Effects of Proteolysis on the Photochemical Activity and Polypeptide Composition of the Photosystem II Core Complex...

The photosystem II core complex (TSF-IIa) is composed of polypeptides of molecular weight 54-, 47-, 42-, and 30 kilodaltons (kD) and cytochrome b-559. After treatment with trypsin or alpha-chymotrypsin for 20 hours, the TSF-IIa particles still retained their photochemical activity and the light-induced cytochrome b-559 signal, although all of the polypeptides of the complexes, except the 30 kD unit were extensively degraded. Proteolytic treatment decreased the apparent molecular weight of the complex from 250,000 to 100,000 daltons as determined by gel filtration, and also decreased the protein to chlorophyll ratio by 40%. Chlorophyll a appeared to be associated with the 47- and 42 kD polypeptides. Proteolysis of the complex produced a single chlorophyll a band with a slightly higher electrophoretic mobility. This band was not equivalent to the 30 kD polypeptide. Proteolysis also reduced the sensitivity of the TSF-IIa particles to 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU), but did not completely abolish it.It was concluded that although the polypeptides of the photosystem II core complex were cleaved by the proteases, many of the peptide fragments do not dissociate from the complex, particularly the chlorophyll a-containing portions, leaving the complex photochemically active. This supports the concept that chlorophyll has a structural function in chlorophyll-protein complexes.