MOLECULAR ORGANIZATION OF A MEMBRANE PROTEIN COMPLEX

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Our research objectives are: 1) to determine the structural topology of the photosystem II (PS II) core complex polypeptides, 2) to characterize the structural relationships between the subunits and 3) to generate site specific mutations of residues which are predicted to coordinate electron transport components, set redox potentials and regulate charge transfer kinetics. Recently, we have developed methods to rapidly locate the position and orientation of transmembrane polypeptide spans using site specific antibodies generated against synthetic peptides. The synthetic peptide antigens correspond to hydrophilic sequences bracketing each potential transmembrane span. By analysis of the antibody binding patterns to inside out and right side out membranes we determine the position and orientation of transmembrane spans. The results of our investigations indicated that the numbers and orientations of transmembrane spans in the D1 protein of the PS II complex did not correspond with that predicted by hydrophobicity plots. In addition it was apparent that the D1 protein was structurally homologous to the L subunit of the Rhodopsuedomonas viridis photosynthetic reaction center. The implication of these results is that the D1 protein coordinates the PS II reaction center primary electron acceptors and donors in a heterodimer complex (D1 and D2) similar to the R. viridis L and M subunits. Using the aforementioned immunological techniques we will continue our analyses of the protein topology of the PS II complex polypeptides including the D2, 43 and 47 kd chlorophyll binding proteins and the cytochrome b559 subunits. These studies will be supplemented by site directed mutagenesis of residues which bind the electron donors and acceptors and which regulate PS II reaction center photochemistry. These investigators will determine whether complex membrane protein structures can be effectively dissected using a variety of immunological and recombinant DNA techniques. We propose that this research will have broad application in many membrane protein systems.