Critical effects from lipid-protein interaction in membranes. II. Interpretation of experimental results

The effects arising from lipid-protein and lipid-cholesterol interaction are discussed within the framework of a general theoretical description presented in the preceding paper. Available experimental results are interpreted, and new experiments are proposed. In the fluid lipid phase proteins and cholesterol increase the lipid orientational order in their neighborhood, in the ordered phase they decrease it. This leads to a decrease of the latent heat at the ordered-fluid transition, which vanishes at a critical concentration of protein or cholesterol. Theoretical predictions for the critical concentrations agree with results from calorimetry. The approach to the critical point is accompanied by an increase of thermal fluctuations of the lipid order and an increase of the lipid response on small perturbations. Thus proteins and cholesterol increase the lipid specific heat, lateral compressibility, permeability, and lateral diffusion on both sides of the phase transition. Notions such as decrease of cooperativity or fluidity due to protein or cholesterol are reviewed in this context.