J. Biol. Chem. doi:10.1074/jbc.M114.607010

Cytochrome c (cytc) has a central role in electron transport, but it can also have a proapoptotic role by interacting with membrane cardiolipin (cdl) and acting as a peroxidase. The conformational landscape of cytc is large, varying in both degree of extension and oligomeric state. To determine the function of particular conformers, Paul et al. have characterized the interaction of single cytc molecules with cdl. Two types of cytc, mammalian (m-cytc) and yeast (y-cytc), were studied. The authors observed that cdl binding changes the conformation of both m-cytc and y-cytc, although the yeast protein is perturbed to a greater extent. Interaction with cdl stimulates m-cytc peroxidase activity, but it suppresses that of y-cytc. Fluorescence correlation spectroscopy was used to follow conformational fluctuations between three states—N (compact), E (extended) and O (oligomeric)—as a function of cdl concentration. For m-cytc, increased cdl prompted a switch from the N to the E state, whereas for y-cytc, cdl leads to a large population of O. In both cases, the abundance of the E state is correlated with peroxidase activity. Whereas in m-cytc cdl induces formation of E (via partial unfolding) to activate the peroxidase, in y-cytc, cdl decreases conformational stability and drives the protein into an inactive oligomer. Computational analysis of the stability of m-cytc and y-cytc shows that although their cores are superimposable, their surface residues have regional variations, with y-cytc containing one positively and one negatively charged cluster compared to m-cytc's more even surface charge distribution. This is reflected in greater root mean square fluctuations (RMSF) of y-cytc relative to m-cytc. Thus, the nonuniform response of cytc from different species to cdl can be attributed to the effect of surface residues and their impact on folding landscape.