RESIDUAL STRUCTURE IN A STAPHYLOCOCCAL NUCLEASE FRAGMENT - IS IT A MOLTEN GLOBULE AND IS ITS UNFOLDING A FIRST-ORDER PHASE-TRANSITION

Temperature-induced unfolding of staphylococcal nuclease and its large fragment, which lacks 13 C-terminal amino acid residues, was studied calorimetrically, and by CD and fluorescence spectroscopy. It was show n that, in contrast to the full length protein which includes two doma ins and unfolds in two distinct stages under some conditions, the frag ment unfolds in one stage. Unfolding of the fragment proceeds in the s ame temperature range in which the N-terminal beta-barrel domain unfol ds in the full length staphylococcal nuclease. Therefore, the fragment is initially partly unfolded. It retains a stable N-terminal domain w hich unfolds co-operatively with significant heat absorption. Unfoldin g of the fragment can be regarded as a first-order phase transition, b ut its initial state certainly does not represent a molten globule, as it was believed.