AtTPR7 is a chaperone-docking protein of the Sec translocon in Arabidopsis

Chaperone-assisted sorting of post-translationally imported proteins is a general mechanism among all eukaryotic organisms. Interaction of some preproteins with the organellar membranes is mediated by chaperones, which are recognised by membrane-bound tetratricopeptide repeat (TPR) domain containing proteins. We have characterised AtTPR7 as an endoplasmic reticulum protein in plants and propose a potential function for AtTPR7 in post-translational protein import. Our data demonstrate that AtTPR7 interacts with the heat shock proteins HSP90 and HSP70 via a cytosol-exposed TPR domain. We further show by in vitro and in vivo experiments that AtTPR7 is associated with the Arabidopsis Sec63 homologue, AtERdj2. Interestingly, AtTPR7 can functionally complement a Delta sec71 yeast mutant that is impaired in post-translational protein transport. These data strongly suggest that AtTPR7 not only has a role in chaperone binding but also in post-translational protein import into the endoplasmic reticulum, pointing to a general mechanism of chaperone-mediated post-translational sorting between the endoplasmic reticulum, mitochondria and chloroplasts in plant cells.