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Cytosolic HSP90 Cochaperones HOP and FKBP Interact with Freshly Synthesized chloroplast Preproteins of Arabidopsis

Molecular Plant 2011年第6期4卷 1133-1145页

作者： Christine Fellerer Regina Schweiger Katharina Schongruber Jurgen Soil Serena Schwenker Department of Biology 1 Botany Ludwig-Maximilians-Universitat Munchen Grobhaderner Strasse 2-4 D-82152 Planegg-Martinsried Germany Munich Center for Integrated Protein Science CiPSM Ludwig-Maximilians-Universitut Munchen Feodor-Lynen-Strasse 25 D-81377 Munich Germany

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. Pre- proteins 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.

关键词： Wheat germ in vitro translation chloroplast import chaperone.

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Nucleus-Encoded Light-Harvesting Chlorophyll a/b Proteins are imported Normally into Chlorophyll b-Free chloroplasts of Arabidopsis

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Molecular Plant 2013年第3期6卷 860-871页

作者： Sabine Nick Jorg Meurer Jurgen Soil Elisabeth Ankele Dept Biologie I-Botanik Biozentrum LMU Munchen Grosshadernerstrasse 2-4 D-82152 Planegg-Martinsried Germany Munich Center for Integrated Protein Science ClPSM Ludwig-Maximilians-Universitat Munchen Feodor-Lynen-Strasse 25 D-81377 Munich Germany

chloroplast-located proteins which are encoded by the nuclear genome have to be imported from the cytosol into the organelle in a posttranslational manner. Among these nuclear-encoded chloroplast proteins are the light- harvesting chlorophyll a/b-binding proteins （LHCPs）. After translation in the cytosol, precursor proteins of LHCPs are imported via the TOC/TIC translocase, processed to their mature size to insert into thylakoid membranes where they recruit chlorophylls a and b to form pigment-protein complexes. The translocation of proteins is a highly regulated process which employs several regulators. To analyze whether CAO （chlorophyll a oxigenase） which converts chlorophyll a to chlorophyll b at the inner chloroplast membrane, is one of these regulators, we performed import reactions utilizing a homozygous loss-of-function mutant （cao-1）. We imported in vitro translated and 35S-labeled precursor proteins of light- harvesting proteins of photosystem II LHCB1, LHCB4, and LHCB5 into chloroplasts isolated from cao-1 and show that import of precursor proteins and their processing to mature forms are not impaired in the mutant. Therefore, regulation of the import machinery cannot be responsible for the decreased steady-state levels of light-harvesting complex （LHC） proteins. Regulation does not take place at the transcriptional level either, because Lhcb mRNAs are not down-regulated. Additionally, reduced steady-state levels of LHCPs also do not occur due to posttranslational turnover of non-functional LHCPs in chloroplasts. Taken together, our data show that plants in the absence of CAO and therefore devoid of chlorophyll b are not influenced in their import behavior of LHC proteins.

关键词： chloroplast import LHCPs cao-1 protein complex assembly thylakoid membranes photosynthesis.

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