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The F1 moiety of the mitochondrial ATP synthase is composed of five different
subunits with stoichiometry alpha 3 beta 3 gamma delta epsilon and exhibits the
capacity to synthesize ATP from ADP and Pi. We have previously crystallized rat
liver F1 and described its structure at 9-A resolution (Amzel, L. M., McKinney,
M., Narayanan, P., and Pedersen, P. L. (1982) Proc. Natl. Acad. Sci. U. S. A.
79, 5852-5856). Here we present an x-ray map of this complex enzyme at 3.6 A,
which provides a much more informative description of its quaternary structure.
The overall dimensions of the F1 molecule are 120 A x 120 A x 74 A. The enzyme
exhibits 3-fold symmetry relating the three copies of each of the two major
subunits, alpha and beta. As the alpha subunits (but not the beta subunits)
contain cysteine residues, it has been possible to identify the alpha subunits
by heavy atom labeling with mersalyl and to relate their positions in the F1
molecule to the beta subunits. Significantly, the alpha and beta subunits each
exist as trimeric arrays which are organized in two slightly offset,
interdigitated layers along the 3-fold axis. In one trimeric layer the alpha
subunits are located close to the axis with homologous subunits interacting with
each other; in the other trimeric layer the beta subunits are far from the axis,
and they interact only with alpha subunits and not with one another. At one end
of the structure, part of the interface between each alpha and beta subunit
encloses a space or "pocket" that is accessible to the solvent; at the other end
the interfaces between the subunits are more open and exposed. The present work
represents the highest resolution map reported to date for the F1 moiety of an
ATP synthase complex.
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