Use of a monoclonal antibody in structural investigations of the 49-kDa polypeptide of photosystem II.

A monoclonal antibody, FAC2, was isolated by immunization of mice with a Photosystem II core preparation followed by splenic fusion and standard monoclonal antibody screening and production techniques. This antibody recognizes the 49-kDa polypeptide of Photosystem II which is the apoprotein of CPal. The antigenic determinant recognized by this antibody lies on a cyanogen bromide fragment which appears as a doublet with an apparent molecular mass of 14.5 kDa. FAC2 was used to follow the effects of trypsin on the 49-kDa polypeptide in a membrane environment. Our results indicate that the extrinsic polypeptides of Photosystem II which are known to be involved in oxygen evolution protect the 49-kDa polypeptide from tryptic attack. Additionally, Photosystem II membranes which are treated with alkaline Tris exhibit a large increase in the ability to bind FAC2. This increase is not observed with membranes treated with calcium chloride or sodium chloride. These results indicate that the 49-kDa polypeptide may be at least structurally associated with the component(s) responsible for oxygen evolution.