Folding of apocytochrome c induced by the interaction with negatively charged lipid micelles proceeds via a collapsed intermediate state

Unfolded apocytochrome c acquires an alpha-helical conformation upon interaction with lipid. Folding kinetic results below and above the lipid's CMC, together with energy transfer measurements of lipid bound states, and salt-induced compact states in solution, show that the folding transition of apocytochrome c from the unfolded state in solution to a lipid-inserted helical conformation proceeds via a collapsed intermediate state (I-C). This initial compact state is driven by a hydrophobic collapse of the polypeptide chain in the absence of the heme group and may represent a hems-free analogue of an early compact intermediate detected on the folding pathway of cytochrome c in solution. Insertion into the lipid phase occurs via an unfolding step of I-C through a more extended state associated with the membrane surface (I-S). While I-C appears to be as compact as salt-induced compact states in solution with substantial alpha-helix content, the final lipid-inserted state (H-mic) is as compact as the unfolded state in solution at pH 5 and has an alpha-helix content which resembles that of native cytochrome c.