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Heat shock protein 90 (Hsp90) is a molecular chaperone required for the activity of many of the most important regulatory proteins of eukaryotic cells (the Hsp90 ‘clients’). Vertebrates have two isoforms of cytosolic Hsp90, Hsp90aand Hsp90b. Hsp90bis expressed constitutively to a high level in most tissues and is generally more abundant than Hsp90a, whereas Hsp90a is stress-inducible and overexpressed in many cancerous cells. | ỊFEBS Journal Expressed as the sole Hsp90 of yeast the a and b isoforms of human Hsp90 differ with regard to their capacities for activation of certain client proteins whereas only Hsp90b generates sensitivity to the Hsp90 inhibitor radicicol Stefan H. Millson1 Andrew W. Truman1 Attila Racz2 Bin Hu3 Barry Panaretou3 James Nuttall1 Mehdi Mollapour1 Csaba Soti2 and Peter W. Piper1 1 Department of Molecular Biology and Biotechnology The University of Sheffield UK 2 Department of MedicalChemistry Semmelweis University Budapest Hungary 3 Division of Life Sciences King s College London UK Keywords chaperone inhibitor human heat shock protein 90 chaperone isoforms of heat shock protein 90 radicicol yeast expression Correspondence P. W. Piper Department of Molecular Biology and Biotechnology The University of Sheffield Firth Court Western Bank Sheffield S10 2TN UK Fax 44 114 222 2800 Tel 44 114 222 2851 E-mail Peter.Piper@sheffield.ac.uk Received 24 April2007 revised 1 June 2007 accepted 3 July 2007 doi 10.1111 j.1742-4658.2007.05974.x Heat shock protein 90 Hsp90 is a molecular chaperone required for the activity of many of the most important regulatory proteins of eukaryotic cells the Hsp90 clients . Vertebrates have two isoforms of cytosolic Hsp90 Hsp90a and Hsp90p. Hsp90b is expressed constitutively to a high level in most tissues and is generally more abundant than Hsp90a whereas Hsp90a is stress-inducible and overexpressed in many cancerous cells. Expressed as the sole Hsp90 of yeast human Hsp90a and Hsp90b are both able to provide essential Hsp90 functions. Activations of certain Hsp90 clients heat shock transcription factor v-src were more efficient with Hsp90a rather than Hsp90b present in the yeast. In contrast activation of certain other clients glucocorticoid receptor extracellular signal-regulated kinase-5 mitogen-activated protein kinase was less affected by the human Hsp90 isoform present in these cells. Remarkably whereas expression of Hsp90b as the sole .
