An essential role for chloroplast heat shock protein 90 (Hsp90C) in protein import into chloroplasts

Abstract

Chloroplast heat shock protein 90 (Hsp90C) represents a highly conserved subfamily of the Hsp90 family of molecular chaperones whose function has not been defined. We identified Hsp90C as a component that interacts with import intermediates of nuclear-encoded preproteins during posttranslational import into isolated chloroplasts. Hsp90C was specifically coprecipitated with a complex of protein import components, including Tic110, Tic40, Toc75, Tic22, and the stromal chaperones, Hsp93 and Hsp70. Radicicol, an inhibitor of Hsp90 ATPase activity, reversibly inhibited the import of a variety of preproteins during translocation across the inner envelope membrane, indicating that Hsp90C functions in membrane translocation into the organelle. Hsp90C is encoded by a single gene in Arabidopsis thaliana, and insertion mutations in the Hsp90C gene are embryo lethal, indicating an essential function for the chaperone in plant viability. On the basis of these results, we propose that Hsp90C functions within a chaperone complex in the chloroplast stroma to facilitate membrane translocation during protein import into the organelle.

Inducible expression of magnesium protoporphyrin chelatase subunit I (CHLI)-amiRNA provides insights into cucumber mosaic virus Y satellite RNA-induced chlorosis symptoms

“…Because Hsp90C has an essential role in protein transport into chloroplasts, the silencing of this gene could affect varying processes of chloroplast biogenesis [14]. By contrast, CHLI has a role only in chlorophyll synthesis, and therefore, the silencing of this gene would have a limited impact on chloroplast biogenesis.…”

Natural Variation in Sensitivity to a Loss of Chloroplast Translation in Arabidopsis

“…Multiple components of the chloroplast protein import system have been identified in Arabidopsis (Jarvis, 2008;Kessler and Schnell, 2009;Shi and Theg, 2013): (1) GTPases in the outer chloroplast membrane that interact with these chaperones and the transit peptide of incoming proteins (Kubis et al, 2004;Inoue et al, 2010); (3) Toc75 outer membrane channel proteins (Baldwin et al, 2005); (4) inner membrane Tic20 channel proteins and associated Tic complex members (Hirabayashi et al, 2011;Kasmati et al, 2011;Kikuchi et al, 2013); (5) stromal chaperones, motor proteins, and heat-shock proteins such as Hsp93 (Kovacheva et al, 2007;Chu and Li, 2012), Hsp90C (Inoue et al, 2013), and chloroplast Hsc70 (Su and Li, 2010); and (6) a stromal processing peptidase (SPP) that cleaves the transit peptide (Trösch and Jarvis, 2011). Features of the N-terminal transit peptide that target proteins to plastids have also been analyzed (Chotewutmontri et al, 2012;Li and Teng, 2013).…”

Molecular Mechanisms and Physiological Significance of Organelle Interactions and Cooperation

“…Moreover, it forms a platform together with the membrane-bound co-chaperone Tic40, which links the pore with a complex protein machinery that supports preprotein import. This machinery consists of Hsp90 (Inoue et al, 2013), the motor chaperone Hsp93 (Chou et al, 2003(Chou et al, , 2006, and the stromal Hsp70 (CpHsp70) (Latijnhouwers et al, 2010). The latter two proteins interact directly with transit peptides in vitro (Ivey et al, 2000) and a lack of these proteins interferes with preprotein import (Su andLi, 2008, 2010).…”