Assistance for a chaperone: Chlamydomonas HEP2 activates plastidic HSP70B for cochaperone binding

The Journal of Biological Chemistry
Felix WillmundMichael Schroda

Abstract

Previous efforts aimed at the biochemical characterization of chloroplast HSP70B were hampered by the observation that recombinant HSP70B was inactive, i.e. incompetent of interacting with its nucleotide exchange factor CGE1. In addition, because heterologously expressed mitochondrial Hsp70 was inactive unless coexpressed with the escort protein Hep1, we wondered whether homologs of Hep1 existed in the chloroplast. Data base searches revealed that algae and higher plants indeed encode at least two HEP homologs, one predicted to be targeted to mitochondria, the others to chloroplasts. Using Chlamydomonas reinhardtii as plant model organism we demonstrate that this alga encodes an HEP homolog (termed HEP2) that is localized to the stroma. HEP2 is expressed constitutively as a low abundance protein with an apparent molecular mass of approximately 21 kDa. In cell extracts HEP2 interacts with HSP70B in an ATP-dependent fashion. Coexpression of HSP70B with HEP2 in Escherichia coli yielded high levels of CGE1-binding competent HSP70B, which also displayed ATPase activity. Inactive HSP70B was more prone to proteolysis than active HSP70B. Although inactive HSP70B interacted with HEP2, it could not be activated. Active HSP70B remained ac...Continue Reading

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Citations

Dec 17, 2009·The Journal of Biological Chemistry·Marta BlamowskaKai Hell
Jul 18, 2008·The Journal of Biological Chemistry·Peng ZhaiJonathan J Silberg
Sep 26, 2012·The Journal of Cell Biology·Marta BlamowskaKai Hell
Dec 14, 2011·Protein Science : a Publication of the Protein Society·Michael T VuJonathan J Silberg
Jun 15, 2015·International Journal of Biological Macromolecules·P R Dores-SilvaJ C Borges
Jan 18, 2015·Biochimica Et Biophysica Acta·Raphael TröschFelix Willmund
Oct 2, 2009·Plant Signaling & Behavior·Michael Schroda, Timo Mühlhaus
May 3, 2011·Biochemical and Biophysical Research Communications·Peng ZhaiJonathan J Silberg

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