Organization and topology of photosystem I subunits.
Intact spinach (Spinacia oleracea) thylakoid membranes were treated with various proteases and photosystem I (PSI) complexes were isolated from these membranes to define the membrane topology of specific PSI subunits. Trypsin treatment caused cleavage of the PSI-D and E subunits. Thermolysin treatment cleaved the PSI-D, E, H, and K subunits, and also caused limited degradation of the reaction center core PSI-A and B subunits. Pronase treatment produced the most dramatic results as the PSI-A and B subunits were cleaved to 47-, 45-, 26-, and 24-kilodalton products. In addition, pronase degraded the PSI-D, E, H, K, and L subunits. Proteolytic cleavage sites for several of the products were identified by amino acid sequencing. The results indicate that PSI-A, B, D, E, H, K, and L subunits all have stroma-exposed regions, and these findings are summarized in a model describing the subunit organization of PSI.
Zilber
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Department of Plant Biology, 111 Genetics and Plant Biology Building, University of California at Berkeley, Berkeley, California 94720.
Malkin
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