Kinetics of the incorporation of cytochrome b5, an integral membrane protein, into unilamellar dimyristoyllecithin liposomes

The kinetics of incorporation of proteins into phospholipid liposomes were studied using fluorescence of the protein tryptophan residue; fluorescence was detd. at varying protein/lipid ratio and at varying temp. Cytochrome b5, purified from liver microsomes, was exposed to unilamellar dimyristoyllecithin liposomes in a fluorescence cuvette. Tryptophan fluorescence of cytochrome b5 increased rapidly as the protein was incorporated into the lipid bilayer. Protein incorporation was biphasic at low lipid/protein ratios, with an initial fast non-exponential phase followed by a slow phase. The fluorescence was detd. at 15, 25, and 35 Deg, and the rigidity of the liposome bilayer was studied as a function of temp. At low temp (15 Deg), protein incorporation was very slow; however, as the lipid phase transition temp. was approached, protein incorporation increased considerably faster. Thus, there is a pos. correlation between liposome fluidity and the ease of protein incorporation into the bilayer. A model is presented. [on SciFinder (R)]