Effect of alterations in lipid packing order by hydrophobic solutes on the association state of protein assemblies in model membranes.

The effects of two hydrophobic solutes which perturb lipid packing order, permethrin and allethrin, on the aggregated state of a lipid membrane-incorporated protein, bacteriorhodopsin (BR), have been determined by resonance energy transfer measurements. As temperature is increased from well below the main gel-fluid phase transition temperature (Tc) of the lipid, patches of aggregated BR dissociate into monomers, a few degrees below the Tc (M.P. Heyn, A. Blume, M. Rehorek and N.A. Dencher, Biochemistry 20 (1981) 7109; M.P. Heyn, R.J. Cherry and N.A. Dencher, Biochemistry 20 (1981) 840). Permethrin and allethrin were found to cause a decrease in the temperature of BR disaggregation which was associated with a decrease in the Tc of the lipid. In gel phase dipalmitoylphosphatidylcholine at 25 degrees C, the pertubing effects of permethrin on lipid packing order were associated with a decrease in the average patch radius from 123 to 33 A. It is concluded that perturbation of lipid packing order by small hydrophobic molecules may alter the stability of protein assemblies in membranes.