Tic62: a protein family from metabolism to protein translocation.
BACKGROUND
The function and structure of protein translocons at the outer and inner envelope membrane of chloroplasts (Toc and Tic complexes, respectively) are a subject of intensive research. One of the proteins that have been ascribed to the Tic complex is Tic62. This protein was proposed as a redox sensor protein and may possibly act as a regulator during the translocation process. Tic62 is a bimodular protein that comprises an N-terminal module, responsible for binding to pyridine nucleotides, and a C-terminal module which serves as a docking site for ferredoxin-NAD(P)-oxido-reductase (FNR). This work focuses on evolutionary analysis of the Tic62-NAD(P)-related protein family, derived from the comparison of all available sequences, and discusses the structure of Tic62.


RESULTS
Whereas the N-terminal module of Tic62 is highly conserved among all oxyphototrophs, the C-terminal region (FNR-binding module) is only found in vascular plants. Phylogenetic analyses classify four Tic62-NAD(P)-related protein subfamilies in land plants, closely related to members from cyanobacteria and green sulphur bacteria. Although most of the Tic62-NAD(P)-related eukaryotic proteins are localized in the chloroplast, one subgroup consists of proteins without a predicted transit peptide. The N-terminal module of Tic62 contains the structurally conserved Rossman fold and probably belongs to the extended family of short-chain dehydrogenases-reductases. Key residues involved in NADP-binding and residues that may attach the protein to the inner envelope membrane of chloroplasts or to the Tic complex are proposed.


CONCLUSIONS
The Tic62-NAD(P)-related proteins are of ancient origin since they are not only found in cyanobacteria but also in green sulphur bacteria. The FNR-binding module at the C-terminal region of the Tic62 proteins is probably a recent acquisition in vascular plants, with no sequence similarity to any other known motifs. The presence of the FNR-binding domain in vascular plants might be essential for the function of the protein as a Tic component and/or for its regulation.
Balsera
Mónica
M
Dep Biologie I, Botanisches Institut, LMU München, 80638 München, Germany. mbalsera@lrz.uni-muenchen.de
Stengel
Anna
A
Soll
Jürgen
J
Bölter
Bettina
B
D017360:Arabidopsis / Q000235:genetics*; D029681:Arabidopsis Proteins / Q000235:genetics* / Q000378:metabolism; D001483:Base Sequence; D001499:Bayes Theorem; D019414:Chlorobi / Q000235:genetics; D002736:Chloroplasts / Q000235:genetics*; D019295:Computational Biology; D000458:Cyanobacteria / Q000235:genetics; D056890:Eukaryota / Q000235:genetics; D019143:Evolution, Molecular; D005287:Ferredoxin-NADP Reductase / Q000378:metabolism; D016013:Likelihood Functions; D026901:Membrane Transport Proteins / Q000235:genetics* / Q000378:metabolism; D008957:Models, Genetic; D008958:Models, Molecular; D008969:Molecular Sequence Data; D005810:Multigene Family / Q000235:genetics*; D010802:Phylogeny; D011725:Pyridines / Q000378:metabolism; D016415:Sequence Alignment
D029681:Arabidopsis Proteins; D026901:Membrane Transport Proteins; D011725:Pyridines; C520728:Tic62 protein, Arabidopsis; D005287:Ferredoxin-NADP Reductase; C023666:pyridine