Folding of apocytochrome c in lipid micelles:: Formation of α-helix precedes membrane insertion

Apocytochrome c, which in aqueous solution is largely unstructured, acquires a highly alpha-helical structure upon interaction with lipid. The alpha-helix content, induced in apocytochrome c depends on the lipid system, and this folding process is driven by both electrostatic and hydrophobic lipid-protein interactions. The folding kinetic mechanism of apocytochrome c induced by zwitterionic micelles of lysophosphatidylcholine (L-PC), predominantly driven by hydrophobic lipid-protein interactions, was investigated by fluorescence stopped-flow measurements of Trp 59 and fluorescein-phosphalidylethanolamine- (FPE) labeled micelles, in combination with stopped-flow far-UV circular dichroism, It was found that formation of the alpha-helical structure of apocytochrome c precedes membrane insertion. The unfolded state in solution (U-W) binds to the micelle surface in a helical conformation (I-S) and is followed by insertion into the lipid micelle, i.e., formation of the final helical state H-L. Binding of apocytochrome c to the lipid micelle (U-W --> I-S) is concurrent with formation of a large fraction (75-100%, depending on lipid concentration) of the alpha-helical structure of the final lipid-inserted state H-L, The highly helical intermediate Is is formed on the time scale of 3-12 ms, depending on lipid concentration, and inserts into the lipid micelle (I-S - H-L) in the time range of similar to 200 ms to > 1 s, depending on lipid-to-protein ratio. The final Lipid-inserted helical state H-L in L-PC micelles has an alpha-helix content similar to 65% of that of cytochrome c in solution and has no compact stable tertiary structure as revealed by circular dichroism results.