protein-protein-interactions-visualized-bimolecular-fluorescence

Protein-protein Interactions Visualized by Bimolecular Fluorescence Complementation in Tobacco Protoplasts and Leaves

Many proteins interact transiently with other proteins or are integrated into multi-protein complexes to perform their biological function. Bimolecular ...

Ludwig-Maximilians-Universität, München

Most chloroplast and mitochondrial proteins are synthesized in the cytosol of the plant cell and have to be imported into the organelles post-translationally. Molecular chaperones play an important role in preventing protein aggregation of freshly translated preproteins and assist in maintaining the preproteins in an import competent state. Preproteins can associate with HSP70, HSP90, and 14-3-3 proteins in the cytosol. In this study, we analyzed a large set of wheat germ-translated chloroplast preproteins with respect to their chaperone binding. Our results demonstrate that the formation of distinct 14-3-3 or HSP90 containing preprotein complexes is a common feature in post-translational protein transport in addition to preproteins that seem to interact solely with HSP70. We were able to identify a diverse and extensive class of preproteins as HSP90 substrates, thus providing a tool for the investigation of HSP90 client protein association. The analyses of chimeric HSP90 and 14-3-3 binding preproteins with exchanged transit peptides indicate an involvement of both the transit peptide and the mature part of the proteins, in HSP90 binding. We identified two partner components of the HSP90 cycle, which were present in the preprotein containing high-molecular-weight complexes, the HSP70/HSP90 organizing protein HOP, as well as the immunophilin FKBP73. The results establish chloroplast preproteins as a general class of HSP90 client proteins in plants using HOP and FKBP as novel cochaperones.

Cytosolic HSP90 cochaperones HOP and FKBP interact with freshly synthesized chloroplast preproteins of 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 Δsec71 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.

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

The secretory system in eukaryotic organisms ensures targeting of proteins to their place of function after they entered the endoplasmic reticulum either co- or post-translationally. Thereby proteins are translocated through the Sec translocon into the endoplasmic reticulum. In the Arabidopsis genome homologs for the three major components of the Sec translocon, the central pore Sec61α and the auxiliary proteins Sec62 and Sec63 are present. Phylogenetic analyses show Sec61α to be the most conserved subunit within the Sec translocon whereas Sec62 and Sec63 show less homology but contain the same functional domains among all organisms. We recently characterized a novel tetratricopeptide repeat domain containing protein, AtTPR7, as part of the Arabidopsis Sec translocon which is probably involved in chaperone assisted post-translational import. In this study we investigated the interaction of AtTPR7 with Sec62 as well as the cytosolic chaperones HSP70 and HSP90 not only in vitro but also in vivo to further strengthen the hypothesis of AtTPR7 being a chaperone docking protein of the Sec translocon for secretory preproteins in Arabidopsis.

AtTPR7 as part of the Arabidopsis Sec post-translocon.

The three tetratricopeptide repeat domain-containing docking proteins Toc64, OM64, and AtTPR7 reside in the chloroplast, mitochondrion, and endoplasmic reticulum of Arabidopsis thaliana, respectively. They are suggested to act during post-translational protein import by association with chaperone-bound preprotein complexes. Here, we performed a detailed biochemical, biophysical, and computational analysis of the interaction between Toc64, OM64, and AtTPR7 and the five cytosolic chaperones HSP70.1, HSP90.1, HSP90.2, HSP90.3, and HSP90.4. We used surface plasmon resonance spectroscopy in combination with Interaction Map® analysis to distinguish between chaperone oligomerization and docking protein-chaperone interactions and to calculate binding affinities for all tested interactions. Complementary to this, we applied pulldown assays as well as microscale thermophoresis as surface immobilization independent techniques. The data revealed that OM64 prefers HSP70 over HSP90, whereas Toc64 binds all chaperones with comparable affinities. We could further show that AtTPR7 is able to bind HSP90 in addition to HSP70. Moreover, differences between the HSP90 isoforms were detected and revealed a weaker binding for HSP90.1 to AtTPR7 and OM64, showing that slight differences in the amino acid composition or structure of the chaperones influence binding to the tetratricopeptide repeat domain. The combinatory approach of several methods provided a powerful toolkit to determine binding affinities of similar interaction partners in a highly quantitative manner.

Article	Year
Cytosolic HSP90 cochaperones HOP and FKBP interact with freshly synthesized chloroplast preproteins of Arabidopsis. Molecular plant\| PubMed ID: 21596689	2011
AtTPR7 is a chaperone-docking protein of the Sec translocon in Arabidopsis. Journal of cell science\| PubMed ID: 22899711	2012
AtTPR7 as part of the Arabidopsis Sec post-translocon. Plant signaling & behavior\| PubMed ID: 23759546	2013
Quantification of interaction strengths between chaperones and tetratricopeptide repeat domain-containing membrane proteins. The Journal of biological chemistry\| PubMed ID: 24036116	2013

Regina Schweiger

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