Transmembrane organization of individual polypeptides of Photosystem I complex and the light-harvesting complex of Photosystem II of thylakoid membranes

Several complementary techniques have been explored to investigate the transmembrane disposition of individual polypeptides of the Photosystem (PS) I complex and the light-harvesting complex of PS II in thylakoid membranes. The immunological technique of Western blotting was used either in combination with antibody depletion by thylakoid vesicles or in combination with surface proteolytic digestion of the membranes. By using mild protease digestion combined with the sensitive immunological technique of Western blotting at least six of the nine subunits of the PS I complex examined were shown to traverse the membrane. Subunits 3 and 4 were accessible to proteases only at the outer surface and subunit 5 was inaccessible from either face. That the PS I complex as a whole spans the membrane was confirmed independently by the ability of an antiserum specific for PS I complex to agglutinate both right-side-out and everted vesicles. For the light-harvesting complex II, both the major 28 and 26.7 kDa subunits have antigenic sites exposed at both the inner and outer surfaces. This shows that not only the whole complex, but both of these individual subunits, span the thylakoid membrane.