STRUCTURE AND PROPERTIES IN SOLUTION OF PSAD, AN EXTRINSIC POLYPEPTIDE OF PHOTOSYSTEM-I

PsaD is a small, extrinsic polypeptide located on the stromal side (cy toplasmic side in cyanobacteria) of the photosystem I reaction centre complex. The gene from the cyanobacterium Nostoc sp. PCC 8009 was expr essed in Escherichia coli and the structure of the recovered protein i n solution investigated. Size-exclusion chromatography, dynamic light scattering and measurement of N-15 transverse relaxation times showed that the protein is a stable dimer in solution, whereas in the reactio n centre complex it is a monomer. NMR experiments showed that the dime r is symmetrical and that there are at least two domains, one structur ed and the remainder unstructured. The structured domain contains a sm all amount of P-sheet. Three-dimensional heteronuclear NMR spectra of [C-13, N-15]PsaD showed that the structured domain is associated with the central part of the sequence while the N- and C-terminal regions a re mobile. Evidence was obtained for a shift in equilibrium between tw o slightly different conformational states at about pH 6, and the prot ein was shown to bind to PsaE preferentially at neutral pH. Addition o f trifluoroethanol was shown to induce the formation of a small amount of a-helix, and the form present in 30% trifluoroethanol appears to b e more closely related to the in situ structure, which has been report ed to contain one short helix in crystals [Schubert, W.-D., Klukas, O. , Krauss, N., Saenger, W., Fromme, P. & witt, H. T. (1997) J. Mol. Bio l. 272, 741-769]. The significance of these findings for the assembly of the complex is discussed.