MODELING OF THE PHOTOSYSTEM-II 33 KDA PROTEIN - STRUCTURE, FUNCTIONS AND POSSIBLE SULFATE-SENSITIVE SITES DERIVED FROM SEQUENCE-ENCODED INFORMATION

A recent study revealed that SO2 inhibits higher plant Photosystem II (PSII) activity via the action of sulfate on two oxygen-evolving complex functions: (1) the binding of chloride, and (2) the binding of the 23 and 18 kDa extrinsic proteins (BEAUREGARD, M. (1991) Envir. exp. Bot. 31, 11-21). The ''33 kDa'' protein of PSII is involved in these two functions and therefore represents a key to the understanding of sulfate action at the molecular level. This protein is, however, poorly characterized, regarding both structure and functions. Here the information content of the 33 kDa protein from seven species is analyzed and used to build a working model for this protein. The prediction presented corroborates structural data available in the literature for spinach 33 kDa protein. Three beta-strands (around L146, S202 and G215), an exposed alpha-helix at Q185 and many chain reversals were predicted for four 33 kDa proteins from higher plants. A similar consensus for secondary structures and exposure propensity is found for cyanobacterial proteins, whereas the ion-binding sites proposed for higher plants are not well conserved in cyanobacteria. For higher plants: (1) a calcium-binding site is proposed around K101, (2) seven domains (E1 to G2, Q12 to T15, Y45 to K49, K101 to E103, K137 to D139, R178 to E182 and D207 to E209) and 1 3 residues are identified as possible sulfate-sensitive sites in spinach 33 kDa protein. The results arc integrated in the first model for the 33 kDa extrinsic protein of Photosystem II that includes structure and functions.