The chlorate‐resistant and photomorphogenesis‐defective mutant cr88 encodes a chloroplast‐targeted HSP90

Abstract

SummaryThe cr88 mutant of Arabidopsis is a novel chlorate-resistant mutant that displays long hypocotyls in red light, but not in far red or blue light, and is delayed in the greening process. In cotyledons and young leaves, plastids are less developed compared with those of the wild type. In addition, a subset of light-regulated genes are under-expressed in this mutant. To understand the pleiotropic phenotypes of cr88, we isolated the CR88 gene through map-based cloning. We found that CR88 encodes a chloroplast-targeted 90-kDa heat shock protein (HSP90). The CR88 gene is expressed at highest levels during early post-germination stages and in leaves and reproductive organs. It is constitutively expressed but is also light and heat shock inducible. Chloroplast import experiments showed that the protein is localized to the stroma compartment of the chloroplast. The possible function of an HSP90 in the chloroplast and a plausible explanation of the pleiotropic phenotypes observed in cr88 are discussed.

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

“…S2). Although Hsp90C is present in chloroplasts from all plant species examined (18,19), its role in chloroplast function and biogenesis has not been examined in detail. Our data suggested that Hsp90C is involved in the import or suborganellar targeting of atTic40.…”

“…Specific subfamilies of the Hsp90s exist in prokaryotes and multiple organelles in eukaryotes, including the cytosol, the ER (glucoseregulated protein 94, Grp94) (33), mitochondria (TNF receptorassociated protein 1, TRAP1) (34), and chloroplasts (Hsp90C) (18). Cytosolic Hsp90 has been studied extensively, and its participation, with a number of cochaperones, in the targeting, assembly, and stability of multiprotein complexes has revealed critical roles in protein folding, cellular signaling, and protein trafficking (35).…”

“…Overexpression of Hsp90C in Arabidopsis reduces tolerance to salt, drought, and oxidative stress (43,44). Additionally, the cr88 mutant, a Gly-to-Arg substitution at position 646 of Hsp90C in Arabidopsis, alters responses to red light and constitutively delays chloroplast development (18). These findings led to proposals that Hsp90C also is involved in stress responses and functions in the transduction of plastid signals that regulate photosynthesisrelated gene expression in a manner reminiscent of the role of cytosolic and other organelle Hsp90 family members in signal transduction pathways (33)(34)(35).…”

“…Hsp90C is a soluble protein that is primarily localized to the chloroplast stroma ( Fig. S3C) (18). Therefore, we tested whether a fraction of Hsp90C associates with the TOC-TIC machinery at envelope membranes as part of the TIC-associated chaperone complex.…”

“…In this study, we attempted to obtain a more complete accounting of the factors associated with protein import intermediates as a foundation for understanding the functions of the chloroplast import-associated chaperone network. We identified chloroplast heat shock protein 90, Hsp90C (18,19), a member of the Hsp90 family of molecular chaperones, as a component of purified complexes containing protein import intermediates. We demonstrate that Hsp90C specifically associates with the TOC-TIC apparatus and several preproteins during protein import.…”