Maturation of steroid receptors: an example of functional cooperation among molecular chaperones and their associated proteins

Abstract

The selective modulation of transcription exerted by steroids depends upon recognition of signalling molecules by properly folded cytoplasmic receptors and their subsequent translocation into the nucleus. These events require a sequential and dynamic series of protein-protein interactions in order to fashion receptors that bind stably to steroids. Central to receptor maturation, therefore, are several molecular chaperones and their accessory proteins; Hsp70, Hsp40, and hip modulate the 3-dimensional conformation of steroid receptors, permitting reaction via hop with Hsp90, arguably the central protein in the process. Binding to Hsp90 leads to dissociation of some proteins from the receptor complex while others are recruited. Notably, p23 stabilizes receptors in a steroid binding state, and the immunophilins, principally CyP40 and Hsp56, arrive late in receptor complex assembly. In this review, the functions of molecular chaperones during steroid receptor maturation are explored, leading to a general mechanistic model indicative of chaperone cooperation in protein folding.

Fig. 1.

Five functional domains of Hsp90 are known. These Hsp90 regions, not drawn to scale, are indicated separately in the figure but there is overlap among them. An amino terminal ATP binding motif rich in glycine occurs within amino-acids 1 to 221 and it may overlap a p23 reactive site, although this is uncertain. A carboxy terminal steroid receptor attachment site encompassed by residues 206–728 overlaps the Hsp90 dimerization region (residues 524–724) and the TPR recognition domain (residues 558–724)

Fig. 2.

Preliminary role of Hsp70 in protein folding and steroid receptor maturation. Nascent proteins, including steroid receptors, are bound by Hsp70·ATP. Upon nucleotide hydrolysis, stimulated by the J-domain of Hsp40, the chaperone undergoes a conformational change. Partially folded proteins are released upon exchange of ADP for ATP on Hsp70, and they may react with other chaperones. However, for steroid receptors, hip prevents release of ADP. This increases the time a partially folded receptor associates with Hsp70 and enhances its interaction with Hsp90 in the next step of maturation.

Fig. 3.

A generalized model of mammalian steroid receptor maturation. Hsp70·ATP binds the nascent steroid receptor, nucleotide hydrolysis is promoted by the J-domain of Hsp40, and the steroid receptor is partially folded. The attachment of hip prevents release of ADP and the receptor from Hsp70. Cytosolic dimers of Hsp90 and hop join the Hsp70·Hsp40·hip·receptor complex to form an intermediate receptor complex. Hsp90 recognizes the steroid receptor hormone binding domain and also interacts with hop through its TPR recognition domain. Hop links Hsp90 to the Hsp70 complex. Hsp40, hip and hop are transient components of the receptor complex and dissociate prior to or concurrent with binding of p23 to Hsp90 which stabilizes the complex. ATP binds Hsp90 late in steroid receptor maturation and may be hydrolyzed. Dissociation of hop frees the TPR recognition domain of Hsp90 and allows association with either of the immunophilins, Hsp56 or CyP40. The role of the immunophilins is uncertain, but Hsp56 may react with the nuclear localization sequence on the steroid receptor, stimulating transport into the nucleus and subsequent activation of transcription. Steroid binding triggers the release of Hsp90, Hsp70 and p23, although some Hsp70 may remain. Imm, immunophilin