3.2 nm resolution from electron micrographs of negativel
y stained specimens. The V-1 complex has a barrel-like structure Il nm in h
eight and 13.5 nm in diameter. It is hexagonal in the top view, whereas in
the side view, the six large subunits A and B are interdigitated for most o
f their length (9 nm). The topology and importance of the individual subuni
ts of the V-1 complex have been explored by protease digestion, resistance
to chaotropic agents, MALDI-TOF mass spectrometry, and CuCl2-induced disulf
ide formation. Treatment of V-1 with trypsin or chaotropic iodide resulted
in a rapid cleavage or release of subunit D from the enzyme, indicating tha
t this subunit is exposed in the complex. Trypsin cleavage of V-1 decreased
the ATPase activity with a time course that was in line with the cleavage
of subunits B, C, G, and F. When CuCl2 was added to V-1 in the presence of
CaADP, the cross-linked products A-E-F and B-H were generated. In experimen
ts where CuCl2 was added after preincubation of CaATP, the cross-linked pro
ducts E-F and E-G were formed. These changes in cross-linking of subunit E
to near-neighbor subunits support the hypothesis that these are nucleotide-
dependent conformational changes of the E subunit.