Role of hydrophobic forces in membrane protein asymmetry

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Abstract
M13 virus [coliphage] coat protein is an integral cytoplasmic membrane protein at all stages of viral infection. The pure virus coat protein can also be incorporated into synthetic lecithin vesicles near the lipid-phase transition temperature (Tm), spanning the bilayer with its N [amino] terminus exposed on the outside and its C[carboxy]-terminus inside. The assembly of coat protein into vesicles in this asymmetric fashion has a sharp maximum near the phase-transition temperature of the lipid fatty acyl chains. At temperatures well below the Tm, coat protein assembles into lecithin vesicles with both termini exposed on the exterior vesicle surface. Unlike this important role of lipid physical state, asymmetry of assembly is unaffected by parameters which govern polar interactions such as pH, ionic strength, lipid polar head group or prior proteolytic removal of the N or C terminus. The orientation of this membrane protein, and perhaps others, is probably determined by interactions of the hydrophobic portion of the protein with the lipid hydrocarbon core.

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