Functional similarity between the chloroplast translocon component, Tic40, and the human co-chaperone, Hsp70-interacting protein (Hip).
Tic40 is a component of the protein import apparatus of the inner envelope of chloroplasts, but its role in the import mechanism has not been clearly defined. The C terminus of Tic40 shares weak similarity with the C-terminal Sti1 domains of the mammalian Hsp70-interacting protein (Hip) and Hsp70/Hsp90-organizing protein (Hop) co-chaperones. Additionally, Tic40 may possess a tetratricopeptide repeat (TPR) protein-protein interaction domain, another characteristic feature of Hip/Hop co-chaperones. To investigate the functional importance of different parts of the Tic40 protein and to determine whether the homology between Tic40 and co-chaperones is functionally significant, different Tic40 deletion and Tic40:Hip fusion constructs were generated and assessed for complementation activity in the Arabidopsis Tic40 knock-out mutant, tic40. Interestingly, all Tic40 deletion constructs failed to complement tic40, indicating that each part removed is essential for Tic40 function; these included a construct lacking the Sti1-like domain (DeltaSti1), a second lacking a central region, including the putative TPR domain (DeltaTPR), and a third lacking the predicted transmembrane anchor region. Moreover, the DeltaSti1 and DeltaTPR constructs caused strong dominant-negative, albino phenotypes in tic40 transformants, indicating that the truncated Tic40 proteins interfere with the residual chloroplast protein import that occurs in tic40 plants. Remarkably, the Tic40:Hip fusion constructs showed that the Sti1 domain of human Hip is functionally equivalent to the Sti1-like region of Tic40, strongly suggesting a co-chaperone role for the Tic40 protein. Supporting this notion, yeast two-hybrid and bimolecular fluorescence complementation assays demonstrated the in vivo interaction of Tic40 with Tic110, a protein believed to recruit stromal chaperones to protein import sites.
Bédard
Jocelyn
J
Department of Biology, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom.
Kubis
Sybille
S
Bimanadham
Sarat
S
Jarvis
Paul
P
D048868:Adaptor Proteins, Signal Transducing; D000595:Amino Acid Sequence; D029681:Arabidopsis Proteins / Q000502:physiology*; D002352:Carrier Proteins / Q000502:physiology*; D002736:Chloroplasts / Q000378:metabolism*; D017353:Gene Deletion; D018506:Gene Expression Regulation, Plant; D005816:Genetic Complementation Test; D006801:Humans; D008565:Membrane Proteins / Q000502:physiology*; D008856:Microscopy, Fluorescence; D008954:Models, Biological; D018832:Molecular Chaperones / Q000502:physiology*; D008969:Molecular Sequence Data; D030821:Plants, Genetically Modified; D017434:Protein Structure, Tertiary; D013045:Species Specificity; D020798:Two-Hybrid System Techniques
D048868:Adaptor Proteins, Signal Transducing; D029681:Arabidopsis Proteins; D002352:Carrier Proteins; C116331:HSPBP1 protein, human; D008565:Membrane Proteins; D018832:Molecular Chaperones; C476397:Tic40 protein, Arabidopsis