SECONDARY STRUCTURE OF THE 33 KDA, EXTRINSIC PROTEIN OF PHOTOSYSTEM-II - A FAR-UV CIRCULAR-DICHROISM STUDY

The 33 kDa extrinsic protein of Photosystem II is an important compone nt of the oxygen-evolving apparatus which functions to stabilize the m anganese cluster at physiological chloride concentrations and to lower the calcium requirement for oxygen evolution. Chou-Fasman analysis of the amino-acid sequence of this protein suggests that this component contains a high proportion of alpha-helical structure and only relativ ely small amounts of beta-sheet structure. A computational study using more sophisticated techniques (Beauregard, M. (1992) Environ. Exp. Be t. 32, 411-429) concluded that the protein contained little periodical ly ordered secondary structure. In this study, we have directly measur ed the relative proportions of secondary structure present in the 33 k Da protein using far-ultraviolet circular dichroism spectroscopy. Our results indicate that, in solution, this protein contains a large prop ortion of beta-sheet structure (38%) and relatively small amounts of a lpha-helical structure (9%). A structural model of the 33 kDa protein based on a constrained Chou-Fasman analysis (Teeter, M.M. and Whitlow, M (1988) Proteins 4, 262-273) is presented.