Hsp90 chaperone activity requires the full-length protein and interaction among its multiple domains

Hsp90 is an abundant and ubiquitous protein involved in a diverse array of cellular processes. Mechanistically we understand little of the apparently complex interactions of this molecular chaperone. Recently, progress has be en made in assigning some of the known functions of hsp90, such as nucleoti de binding and peptide binding, to particular domains within the protein. W e used fragments of hsp90 and chimeric proteins containing functional domai ns from hsp90 or its mitochondrial homolog, TRAP1, to study the requirement s for this protein in the folding of firefly luciferase as well as in the p revention of citrate synthase aggregation. In agreement with others who hav e found peptide binding and Limited chaperone ability in fragments of hsp90 , we see that multiple fragments from hsp90 can prevent the aggregation of thermally denatured citrate synthase, a measure of passive chaperoning acti vity. However, in contrast to these results, the luciferase folding assay w as found to be much more demanding. Here, folding is mediated by hsp70 and hsp40, requires ATP, and thus is a measure of active chaperoning. Hsp90 and the cochaperone, Hop, enhance this process. This hsp90 activity was only o bserved using full-length hsp90 indicating that the cooperation of multiple functional domains is essential for active, chaperone-mediated folding.