Characterization and N‐terminal sequence of a 5 kDa polypeptide in the photosystem I core complex from spinach

Abstract

Photosystem I core complexes were isolated from spinach photosystem I particles after heat treatment in the presence of 50% (v/v) ethylene glycol (heat/EG treatment). The core complex from 58°C/EG‐treated particles was composed of polypeptides with apparent molecular masses of 63, 60 and 5 kDA; this complex contained the iron sulfur center F_x but lacked center F_A and F_B. The core complex obtained from the 70°C/EG‐treated preparation lacked F_x and contained a lesser amount of the 5 kDa polypeptide. The N‐terminal amino acid sequence of the 5 kDa polypeptide did not correspond to the sequence derived from any possible reading frame in the chloroplast DNA of liverwort or tobacco. Twelve of the first 29 N‐terminal amino acids were hydrophobic, suggesting that this protein is intrinsic to the photosystem I reaction center.