Plant endoplasmin supports the protein secretory pathway and has a role in proliferating tissues

Abstract

SummaryEndoplasmin is a molecular chaperone of the heat-shock protein 90 class located in the endoplasmic reticulum and its activity is poorly characterized in plants. We assessed the ability of endoplasmin to alleviate stress via its transient overexpression in tobacco protoplasts treated with tunicamycin, an inhibitor of glycosylation and inducer of the unfolded protein response (UPR). Endoplasmin supported the secretion of a model secretory protein but was less effective than BiP, the endoplasmic reticulum member of the heat-shock protein 70 family. Consistently, immunoprecipitation experiments with in vivo radioactively labelled proteins using an antiserum prepared against Arabidopsis endoplasmin showed that a much smaller number of newly synthesized polypeptides associated with endoplasmin than with BiP. Synthesis of endoplasmin was enhanced by UPR inducers in tobacco seedlings but not protoplasts. As BiP synthesis was induced in both systems, we conclude that the UPR acts differently, at least in part, on the expression of the two chaperones. Endoplasmin was not detectable in extracts of leaves and stems of the Arabidopsis endoplasmin T-DNA insertion mutant shepherd. However, the chaperone is present, albeit at low levels, in shepherd mutant callus, mature roots and tunicamycin-treated seedlings, demonstrating that the mutation is leaky. Reduced endoplasmin in the shepherd mutant has no effect on BiP protein levels in callus or mature roots, leaves and stems, but is compensated by increased BiP in seedlings. This increase occurs in proliferating rather than expanding leaf cells, indicating an important role for endoplasmin in proliferating plant tissues.

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

“…TRAP1 is implicated in mitochondrial homeostasis of tumor cells (36), and Grp94 function is essential in mouse embryo development (37). In addition, Arabidopsis Grp94 is required for meristem function and supports the synthesis of a secretory protein in vivo (38,39). Despite the fact that insertional mutants lacking Hsp90C expression are lethal in plants ( Fig.…”

Endoplasmic Reticulum Protein Quality Control and Its Relationship to Environmental Stress Responses in Plants

“…SHEPHERD (SHD) appears to encode the only ortholog of GRP94 in Arabidopsis, which is highly induced by tunicamycin and DTT (Martínez and Chrispeels, 2003;Liu and Howell, 2010). Biochemical and cell biology studies indicated that At-SHD has in vivo chaperone activity (Klein et al, 2006). Interestingly, the substrates/targets of At-SHD seem to be very specific.…”

Arabidopsis bZIP60 Is a Proteolysis-Activated Transcription Factor Involved in the Endoplasmic Reticulum Stress Response

“…The relative changes in gene expression for these 129 genes in wild-type and bzip60 seedling are shown in Supplemental Table 1 online. The numerically dominant component of the tunicamycin response comprises 25 genes encoding ER chaperones and folding enzymes, including BiP, GRP94 (Klein et al, 2006), J protein (Yamamoto et al, 2008), calreticulin (Christensen et al, 2008), calnexin, protein disulfide isomerase (Houston et al, 2005), ERO1, and ROC7, and enzymes involved in modification, such as glycosylation. An additional 21 of the tunicamycin-induced genes code for proteins involved in protein transport into or through the secretory pathway.…”