Solution structure and dynamics of a heat shock protein assembly probed by hydrogen exchange and mass spectrometry

The solution conformation and dynamics of the 16.9 kDa small heat shock protein from wheat have been studied using a combination of hydrogen/deuterium exchange, proteolytic digestion, and mass spectrometry. At room temperature, HSP16.9 exists as a dodecameric assembly. Regions of HSP 16.9 that form extensive and essential intersubunit contacts in the assembly, including residues 1-40 and 131151, show little or no protection against hydrogen/deuterium exchange after incubation in D2O for 5 s. The high levels of hydrogen/deuterium exchange indicate that these regions have experienced large conformational fluctuations in solution, breaking intersubunit contacts and exposing buried amide hydrogens to solvent. When HSP16.9 is pulse labeled for 10 ms, residues 1-40 and 131-151 are substantially more protected than they are after 5 s. Thus, the breaking of intersubunit contacts occurs on a time scale between 10 milliseconds and 5 s. At 42 degreesC, HSP16.9 exists in a suboligomeric form. When the intrinsic temperature dependence of hydrogen/deuterium exchange is taken into account, exchange patterns at 25 and 42 degreesC are identical within experimental error, suggesting that the conformation of individual HSP16.9 subunits is the same in both the dodecameric and subdodecameric forms. Significant protection is seen in regions that form the dimeric interface, suggesting that the stable suboligomeric form is a dimer. Taken together, these results suggest that heat activation of HSP16.9 occurs by shifting the dodecamer <----> dimer equilibrium in favor of free dimers. The conformation of the dimers themselves does not appear to be altered with an increase in temperature.