Thermally-induced structural changes in an armadillo repeat protein suggest a novel thermosensor mechanism in a molecular chaperone

• The chaperone UNC-45B is involved in myosin folding in response to heat stress.
• UCS domain undergoes structural changes in response to physiological temperatures.
• Buried hydrophobic moieties are exposed in the UCS domain in response to heat stress.
• At higher temperatures there is an increase in the overall flexibility of UCS.
• These structural changes would allow the UCS domain to perform its chaperone function.

Molecular chaperones are commonly identified by their ability to suppress heat-induced protein aggregation. The muscle-specific molecular chaperone UNC-45B is known to be involved in myosin folding and is trafficked to the sarcomeres A-band during thermal stress. Here, we identify temperature-dependent structural changes in the UCS chaperone domain of UNC-45B that occur within a physiologically relevant heat-shock range. We show that distinct changes to the armadillo repeat protein topology result in exposure of hydrophobic patches, and increased flexibility of the molecule. These rearrangements suggest the existence of a novel thermosensor within the chaperone domain of UNC-45B. We propose that these changes may function to suppress aggregation under stress by allowing binding to a wide variety of aggregation prone loops on its client.