Abstract
We analyzed a eukaryotically encoded rubredoxin from the cryptomonad Guillardia theta and identified additional domains at the N- and C-termini in comparison to known prokaryotic paralogous molecules. The cryptophytic N-terminal extension was shown to be a transit peptide for intracellular targeting of the protein to the plastid, whereas a C-terminal domain represents a membrane anchor. Rubredoxin was identified in all tested phototrophic eukaryotes. Presumably facilitated by its C-terminal extension, nucleomorph-encoded rubredoxin (nmRub) is associated with the thylakoid membrane. Association with photosystem II (PSII) was demonstrated by co-localization of nmRub and PSII membrane particles and PSII core complexes and confirmed by comparative electron paramagnetic resonance measurements. The midpoint potential of nmRub was determined as +125 mV, which is the highest redox potential of all known rubredoxins. Therefore, nmRub provides a striking example of the ability of the protein environment to tune the redox potentials of metal sites, allowing for evolutionary adaption in specific electron transport systems, as for example that coupled to the PSII pathway.
Highlights
We analyzed a eukaryotically encoded rubredoxin from the cryptomonad Guillardia theta and identified additional domains at the N- and C-termini in comparison to known prokaryotic paralogous molecules
In the course of an international effort to sequence the nucleomorph genome of the cryptomonad Guillardia theta [7], a nucleomorph-located and eukaryotically encoded rubredoxin gene was identified [8, 9]
In the conserved positions rubredoxin has high identity to prokaryotic paralogous molecules (Fig. 1), which were identified in several anaerobic and aerobic eubacteria [8]
Summary
We analyzed a eukaryotically encoded rubredoxin from the cryptomonad Guillardia theta and identified additional domains at the N- and C-termini in comparison to known prokaryotic paralogous molecules. Facilitated by its C-terminal extension, nucleomorph-encoded rubredoxin (nmRub) is associated with the thylakoid membrane. In contrast to other bacterial rubredoxins, the Synechocystis gene encodes a C-terminal extension, which was predicted to be a membrane anchor. In the course of an international effort to sequence the nucleomorph genome of the cryptomonad Guillardia theta [7], a nucleomorph-located and eukaryotically encoded rubredoxin gene was identified [8, 9]. We present data concerning the characterization of the encoded protein by means of immunological and biochemical methods and show the intracellular localization of rubredoxin as well as its redox properties. We present evidence that rubredoxin is associated with photosystem II (PSII) by colocalization of rubredoxin in BBYs and OG and DM cores
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