Polypeptide Composition of Higher Plant Photosystem I Complex. Identification of PsaI, PsaJ and PsaK Gene Products

High resolution gel electrophoresis of the native photosystem I complex retaining light-harvesting chlorophyll complex revealed the presence of three low-molecular-mass proteins of 7, 4.1 and 3.9 kDa in spinach, and 6.8, 4.4 and 4.1 kDa in pea, in addition to the other well-characterized higher-molecular-mass components. Upon further detergent treatment to deplete light-harvesting chlorophyll complex, the 7 kDa and 4.1 kDa proteins were removed from the photosystem I core complex of spinach, while the 3.9 kDa protein was retained. N-terminal sequencing demonstrated that the 4.1 kDa proteins from both spinach and pea correspond to the gene product of ORF42/44 in chloroplast genome of liverwort and higher plants, which was previously hypothesized as a photosystem I gene (psaJ) based on sequence homology with the cyanobacterial photosystem I component of 4.1 kDa [(1989) FEBS Lett. 253, 257-263]. N-terminal sequence of the spinach 3.9 kDa and pea 4.4 kDa proteins fitted with chloroplast ORF36/40 (psaI) although no homologue has been found in cyanobacteria. The spinach 7 kDa and pea 6.8 kDa proteins correspond to the nuclear-encoded psaK product and significantly matched with the N-terminal sequence of the cyanobacterial 6.5 kDa subunit. The evolutional conservation of the psaJ and psaK seems to suggest their intrinsic role(s) in photosystem I.