Subunit stoichiometry and juxtaposition of the photosynthetic coupling factor 1: Immunoelectron microscopy using monoclonal antibodies

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Abstract
Monoclonal antibodies specific to the .alpha. subunits of the photosynthetic coupling factor 1 (CF1) were used as marker molecules in an electron microscopic analysis of the subunit organization of this enzyme. Immune complexes were obtained by incubation of CF1 with saturating amounts of anti-.alpha.-subunit IgG, isolated by gel filtration, and visualized by electron microscopy. The maximum number of antibodies bound to a CF1 molecule was three, the angle defined by a neighboring pair of antibodies characteristically being 120.degree.. These results are interpreted as direct evidence for the presence of three .alpha. subunits in the CF1 complex, the relative orientation of them being described by 3-fold rotary symmetry. Our observations thus favor an overall subunit stoichiometry of .alpha.3.beta.3.gamma..delta..epsilon.