Complexity of Hsp90 in organelle targeting

Abstract

Heat shock protein 90 (Hsp90) is an abundant and highly conserved molecular chaperone. In Arabidopsis, the Hsp90 gene family consists of seven members. Here, we report that the AtHsp90-6 gene gives rise to two mRNA populations, termed AtHsp90-6L and AtHsp90-6S due to alternative initiation of transcription. The AtHsp90-6L and AtHsp90-6S transcription start sites are located 228 nucleotides upstream and 124 nucleotides downstream of the annotated translation start site, respectively. Both transcripts are detected under normal or heat-shock conditions. The inducibility of AtHsp90-6 mRNAs by heat shock implies a potential role of both isoforms in stress management. Stable transformation experiments with fusion constructs between the N-terminal part of each AtHsp90-6 isoform and green fluorescent protein indicated import of both fusion proteins into mitochondria. In planta investigation confirmed that fusion of the AtHsp90-5 N-terminus to green fluorescent protein (GFP) did result in specific chloroplastic localization. The mechanisms of regulation for mitochondria- and plastid-localized chaperone-encoding genes are not well understood. Future work is needed to address the possible roles of harsh environmental conditions and developmental processes on fine-tuning and compartmentalization of the AtHsp90-6L, AtHsp90-6S, and AtHsp90-5 proteins in Arabidopsis.

Conserved Alternative Splicing of Arabidopsis Transthyretin-Like Determines Protein Localization and S-Allantoin Synthesis in Peroxisomes

“…In general, the majority of subcellular localization signals, including signals for the endoplasmic reticulum, mitochondria, and chloroplast, are located near the protein N terminus. Thus, the production of mRNAs encoding proteins with different cellular localization often depends on the use of alternative transcription start sites rather than alternative splicing (Prassinos et al, 2008;Puyaubert et al, 2008). However, examples exist in plants of alternative splicing involving subcellular localization signals.…”

Overexpression of AtHsp90.2, AtHsp90.5 and AtHsp90.7 in Arabidopsis thaliana enhances plant sensitivity to salt and drought stresses

“…AtHsp90.5 and AtHsp90.7 proteins are localized in chloroplast (Cao et al 2003) and endoplasmic reticulum (ER) (Ishiguro et al 2002), respectively. AtHsp90.6 is localized in mitochondria (Prassinos et al 2008). However, until recently, only a few literatures have reported the physiological roles of Hsp90 chaperone complexes in plants.…”

Overexpression of Organellar and Cytosolic AtHSP90 in Arabidopsis thaliana Impairs Plant Tolerance to Oxidative Stress

“…AtHSP90.1 to AtHSP90.4 proteins correspond to the cytoplasmic subfamily (Milioni and Hatzopoulos 1997;Krishna and Gloor 2001), while AtHSP90.5 and AtHSP90.7 proteins are located in the chloroplast and endoplasmic reticulum (ER) (Cao et al 2003;Ishiguro et al 2002), respectively, and AtHSP90.6 is located within the mitochondria (Prassinos et al 2008). The AtHSP90.2, AtHSP90.3, and AtHSP90.4 protein sequences are highly similar with at least 96% identity.…”