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 fi laments are attached to the cell membrane through integral membrane ac tin-binding proteins. We have challenged this concept, using an in vit ro system composed of pure actin and liposomes, and have found that ac tin may also interact with membrane lipids. Differential scanning calo rimetry (DSC) shows that when the actin molecule is in contact with su ch lipids, it undergoes a major conformational change which results in the complete disappearance of its phase transition. Conversely, DSC s cans reveal that the phase transition of the membrane lipids is only w eakly affected by the presence of actin. Indeed, the lipids' main tran sition shows only slight shifts in T(m), from 56.6 to 57-degrees-C, an d DELTAH(cal), from 10.1 to 8.8 kcal/mol. In the lipids' pretransition , T(p) is shifted from 52.7 to 53.7-degrees-C, and DELTAH(cal) is shif ted from 0.75 to 0.33 kcal/mol. This interaction between purified acti n 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 li posomes. We therefore propose a model in which a limited number of lip id molecules may interact with specific sites on the actin molecule, r esulting in the protein's observed conformational change.