Characterization of a Plant Homolog of Hop, a Cochaperone of Hsp90

Abstract

The 90-kD molecular chaperone hsp90 is the key component of a multiprotein chaperone complex that facilitates folding, stabilization, and functional modulation of a number of signaling proteins. The components of the animal chaperone complex include hsp90, hsp70, hsp40, Hop, and p23. The animal Hop functions to link hsp90 and hsp70, and it can also inhibit the ATPase activity of hsp90. We have demonstrated the presence of an hsp90 chaperone complex in plant cells, but not all components of the complex have been identified. Here, we report the isolation and characterization of soybean (Glycine max) GmHop-1, a soybean homolog of mammalian Hop. An analysis of soybean expressed sequence tags, combined with preexisting data in literature, suggested the presence of at least three related genes encoding Hop-like proteins in soybean. Transcripts corresponding to Hop-like proteins in soybean were detected under normal growth conditions, and their levels increased further in response to stress. A recombinant GmHop-1 bound hsp90 and its binding to hsp90 could be blocked by the tetratricopeptide repeat (TPR) domain of rat (Rattus norvegicus) protein phosphatase 5. Deletion of amino acids 325 to 395, adjacent to the TPR2A domain in GmHop-1, resulted in loss of hsp90 binding. In a minimal assembly system, GmHop-1 was able to stimulate mammalian steroid receptor folding. These data show that plant and animal Hop homologs are conserved in their general characteristics, and suggest that a Hop-like protein in plants is an important cochaperone of plant hsp90.The highly conserved and abundant molecular chaperone hsp90 is distinct from other chaperones in that it plays a key role in signal transduction networks, cell cycle control, protein degradation, and genomic silencing (Young et al., 2001). A critical dependence on hsp90 has been established for animal steroid hormone receptors (SRs), several Ser/Thr and Tyr kinases, and other distinct proteins (Pratt and Toft, 1997;Buchner, 1999). Two key features regarding the mechanism of hsp90 action have emerged over the last decade: (a) hsp90 lies at the center of a multiprotein chaperone complex that facilitates the folding of client proteins into their stable or activatable conformations, and (b) hsp90 is an ATPdependent chaperone. The study of hsp90 complexes, in particular those involving animal SRs, has revealed that five crucial chaperone components participate in the conformational regulation of hsp90 client proteins. These include hsp90 and hsp70 and their cochaperones Hop, p23, and hsp40. The high-M r immunophilins are also recovered in hsp90 complexes, but these appear to be nonessential in receptor folding assays (Pratt and Toft, 1997). The cochaperone Cdc37p/p50 cdc37 is predominantly found in hsp90-kinase complexes (Pratt and Toft, 1997).Hop (hsp70-and hsp90-organizing protein) derives its name from its role as an adapter protein that can bind to both hsp90 and hsp70 simultaneously, bringing them into close proximity (Chen and Smith, 1998). It is proposed that hsp70 first co...

Characterization of orchardgrass p23, a flowering plant Hsp90 cohort protein

“…In addition, plant Hsp70 and plant Hop exhibited the same functional abilities as the homologous animal proteins to form a receptor-Hsp90 heterocomplex (Stancato et al 1996;Zhang et al 2003). Similarly, rape Hsp90 exhibited the same activity as rabbit Hsp90 in assembly with rabbit Hsp70/Hsp40, human Hop, and human p23 (Dittmar et al 1997).…”

“…Hsp90 also plays important roles in development, innate immunity, and buffering of genetic variation in plants (Queitsch et al 2002;Sangster and Queitsch 2005). Plant Hsp90 acts in cooperation with other co-chaperones such as FKBP42 (Kamphausen et al 2002), SGT1 (Muskett and Parker 2003), RAR1 , RPM1 (Hubert et al 2003), Hop (Zhang et al 2003), PP5 (de la Fuente van Bentem et al 2005), FKBP62, and FKBP65 (Aviezer-Hagai et al 2007). Nonetheless, p23, a well-conserved co-chaperone of Hsp90 heterocomplex chaperone machinery in mammals, is not yet characterized in plants.…”

Characterization of TcSTI-1, a homologue of stress-induced protein-1, in Trypanosoma cruzi

“…Heat shock and other stress conditions have been shown to induce STI-1 expression in several organisms (Nicolet & Craig 1989, Honoré et al 1992, Lässle et al 1997, Heine et al 1999, Zhang et al 2003, Song et al 2009). We evaluated the stress-induced expression of the TcSTI-1 gene in exponentially growing T. cruzi epimastigotes subjected to nutritional, acidic or heat stress for 2 h. We found that the incubation of exponentially growing parasites under these stress conditions resulted in no significant change in the level of the TcSTI-1 protein.…”

“…Total mRNA levels have been analyzed in several organisms to evaluate the induction of STI-1 gene expression by stress conditions (Lässle et al 1997, Webb et al 1997, Zhang et al 2003, Song et al 2009). We chose to analyze only the polysome-associated mRNA fraction because gene expression in T. cruzi is predominantly regulated at the posttranscriptional level (Teixeira 1998, Clayton 2002.…”

“…Like their counterpart in yeast, the mouse and Caenorhabditis elegans STI-1 homologues are induced by heat shock (Lässle et al 1997, Song et al 2009). Interestingly, a soybean homologue has been shown to be induced by either heat or cold stress (Zhang et al 2003). In addition, the upregulation of STI-1 observed in human SV-40-transformed fibroblasts or mouse macrophages exposed to bacterial lipopolysaccharide demonstrates that stress conditions unrelated to temperature may also lead to the induction of STI-1 expression (Honoré et al 1992, Heine et al 1999.…”