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

Abstract

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.

Cell Surface-Dependent Generation of Angiostatin4.5

“…The mechanism of actin trafficking and binding to the cell surface is not well understood. However, actin can bind to membrane lipids such as phospholipids (51)(52)(53)(54)(55), and direct interactions between actin and cell membranes are possible in vivo although actin does not have a transmembrane domain. Furthermore, studies by Hu et al (50) also showed that cellsurface actin was not derived from the serum in tissue culture experiments.…”

Interaction of myosin subfragment 1 with F‐actin studied by differential scanning calorimetry

“…Here, we show the application of DSC for characterization of a complex of isolated myosin head, or myosin subfragment 1 (S1), with F-actin. Previously, this method was successfully used to study actin polymerization [3] and interaction of F-actin with membrane lipids [4] and with phosphate analogues [5]. The thermal unfolding and domain structure of S1 and their changes induced by nucleotide binding have also been studied by the DSC method [6][7][8].…”

Study of F‐Actin Interaction with Planar and Liposomal Bilayer Phospholipid Membranes

“…In that respect, F-actin molecules display the same way as many polycations do, as has been long known (17). The presence of negatively charged lipid in the lipid mixture used seems very important for the interaction of F-actin with the lipid bilayer because F-actin cannot interact with neutral phospholipids (4,18,19). We also failed in measuring the ion conductivity of BLM made up of only neutral lipids (results not shown).…”

“…The rst is the expected electrostatic adsorption of long, brillar protein molecules on the membrane surface, a result of the polyelectrolyte nature of F-actin and the negative charge of the membrane (4,19,26). The interaction is strong enough to produce long furrows on the membrane surface just beneath the protein molecule and is visible also on the hydrophobic fracture surface of membrane.…”

“…Although numerous studies support the idea that actin laments are anchored to the membrane through actin-binding proteins (3,28), abundant information indicates that actin can also interact directly with lipids without an intermediate linker protein (4,5,18,19). As shown in the schematic drawings, F-actin can (1) initiate the highly curved folds of membranes where local dislocations of the lipid bilayer could occur (Fig.…”