H-1 AND N-15 NMR ASSIGNMENTS OF PSAE, A PHOTOSYSTEM-I SUBUNIT FROM THE CYANOBACTERIUM SYNECHOCOCCUS SP STRAIN PCC-7002

PsaE is a highly conserved, water-soluble protein of the photosystem I reaction center complexes of cyanobacteria, algae, and green plants. Along with the PsaC and PsaD proteins, the PsaE protein binds to the stromal surface of photosystem I and is required for cyclic electron transport in Synechococcus sp. strain PCC 7002 [Yu, L., Zhao, J., Muhlenhoff, U., Bryant, D. A., and Golbeck, J. H. (1993) Plant Physiol. 103, 171-180]. The psaE gene from this cyanobacterium encodes a mature protein of 69 amino acid residues and has recently been overexpressed in Escherichia coli [Zhao, J., Snyder, W. B., Muhlenhoff, U., Rhiel, E., Warren, P. V., Golbeck, J. H., and Bryant, D. A. (1993) Mol. Microbiol. 9, 183-194]. By using both unlabeled and uniformly N-15-labeled protein in a series of two- and three-dimensional NMR experiments, complete H-1 and N-15 amide resonance assignments were made. The major secondary structural element of PsaE is a five-stranded antiparallel beta-sheet. The five strands extend as follows: beta A, residues 7-10; beta B, residues 21-26; beta C, residues 36-39; beta D, residues 57-60; and beta E, residues 65-68. The topology is represented by (+1, +1, +1, -4x); it brings the first and last strands, and consequently the N- and C-termini, together. The protein has an extensive hydrophobic core organized around a conserved phenylalanine residue (Phe-40); another of its distinctive features is a segment extending from residue 42 to residue 56 devoid of dipolar contacts with the beta-sheet. The pK(1/2) of the sole histidine residue (His-63) was determined to be 5.4.