Binding of Antibodies onto the Thylakoid Membrane VII. Localization of Coupling Factor o f Photophosphorylation in the Lamellar System o f Chloroplasts from Antirrhinum majus

The maximal binding of antibodies against the subunits of the coupling factor of photo­ phosphorylation onto the ultrasonic sediment and to stroma-freed chloroplasts o f Antirrhinum majus was determined. Different surfaces o f the thylakoid membrane are accessible to antibodies in both chloroplast preparations. Stroma-freed chloroplasts bind antibodies only at the outer sur­ face, which is directed towards the stroma in intact chloroplasts. In the ultrasonic sediment, which is the product of ultrasonication and centrifugation, as was demonstrated by electronmicroscopy, the major part of the surface directed towards the inside of the thylakoids is accessible. Both chloroplasts preparations are able to bind different amounts o f antibodies to the five sub­ units of the coupling factor. While antigenic determinants o f all five subunits are present on the surface directed to the outside, the «^-components seems to be lacking on the surface directed towards the inside. The orand e-component bind on both surfaces nearly the same amount of antibodies. Antigenic determinants o f the /?-subunit seem to exist in higher concentrations on the surface towards the outside and those o f the y-subunit on the surface towards the inside. From the serological investigations it follows that coupling factor molecules span the thylakoid membrane from the outside to the inside. They are not only present in those parts of the thylakoid membrane, exposed in intact chloroplasts to the stroma, but also in stacked thylakoids. The differences between the results of other authors and the present results are discussed.