Identification of chloroplast membrane peptides with subunits of coupling factor and ribulose-1,5 diphosphate carboxylase

Abstract

Coupling factor 1 and ribulose-diphosphate carboxylase are the main peripheral proteins associated with chloroplast internal membranes. The two proteins were sequentially solubilized and purified by gel filtration and their subunit structure was characterized by sodium dodecyl sulfate polyacrylamide gel electrophoresis. The differences between the polypeptide profiles of the insoluble membrane fraction, before and after extraction of these oligomeric proteins, allowed identification of original membrane peptides with specific protein subunits. The 52,000 and 14,000 molecular weight peaks are identical to the large and small subunits, respectively, of ribulose-diphosphate carboxylase; the 56,000 and 53,000 peaks are identified with the α and β subunits, respectively, of the coupling factor protein. These identifications, together with earlier studies on the 25,000 M r band, assign a physiological role to the most prominent peptides of chloroplast internal membranes. Now it becomes apparent that the major membrane polypeptides do not directly relate to photosynthetic electron transport components, but rather to enzymatic capacities associated with this process and to the light-gathering antenna of the photosynthetic unit. The observation that chloroplast coupling factor 1 dissociates during gel filtration, with preferential loss of the smaller subunits ( M r < 50,000) is discussed in relation to the possible function of these subunits in situ in the thylakoid membrane.