The abundant presence of cardiolipin (CL) in mitochondria membranes has given rise to the suspicion that this lipid play be an essential role in triggering cell apoptosis, possibly by mechanically destabilizing the host membrane and thus enhancing the effect of the tBid apoptosis protein. Therefore the mechanical effect of the presence of CL in model bilayer membranes was investigated by a combination of micromechanical deformation studies and differential calorimetry (DSC). The calorimetry data are not easy to interpret due to the continuous chemical decay of the system upon hydrolysis. The induction of hexagonal phases was not observed in the low contents regime (CL<5%). From the initial scans of the fresh samples, a phase diagram could be constructed (CL <5%) that exhibits a continuous increase of melting temperatures as CL contents increases and that is accompanied by a loss in transition collaborativity. Giant vesicles as used for micromanipulation seemed to be stable for hours; light scattering on extruded vesicles revealed a conflicting result and showed a preference of stable radii of ~80nm independent on the initial preparation radius. The high shearing forces during extrusion may make use of a general CL induced instability and kick the system into a favourite curvature conformation. The presence of CL seems to make membranes more expandable at essentially constant limiting tension. As the protein adsorbs to the interface, the expansion modulus is apparently increased, both in the presence and absence of CL. We interpret this as a formation of patches of protein-lipid clusters that in effect reduce the amount of expandable fluid membrane area. The rupture tension falls significantly as soon as tBid is present on the outer vesicle membrane.
|Publication status||Published - 2007|