3-DIMENSIONAL SOLUTION STRUCTURE OF PSAE FROM THE CYANOBACTERIUM SYNECHOCOCCUS SP STRAIN PCC-7002, A PHOTOSYSTEM-I PROTEIN THAT SHOWS STRUCTURAL HOMOLOGY WITH SH3 DOMAINS

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. s train PCC 7002 was determined at pH 5.8 and room temperature using ove r 900 experimental restraints derived from two- and three-dimensional NMR experiments. The structure is comprised of a well-defined five-str anded beta-sheet with (+1, +1, +1, -4x) topology. There is no helical region except for a single turn of 3(10) helix between the beta D and beta E strands. PsaE also exhibits a large unrestrained loop spanning residues 42-56. A comparison to known protein structures revealed simi larity with the Src homology 3 (SH3) domain, a membrane-associated pro tein involved in signal transduction in eukaryotes. The match is remar kable as 47 of the alpha-carbons of PsaE can be superimposed onto thos e of the SH3 domain from chicken brain alpha-spectrin with a root-mean -square deviation of 2.3 Angstrom. Although the amino acid sequences h ave low identity and the loops are different in both proteins, the top ology of the beta-sheet and the 3(10) turn is conserved. SH3 domains f rom other sources show a similar structural homology. The structure of PsaE was used to suggest approaches for elucidating its roles within photosystem I.