Heat Shock Protein 90

Introduction

Hsp90 is one of the most abundant proteins expressed in cells accounting for 1–2% of all cellular proteins within non-stressed cells. It acts as a molecular chaperone and when coupled with other co-chaperones, it aids the folding of newly synthesized proteins, enabling them to be folded correctly as well as helping to degrade misfolded proteins. Hsp90 also facilitates the transport of proteins across the membranes of various organelles including the endoplasmic reticulum and prevents aggregation of proteins which have been partially or fully unfolded. It is found within most organisms, with the notable absence of archaea (PMID:15218707) where it does not appear to be present and functioning. The Hsp90 homologue HtpG is present within most species of bacteria, but not in archaea and none of the Hsp90 isoforms present in organelles have been shown to ‘derive from endosymbionts of early eukaryotes’ (PMCID:1525184). This Within E.coli, this Hsp90 ortholog with an associated 50S ribosomal protein L2 activates the ATPase function of HtpG to autophosphorylate serine and threonine residues (PMID:8419347). Since HSP90 is highly conserved, those discoveries may apply to humans.

There are many different isoforms of Hsp90 within species, homo sapiens in particular have an alpha and beta class and within these classes there are different isoforms. The functions of each different isoform is largely the same, however there is some variation in structure which enables it to carry out its function effectively in different parts of the body. The function of this molecular chaperone within species is very similar with some particular features being more noticeable in some than others. HSP90 in mouse is expressed at a higher level in the testis and helps mouse embryo development in germ cells (PMID:2342473) as well as possibly preventing mutations caused by environmental changes during embryo development (PMID:12136260). HSP90 in Xenopus laevis is found to help the export of 60S ribosomal subunits from the nuclei (PMCID:1222432). and in prokaryotes, the homolgue of Hsp90 (HtpG) can be found.

In terms of structure, Hsp90 is a homodimer consisting of three domains – the N-terminus, middle and C-terminus. The N-terminus contains a GKHL ATP-binding domain, found in all Hsp90 classes and the bacterial homologue HtpG. The same binding domain is also found in other GKHL proteins, such as Type II topoisomerases, CheA-family histidine kinases, and the MutL DNA repair factor. (PMID:20635416). The middle domain is associated with client protein binding, regulated by the conformational cycle induced by ATP-binding of the N-terminal.  This middle terminal shows evolutionary convergence from corresponding domains found in other GHKL family proteins, inferably because of its role in defining the client proteins that Hsp90 works with (PMID:12667448). The C-terminus domain contains another, less well understood, ATP-binding domain. Binding of ATP at the C-terminus relies on ATP also being bound at the N-terminus site. (PMID:11751878) The C-terminus also contains a co-chaperone binding site that helps regulate Hsp90 activity(PMID:18991631).

HSP90 in Signalling

Hsp90 is a chaperone for unstable signal transducers, keeping them poised for activation. Interacts with RIP and Akt and promotes NF-kappa B mediated inhibition of apoptosis; in addition it also blocks some steps in the apoptotic pathways.

Src is non-receptor tyrosine kinase involved in downstream production of bcl-2 apoptosis regulator gene. Hsp90 stabilizes Src allowing it to activate transcription factor STAT3 that enters the nucleus and promotes transcription of bcl-2 and bcl-xL. A constituently active STAT3 leads to oncogenesis (1), which is why Hsp90’s rate of stabilization of Src is crucial in maintaining an acceptable level of STAT3 activity. Bcl’s act by inhibiting release of cytochrome C from mitochondria in the cytoplasm, cytoplasmic cytochrome C leads to apoptosis via a caspases cascade.

Apaf1 inhibition + Hsp70. Hsp90/Hsp70 complex inhibit of the action of Apaf1, a protease key in the activation of caspase-9. They do this by binding to the Apaf1 complex and changing the conformation of procaspase-9 binding. Activation of caspase-9 leads to activation of more caspases eventually leading to apoptosis.

Akt – Hsp90 maintains Akt activity by preventing PP2A-mediated dephosphorylation (2). Hsp90 protects Akt from inactivation by dephosphorylation, and cells with inhibition of Hsp90-Akt complex formation have increased sensitivity to induced apoptosis.

Bid -> tBid inhibition – Hsp90 inhibits cleavage of Bid to tBid (3). In healthy cells Bid acts by interacting with Bcl2, which mediates the release of cytochrome C from mitochondria (4). Loss of function of Hsp90 leads to the cleavage of Bid and easier induction into apoptosis by cytochrome C release.

Ikb -> HSP maintains the IκB kinase (IKK) complex, mainly in cardiac cells. This complex is important in the regulation of angiotensin II-induced cardiac hypertrophy as well as apoptosis. IκBα kinase inhbits NF-κB kinase, extracellular signals activate IκB kinase which ubiquitinates IκBα kinase. NF-κB is activated and enters the nucleus where it acts as a transcription factor.

References

PMID:17536179
(1) Yin W, Cheepala S, Roberts JN, Syson-Chan K, Digiovanni J and Clifford JL (2006). Active Stat3 is required for survival of human squamous cell carcinoma cells in serum-free conditions. Mol Cancer 5 (1): 15. 
(2) Saori S, Naoya F and Takashi T (2000). Modulation of Akt kinase activity by binding to Hsp90. Cell biology.
(3) Chen Z and Enhua W (2003). Heat shock protein 90 suppresses tumor necrosis factor alpha induced apoptosis by preventing the cleavage of Bid in NIH3T3 fibroblasts. Cellular Signalling 5, Issue 3.
(4) Luo X, Budihardjo I, Zou H, Slaughter C, Wang X (1998). Bid, a Bcl2 interacting protein mediates cytochrome c release from mitochondria in response to activation of cell surface death receptors. Cell 1998 Aug 21;94(4):481-90.