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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., Mühlenhoff, U., Bryant, D.A., & 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., Mühlenhoff, U.,
Rhiel, E., Warren, P. V., Golbeck, J. H., & Bryant, D. A. (1993) Mol.
Microbiol. 9, 183-194]. By using both unlabeled and uniformly 15N-labeled
protein in a series of two- and three-dimensional NMR experiments, complete 1H
and 15N 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 pK1/2 of the sole
histidine residue (His-63) was determined to be 5.4.
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