Control of steroid receptor dynamics and function by genomic actions of the cochaperones p23 and Bag-1L

Abstract

Molecular chaperones encompass a group of unrelated proteins that facilitate the correct assembly and disassembly of other macromolecular structures, which they themselves do not remain a part of. They associate with a large and diverse set of coregulators termed cochaperones that regulate their function and specificity. Amongst others, chaperones and cochaperones regulate the activity of several signaling molecules including steroid receptors, which upon ligand binding interact with discrete nucleotide sequences within the nucleus to control the expression of diverse physiological and developmental genes. Molecular chaperones and cochaperones are typically known to provide the correct conformation for ligand binding by the steroid receptors. While this contribution is widely accepted, recent studies have reported that they further modulate steroid receptor action outside ligand binding. They are thought to contribute to receptor turnover, transport of the receptor to different subcellular localizations, recycling of the receptor on chromatin and even stabilization of the DNA-binding properties of the receptor. In addition to these combined effects with molecular chaperones, cochaperones are reported to have additional functions that are independent of molecular chaperones. Some of these functions also impact on steroid receptor action. Two well-studied examples are the cochaperones p23 and Bag-1L, which have been identified as modulators of steroid receptor activity in nuclei. Understanding details of their regulatory action will provide new therapeutic opportunities of controlling steroid receptor action independent of the widespread effects of molecular chaperones.

Keywords: cellular transport; chaperones; nuclear receptors; steroid receptors.

Figure 1. A model depicting some of the key steps of the maturation pathway of steroid receptors.

(A) Binding of the Hsp90, p23 and a preassembled complex of Hop, Hsp70 and Hsp40 assists a mature folding of the steroid receptor (SR). Cytoplasmic Bag-1 isoforms (Bag-1, -1M, -1S) control this process and mediate proteasomal degradation of misfolded SRs. Addition of Hsp90-dimers and p23 complete the assembled complex, termed the “foldosome” (B). Release of Hop, Hsp70 and Hsp40 and addition of any one of the TPR-containing cochaperones, for example FKBP51 (as shown here), further stabilize the SR in a high affinity form (C). After ligand binding FKBP51 is replaced by FKBP52, which mediates translocation to the nucleus via the microtubuli system (via dynein and dynamitin) in a molecular complex termed the “transportosome” (D). Within the nucleus FKBP52 is released and the receptor binds the response elements as an active dimer. Cochaperones, such as p23 and Bag-1L (that has been described to bind to chromatin prior to the nuclear entry of the receptor), enhance the activity of the SR most likely by stabilizing the active state of the receptor. The molecular chaperones Hsp90 and Hsp70 possibly also play a role in this process (E).

Figure 2. The structure of TPR-containing and TPR-lacking cochaperones.

Top: Domain structure of cochaperones containing tetratricopeptide repeats (TPR) motifs. All TPR motifs are shown in yellow. Other important protein domains are indicated. Bottom: The domain structures of three cochaperones (p23, AHA1 and BAG-1) lacking the classical TPR motifs, with their Hsp90/Hsp70-binding domains highlighted in blue. All domain information (including residue numbers) were obtained from the RefSeq database (NCBI) [8]). STI1: Stress inducible protein 1 (Heat shock protein binding motif); NLS: Nuclear localization signal; PPlase: Peptidyl-prolyl cis-trans isomerase. ARM: Armadillo; CS: CHORD-containing protein SGT1; Aha1: Activator of Hsp90 ATPase; SRPBCC: START/RHOs_alpha_C/PITP/Bet v 1/CoxG/CalC: UBQ: Ubiquitin-like domain; BAG: Bcl-2-associated athanogene (Heat shock protein binding motif). Single alphabet letters (with or without separation by a slash) correspond to particular amino acids (or amino acid sequences) that are over-represented in a certain region.

Figure 3. The Bag-1 protein family and their structural domains.

A. Top: Intron-exon structure of the human Bag-1 gene and corresponding transcript. The start codons for the different Bag-1 transcripts are indicated by arrows. Note, Bag-1L, the longest family member, is the only one with a CUG start codon. Bottom: The domain structures of the four human Bag-1 isoforms (left) and the two isoforms of Schizosaccharomyces pombe (right), with their Hsp70/Hsc70-binding domains (BAG) highlighted in blue. The TR/QSEEX repeat region is shown as vertical lines and other functional domains are indicated. The domain information (including residue numbers) for the human Bag-1 isoforms were obtained from the RefSeq database (NCBI), while the domain information for the yeast homologues were taken from Kriegenburg et al. 2014 [72]. NLS: Nuclear localization signal; UBQ: Ubiquitin-like domain; TM: Transmembrane domain. B, C. Phylogenetic tree (B) and sequence alignment (C) of the first 80 N-terminal amino acids of the human Bag-1L protein compared with Bag-1 isoforms in other organisms. Both graphs were generated using the MultAlin website [73].