Identification of a 19-kDa polypeptide as an Fe-S center apoprotein in the photosystem I primary electron acceptor complex.

Treatment of Photosystem I (PSI) with sodium thiocyanate, a chaotropic agent, results in the selective depletion of certain low-molecular-weight polypeptides. A PSI complex obtained following treatment with 0.5 M sodium thiocyanate is significantly depleted of polypeptides of approximately 8, 10, 14, and 16 kDa, relative to an untreated control, but retains approximately 90% of the EPR signal amplitude associated with the iron-sulfur Centers A and B. The only peptides remaining that could not be depleted without a parallel decrease in the signal amplitude of the Fe-S Centers A and B are the 62-kDa reaction center-containing polypeptide and a 19-kDa polypeptide. These results are considered in relation to the identity of the apoprotein of the Fe-S Centers A and B.