CONFORMATIONAL ACTIVATION OF A BASIC HELIX-LOOP-HELIX PROTEIN (MYOD1) BY THE C-TERMINAL REGION OF MURINE HSP90 (HSP84)

A murine cardiac lambdagt11 expression library was screened with an amphipathic helix antibody, and a recombinant representing the C-terminal 194 residues of murine HSP90 (HSP84) was cloned. Both recombinant and native HSP90s were then found to rapidly convert a basic helix-loop-helix protein (MyoD1) from an inactive to an active conformation, as assayed by sequence-specific DNA binding. The conversion process involves a transient interaction between HSP90 and MyoD1 and does not result in the formation of a stable tertiary complex. Conversion does not require ATP and occurs stoichiometrically in a dose-dependent fashion. HSP90 is an abundant, ubiquitous, and highly conserved protein present in most eukaryotic cells. These results provide direct evidence that HSP90 can affect the conformational structure of a DNA-binding protein.