Solution structure of a zinc substituted eukaryotic rubredoxin from the cryptomonad alga Guillardia theta

The rubredoxin from the cryptomonad Guillardia theta is one of the first ex amples of a rubredoxin encoded in a eukaryotic organism. The structure of a soluble zinc-substituted 70-residue G. theta rubredoxin lacking the membra ne anchor and the thylakoid targeting sequence was determined by multidimen sional heteronuclear NMR, representing the first three-dimensional (3D) str ucture of a eukaryotic rubredoxin. For the structure calculation a strategy was applied in which information about hydrogen bonds was directly inferre d from a long-range HNCO experiment, and the dynamics of the protein was de duced from heteronuclear nuclear Overhauser effect data and exchange rates of the amide protons. The structure is well defined, exhibiting average roo t-mean-square deviations of 0.21 Angstrom for the backbone heavy atoms and 0.67 Angstrom for all heavy atoms of residues 7-56, and an increased flexib ility toward the termini. The structure of this core fold is almost identic al to that of prokaryotic rubredoxins. There are, however, significant diff erences with respect to the charge distribution at the protein surface, sug gesting that G. theta rubredoxin exerts a different physiological function compared to the structurally characterized prokaryotic rubredoxins. The ami no-terminal residues containing the putative signal peptidase recognition/c leavage site show an increased flexibility compared to the core fold, but s till adopt a defined 3D orientation, which is mainly stabilized by nonlocal interactions to residues of the carboxy-terminal region. This orientation might reflect the structural elements and charge pattern necessary for corr ect signal peptidase recognition of the G. theta rubredoxin precursor.