EQUILIBRIUM FOLDING INTERMEDIATES OF A GREEK KEY BETA-BARREL PROTEIN

Protein S is a calcium-binding protein comprising two Greek key beta-b arrel domains. We have used NMR and optical spectroscopies to show tha t, in the absence of calcium, the N-terminal domain of protein S forms two equilibrium folding intermediates that are in slow exchange. The intermediates arise from differential calcium-dependent folding of sub domains which are not contiguous along the polypeptide chain. The stru ctures of these intermediates are incompatible with several previously proposed folding mechanisms for Greek key beta-barrel domains. We pro pose a different mechanism that involves multiple nucleation sites for folding and sequential acquisition of native long-range interactions. (C) 1998 Academic Press Limited.