THERMOSTABILITY OF THE BARNASE-BARSTAR COMPLEX

Scanning microcalorimetry was used to study heat denaturation of barnase in complex with its intracellular inhibitor barstar. The heat denaturation of the barnase-barstar complex is well approximated by two two-state transitions with the lower temperature transition corresponding to barstar denaturation and the higher temperature one to barnase denaturation. The temperature of barnase melting in its complex with barstar is 20 degrees C higher than that of the free enzyme. The barstar melting temperature is almost the same in the complex or alone (71 degrees C at pH 6.2 and 68 degrees C at pH 8.0). It seems possible that when barstar unfolds it can remain bound to barnase, while the latter unfolds only on dissociation of the denatured barstar.