Three steps in the thermal unfolding of F-actin: an experimental evidence.

The development of differential scanning calorimetry has resulted in an increased interest in studies of the unfolding process in proteins with the aim of identifying domains and interactions with ligands or other proteins. Several of these studies were done with actin and showed that the thermal unfolding of F-actin occurs in at least three steps; this was interpreted as the denaturation of independent domains. In the present work, we have followed the thermal unfolding of F-actin using differential scanning calorimetry (DSC), CD spectroscopy, and probe fluorescence.

F-actin has a very high calorimetric unfolding enthalpy.

The thermal unfolding of F-actin was studied using differential scanning calorimetry. Heat denatures F-actin in two steps. The first is endothermic and corresponds to the unfolding of the peptide chain, while the second is exothermic and is due to the aggregation of the unfolded molecules.

Effect of the polar headgroup of phospholipids on their interaction with actin.

It is generally admitted that actin filaments are anchored to a membrane by membranar actin-binding-proteins. However, we found that actin may also interact directly with membrane phospholipids. The actin-phospholipid complex has been investigated at the air-water interface using a film balance technique.

Adsorption of actin at the air-water interface: a monolayer study.

The intrinsic surface activity of the contractile protein actin has been determined from surface tension measurements using the Wilhelmy hanging-plate method. Actin, a very soluble protein, moves from the subphase to the air-water interface to make a film. In the absence of magnesium, actin is monomeric and is known as G-actin.

Surface film pressure of actin: interactions with lipids in mixed monolayers.

The interactions of actin with neutral lipid films made from DLPC, and with positively charged films built from DLPC and stearylamine (SA), have been characterized by the monolayer technique. Injection of actin underneath an expanded lipid film produces an increase in the surface pressure that is consistent with a penetration of the lipid molecules by actin. This adsorption of actin to the lipid is more pronounced either with positively charged films or with Mg(2+) present in the sub-phase, suggesting that the mechanism involves an electrostatic attraction.