Actin conformation is drastically altered by direct interaction with membrane lipids: a differential scanning calorimetry study.

One of the current dogmas in cytoskeleton research holds that actin filaments are attached to the cell membrane through integral membrane actin-binding proteins. We have challenged this concept, using an in vitro system composed of pure actin and liposomes, and have found that actin may also interact with membrane lipids. Differential scanning calorimetry (DSC) shows that when the actin molecule is in contact with such lipids, it undergoes a major conformational change which results in the complete disappearance of its phase transition. Conversely, DSC scans reveal that the phase transition of the membrane lipids is only weakly affected by the presence of actin. Indeed, the lipids' main transition shows only slight shifts in Tm, from 56.6 to 57 degrees C, and delta Hcal, from 10.1 to 8.8 kcal/mol. In the lipids' pretransition, Tp is shifted from 52.7 to 53.7 degrees C, and delta Hcal is shifted from 0.75 to 0.33 kcal/mol. This interaction between purified actin and membrane lipids is inhibited by high concentrations of KCl, thus indicating that the phenomenon is primarily electrostatic in nature. The ultrastructural consequences of this change in actin conformation were investigated by electron microscopy, which revealed the formation of paracrystalline arrays of actin filaments at the surface of the liposomes. We therefore propose a model in which a limited number of lipid molecules may interact with specific sites on the actin molecule, resulting in the protein's observed conformational change.