Electron microscopy and single molecule averaging of subunit-deficient F1-ATPases from Escherichia coli and spinach chloroplasts.

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Abstract
The morphology of F1‐ATPases lacking one or more small subunits has been investigated by minimal‐beam electron microscopy of close‐packed monolayers of molecules. Computer‐based rotational analyses of single molecules were performed on reconstituted 3‐subunit F1‐ATPase (‐delta epsilon) from Escherichia coli and both 3‐subunit (‐delta epsilon) and 4‐subunit (‐delta) F1‐ATPase from chloroplasts. Optical diffraction measurements of close‐packed arrays revealed maximal dimensions of 122 +/‐ 4 A and 129 +/‐ 9 A for 3‐subunit ECF1 and 4‐subunit CF1, respectively. Molecules which displayed either hollow or solid hexagonal morphologies were observed in all preparations. Averaged reconstructions were obtained from molecules with hollow morphologies in 3‐subunit preparations and demonstrated strong hexagonal symmetry in projection with a central, stain‐filled cavity. The average reconstruction obtained from molecules with the solid morphology in 4‐subunit CF1 preparations, was also strongly hexagonal with six peripheral units ringed about a central subunit. Differences between hollow and solid morphologies cannot be attributed solely to the presence or absence of the delta and epsilon subunits; therefore, the two image types may represent staining variants of a common structure. Overall, the reconstructions are consistent with an alpha 3 beta 3 gamma stoichiometry for the coupling factors from both E. coli and chloroplasts.