ORGANIZATION OF PHOTOSYSTEM-I POLYPEPTIDES - A STRUCTURAL INTERACTIONBETWEEN THE PSAD AND PSAL SUBUNITS

The wild-type, PsaD-less, and PsaL-less strains of the cyanobacterium Synechocystis sp. PCC 6803 were used to study subunit interactions in photosystem I (PSI). When the membranes of a PsaD-less strain were sol ubilized with Triton X-100 and PSI was purified using ion-exchange chr omatography and sucrose-gradient ultracentrifugation, the PsaL subunit was substantially removed from the core of PSI, whereas other subunit s, such as PsaE and PsaF, were quantitatively retained during purifica tion. When the wild-type PSI was exposed to increasing concentrations of Nal, the PsaE, PsaD, and PsaC subunits were gradually removed, wher eas PsaF, PsaL, PsaK, and PsaJ resisted removal by up to 3 M Nal. The absence of PsaL enhanced the accessibility of PsaD to removal by Nal. Treatment of the wild-type PSI complexes with glutaraldehyde at 4 degr ees C resulted in a 29-kD cross-linked product between PsaD and PsaL. The formation of such cross-linked species was independent of PSI conc entrations, suggesting an intracomplex cross-linking between PsaD and PsaL. Taken together, these results demonstrate a structural interacti on between PsaD and PsaL that plays a role in their association with t he PSI core.