Extrinsic polypeptides of spinach photosystem I.

By combining Triton X-114 partitioning with alkaline-salt and chaotropic washings of thylakoid membrane vesicles and photosystem I particles, we have studied the protein subunit composition and organization of spinach photosystem I. Upon fractionation of photosystem I particles with Triton X-114, 6 polypeptides of 5.0, 8.2 (psaE), 10.5, 16.6 (psaG), 19.3 and 22.1 kDa (psaD) were considered to be extrinsic membrane proteins. By combining this partitioning with salt washes of thylakoid membranes, the polypeptides of 8.2, 11.6 (psaH), 19.3 and 22.1 kDa were directly shown to be stromally oriented and extrinsic while no extrinsic subunits were identified at the inner thylakoid surface. The 5.0, 8.2, 10.5, 17.2, 19.3 and 22.1 kDa polypeptides appear to have regulatory rather than catalytic functions as their release from photosystem I particles upon high salt-alkali treatment does not affect photosystem I-mediated electron transport.