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.