The solution structure of photosystem I accessory protein E from the cyanobacterium Nostoc sp strain PCC 8009

PsaE is a small basic subunit located on the stromal (cytoplasmic) side of photosystem I. In cyanobacteria, this subunit participates in cyclic electr on transport and modulates the interactions of the complex with soluble fer redoxin. The PsaE protein isolated from the cyanobacterium Synechococcus sp . strain PCC 7002 adopts the beta topology of an SH3 domain, with five beta strands (beta A through beta E) and a turn of 3(10) helix between strands beta D and beta E [Falzone, C. J., Kao, Y.-H., Zhao, J., Bryant, D. A., and Lecomte, J. T. J. (1994) Biochemistry 33, 6052-6062]. The primary structur e of the PsaE protein is strongly conserved across all oxygen-evolving phot osynthetic organisms. However, variability in loop lengths, as well as N- o r C-terminal extensions, suggests that the structure of a second representa tive PsaE subunit would be useful to characterize the interactions among ph otosystem I polypeptides. In this work, the solution structure of PsaE from the cyanobacterium Nostoc sp. strain PCC 8009 was determined by NMR method s. Compared to PsaE from Synechococcus sp. strain PCC 7002, this PsaE has a seven-residue deletion in the loop connecting strands beta C and beta D, a nd an eight-residue C-terminal extension. Angular and distance restraints d erived from homonuclear and heteronuclear NMR experiments were used to calc ulate structures by a distance-geometry/simulated-annealing protocol. A fam ily of 20 structures (rmsd of 0.24 Angstrom in the regular secondary struct ure) is presented. Differences between the two cyanobacterial proteins are mostly confined to the CD loop region; the C-terminal extension is disorder ed. The thermodynamic stability of Nostoc sp. strain PCC 8009 PsaE toward u rea denaturation was measured by circular dichroism and fluorescence spectr oscopy, and thermal denaturation was monitored by UV absorption spectroscop y. Chemical and thermal denaturation curves are modeled satisfactorily with two-state processes. The Delta G degrees of unfolding at room temperature is 12.4 +/- 0.3 kJ mol(-1) (pH 5), and the thermal transition midpoint is 5 9 +/- 1 degrees C (pH 7). Interactions with other proteins in the photosyst em I complex may aid in maintaining PsaE in its native state under physiolo gical conditions.