Constrained modeling of spin-labeled major coat protein mutants from M13 bacteriophage in a phospholipid bilayer

The family of three-dimensional molecular structures of the major coat prot ein from the M13 bacteriophage, which was determined in detergent micelles by NMR methods, has been analyzed by constrained geometry optimization in a phospholipid environment. A single layer solvation shell of dioleoyl phosp hatidylcholine lipids was built around the protein, after replacing single residues by cysteines with a covalently attached maleimide spin label. Both the residues substituted and the phospholipid were chosen for comparison w ith site-directed spin labeling EPR measurements of distance and local mobi lity made previously on membranous assemblies of the M13 coat protein purif ied from viable mutants. The main criteria for identifying promising candid ate structures, out of the 300 single-residue mutant models generated for t he membranous state, were 1) lack of steric conflicts with the phospholipid bilayer, 2) good match of the positions of spin-labeled residues along the membrane normal with EPR measurements, and 3) a good match between the seq uence profiles of local rotational freedom and a structural restriction par ameter for the spin-labeled residues obtained from the model. A single subc lass of structure has been identified that best satisfies these criteria si multaneously. The model presented here is useful for the interpretation of future experimental data on membranous M13 coat protein systems. It is also a good starting point for full-scale molecular dynamics simulations and fo r the design of further site-specific spectroscopic experiments.