Heat shock protein native state

Untransformed 8-10S receptors are stabilized in vitro by sodium molybdate. Several laboratories have purified native PR in both transformed and untransformed states and have examined their protein composition. Molybdate-stabilized PR contain, in addition to A- or B-proteins, a non-steroid binding of 90 kDa, which is a heat-shock protein (hsp). This has been observed for PR of different target tissues and species [55-58] and also for molybdate stabilized glucocorticoid receptors [59,60]. Since the 90 kDa hsp associates only with molybdate-stabilized receptors it has been suggested that transformation and receptor conversion from 8-10S to 4S is due to 90 kDa dissociation and unmasking of receptor DNA binding sites. If so, the 90 kDa component of 8S receptors would function to maintain receptors in an inactive state in the absence of hormone. As with other hsps, 90 kDa is a ubiquitous and abundant protein and only a small fraction is found associated with 8-10S receptors. Because of this there is concern that 90 kDa-receptor associations may be in vitro artifacts it is not known whether these associations occur in vivo and whether these interactions are of physiological relevance to receptor function. 

Based on physiochemical properties, single-step immunoaffinity purified B-receptors were in the native transformed state. Isolated receptors were maintained as undegraded 120 kDa doublets (Fig. 2) they retained their hormone binding activity and they displayed the correct steroid specificity for PR. Isolated B-receptors also bound efficiently to DNA-cellulose requiring 0.25 M salt for their release. They sedimented as 4S monomers on salt-containing sucrose gradients and as a 6S peak in the absence of salt. All these confirm their transformed state. In addition, under these conditions, purified B-receptors were free of the 90 kDa receptor-associated heat shock protein that always copurifies with 8S untransformed receptors in other... 

In vivo, the correct assembly of proteins is guided by a family of cellular proteins termed molecular chaperones, e.g., heat shock protein (HSP), nuleoplasmins, and chaperonins. Chaperones bind to the intermediate that tends to aggregate, and either assembles the intermediate to the native state or renders the intermediate void of further reaction to form an aggregate. Normally, all proteins should fold without molecular chaperones. Proteins that tend to form aggregates, like those shown in the above mechanisms, bind to a chaperone to yield the native state. 

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