THERMOSTABILITY OF THE BARNASE-BARSTAR COMPLEX

Scanning microcalorimetry was used to study heat denaturation of barna se in complex with its intracellular inhibitor barstar. The heat denat uration 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 den aturation. The temperature of barnase melting in its complex with bars tar is 20 degrees C higher than that of the free enzyme. The barstar m elting temperature is almost the same in the complex or alone (71 degr ees C at pH 6.2 and 68 degrees C at pH 8.0). It seems possible that wh en barstar unfolds it can remain bound to barnase, while the latter un folds only on dissociation of the denatured barstar.