Hsp70-Hsp90 Organizing Protein (HOP, p60) is an ~60kD protein that is a critical intermediate component for the efficient maturation of steroid receptor complexes (1–3), serving to recruit Hsp90 to Hsp70-containing complexes. Unactivated steroid hormone receptors are found in hetero-oligomeric complexes that are thought to stabilize a partially folded receptor polypeptide prior to hormone-dependent activation (4–6). Hsp70 and Hsp90 are both known to associate with steroid hormone receptors. Numerous studies (8–13) have shown that Hsp70 functions in an ATP-dependent manner through transient interactions to mediate folding or unfolding of polypeptide chains. Hsp90 is thought to perhaps also function in some capacity related to folding or protein-protein interactions (14). HOP contains three tetratricopeptide repeat (TPR) domains, TPR1, TPR2a and TPR2b. Hsp70 binding has been localized to TPR1, and Hsp90 binding has been localized to TPR2a. Importantly, the highly conserved ñEEVD sequence that terminates many Hsp70 family members, and the similar ñMEEVD sequence that terminates Hsp90, are important recognition sites for the TPR domains. The co-crystal structures for TPR1 plus a GPTIEEVD octapeptide (Hsp70 sequence) and TPR2a plus the MEEVD pentapeptide (Hsp90) have been solved (15). HOP is closely related to a human 63kD protein that is sensitive to simian virus SV 40 transformation (16) and is related to the yeast heat shock-responsive STI1 gene product (17).
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