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Protein Classes
HSP90 in Homo sapiens (Human)There are many different forms of the protein Hsp90 within homo sapiens, with two major classes - the alpha isoform and the beta isoform. However, within the alpha isoform alone there are two classes, A and B, with class A containing two different isoforms (1 and 2) and class B containing one. 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. Alpha Isoform, Class AWithin class A of the alpha isoform of Hsp90, there are two members: HSP90AA1 (isoform 1) (Flat file) and HSP90AA2 (isoform 2) (Flat file). These are the two which will be sequentially and structurally analysed. HSP90AA1 has the gene map locus 14q32.2 (OMIM: 140571) and is 854 amino acids in length. Aside form having an N-terminal extension, this isoform is identical to HSP90AA2. HSP90AA2 has a gene map locus of 11p14.1 (OMIM: 140575) and is 732 amino acids in length. Apart from the N-terminal extension in Hsp90AA1, the remainder of the structure is the same for both isoforms. They both consist of a highly conserved N-terminal domain, a charged domain, a middle domain involved in ATPase activity, a second charged domain, and a C-terminal domain. There is also a 4-helical cytokine motif, a glutamine-rich region, and a C-terminal MEEVD motif characteristic of cytosolic HSP90 proteins. Both these isoforms are inducible cytosolic proteins which are important molecular chaperones. Hsp90 alpha isoforms have been identified as being expressed extracellularly on breast cancer cells causing interactions which are crucial for tumour invasiveness (1). Alpha Isoform, Class BThere is one isoform type in this class, Hsp90AB1 which has a gene locus 6p21.1 (OMIM: 140572) and has a length of 724 amino acids, but other variants on amino acid length have been found as well. Hsp90AB1 is a constitutively expressed form of the Hsp90 protein. This isoform is identical to the Hsp90AA1 isoform except for a short C-terminal extension. The gene consists of 12 exons and 11 introns. Some heat shock proteins, including this isoform are known to be antiapoptotic and are one of the main targets for cancer killing drugs. It helps by increasing the activity of Inositol hexakisphosphate kinase 2, sensitizing cells to stressors whereas usually, depletion of IP6K2 results in the blockage of cell death. By targeting the Hsp90-IP6K2 binding complex, anti-cancer drugs work to promote cell death of the tumour, because high Hsp90 activity increases malignancy of tumours, so when it is disrupted, cell death can take place. Beta IsoformThe beta isoform HSP90B1 has the gene locus 12q23.3 (OMIM: 191175) and is 803 amino acids in length. This isoform is found within the endoplasmic reticulum and has an important function in protein folding in secretory pathways such as toll like receptors and integrins. It is an essential immune chaperone which helps regulate both the innate and adaptive immunity. A tumour-derived form of HSP90B1 called vitespan has entered clinical trials for cancer immunotherapy due to these features it has. Structurally HSP90B1 contains a 21-amino acid signal peptide and 5 potential N-linked glycosylation sites, along with a highly conserved N-terminal domain, a charged domain, a middle domain involved in ATPase activity, a second charged domain, and a C-terminal domain. It also has an incomplete 4-helical cytokine motif, a glutamine-rich region, and a signal peptide.
HSP90 in Pan troglodytes (Chimpanzee)The HSP90 alpha protein (Flat file) found in Chimpanzees is 733 amino acids in length and spans across the region 102520- 102590K bp of chromosome 14. It is a homodimer with ATPase activity found in the cytosol of cells where it acts as a chaperone and is involved in many cell processes including protein folding and response to stress, in a very similar way to how it works in homo sapiens.
HSP90 in Mus musculus (House Mouse)Mus musculus also has two isoforms, alpha and beta. They are encoded by separate genes on different chromosomes and when cells are under unstressed conditions, both of the isoforms are present in the cytoplasm at a basal level. The accession number of Hsp90-alpha is NP_034610. It is a cytosolic protein and is composed of 733 amino acids with a molecular weight of 86KDa (UniProt). Its located on chromosome 12 and the location is 12 F1; 12 48.0 cM. Hsp90-alpha usually exists as a homodimer. It acts as a molecular chaperone having ATPase activity. Mouse Hap90-alpha has nearly all functions that human Hsp90 has since a protein BLAST shows that they are 100% identical. However, mouse Hsp90-alpha has shown a special characteristic as that it helps embryo development in germ cells. In the testes, the amount of Hsp90-alpha mRNA is a lot greater than other tissues and Hsp90-alpha is highly expressed in germ cells reaching the meiotic prophase (2). Since one of the major function of Hsp90 is to help protein stability under stressed conditions, the extremely large amount of Hsp90-alpha present in germ cell can provide a way to prevent phenotypic mutations during the embryo development if the environments change (3). HSP90 in Xenopus laevis (African clawed frog)Xenopus laevis has two isoforms, Hsp90-alpha and Hsp90-beta. They are both cytosolic proteins and they have nearly identical functions as human Hsp90. Xenopus laevis Hsp90-alpha is used to compare with Hsp90 from other species. The accession number of Hsp90-alpha is NP_001085598 (Flat file). It contains 722 amino acids. From unfertilized eggs to the cleavage stage, Xenopus laevis embyros express Hsp90-alpha mRNA constitutively at all stages. This indicates that the information of Hsp90-alpha comes from the mother (4). Xenopus laevis embryogenesis also shows how Hsp90 is affected by temperature. At control temperature, the amount Hsp90-alpha mRNA is low but it increases greatly after a heat shock (5). The experiment done by using microinjection techniques on Xenopus oocytes suggests that Hsp90-alpha functions to help the export of 60S ribosomal subunits from nuclei. Nuclear pore complexes on the nuclear membrane have the binding sites for Hsp90-alpha. This can be reinforced by the fact that if geldanamycin (a Hsp90-binding drug) is used, the amount of 60S ribosomal subunits exported to the cytoplasm is reduced (6).
HtpG in Escherichia coli (str. K-12 substr. MG1655)Instead of Hsp90, prokaryotes have HtpG, which is the ortholog of Hsp90. HtpG stands for High Temperature Protein G. HtpG found in this E.coli strain is made of 624 amino acids with the accession number of NP_415006. Its molecular weight is 71.424KDa. It is encoded by the htpG gene at 11.1 min on the chromosome. Similar to eukaryotic Hsp90, HtpG is a molecular chaperone and has ATPase activity with a kcat value of 3 min-1 and a KM for ATP of 500 µM. In solution, HtpG exists as a homodimer and it becomes an oligomer at 65°C (UniProt). HtpG has two distinct functions. One is that it autophosphorylates at serine and threonine residues (7). The other is that HtpG has an associated protein called 50S ribosomal protein L2, which activates the ATPase function of HtpG (8).
References2. Expression of HSP86 in Male Germ Cells 3. Heat shock protein HSP86 expression during mouse embryo development, especially in the germ-line 5. Expression of hsp90 and hsp90 during Xenopus laevis embryonic development 8. Ribosomal protein L2 associates with E. coli HtpG and activates its ATPase activity |
