PsaE is a 69 amino acid polypeptide from photosystem I present on the stromal side of the thylakoid membrane. The three-dimensional solution structure of this protein from the cyanobacterium Synechococcus sp. strain PCC 7002 was determined at pH 5.8 and room temperature using over 900 experimental restraints derived from two- and three-dimensional NMR experiments. The structure is comprised of a well-defined five-stranded β-sheet with (+1, +1, +1, −4x) topology. There is no helical region except for a single turn of 310 helix between the βD and βE strands. PsaE also exhibits a large unrestrained loop spanning residues 42–56. A comparison to known protein structures revealed similarity with the Src homology 3 (SH3) domain, a membrane-associated protein involved in signal transduction in eukaryotes. The match is remarkable as 47 of the α-carbons of PsaE can be superimposed onto those of the SH3 domain from chicken brain α-spectrin with a root-mean-square deviation of 2.3 Å. Although the amino acid sequences have low identity and the loops are different in both proteins, the topology of the β-sheet and the 310 turn is conserved. SH3 domains from other sources show a similar structural homology. The structure of PsaE was used to suggest approaches for elucidating its roles within photosystem I.
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