Interaction of the 33-kDa extrinsic protein with photosystem II: identification of domains on the 33-kDa protein that are shielded from NHS-biotinylation by photosystem II.

The structural association of the spinach 33-kDa extrinsic protein of photosystem II with the membrane-bound components of the photosystem was investigated by labeling the 33-kDa extrinsic protein with the amino group-specific reagent N-hydroxysuccinimidobiotin both on NaCl-washed photosystem II membranes and free in solution. After quenching of the labeling reagent and isolation of the biotinylated molecules, the biotinylation sites were identified by Staphylococcus V8 protease digestion and analysis of the resultant peptide fragment mixture by matrix-assisted laser desorption/ionization mass spectrometry. When the 33-kDa extrinsic protein was modified on PS II membranes, three domains were biotinylated: 14K, 41K-76K, and 190K-236K. When the 33-kDa extrinsic protein was modified in solution, four additional domains were biotinylated: 1E-4K, 20K, 101K-105K, and 159K-186K. These additional modified domains reside in portions of the 33-kDa protein that are not accessible to the bulk solvent when the protein is associated with PS II and may define regions of interaction with the photosystem.