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

Monoclonal antibodies specific to the α 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-α-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° . These results are interpreted as direct evidence for the presence of three α 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 α₃β₃γδε.