Structure and dynamics of phospholamban in solution and in membrane bilayer: computer simulations

We have performed molecular dynamics simulations of phospholamban (PLB), a 52-residue integral membrane protein that inhibits calcium ATPase in the cardiac sarcoplasmic reticulum. We present a microscopic description of the structure and dynamics of PLB in solution and membrane environments, based on 10 ns molecular dynamics simulations of PLB in lipid bilayer and 5 ns simulations in methanol and water, and a water-soluble model of PLB in water. Throughout the simulations, PLB retains its "L"shape, with two well-defined helical domains at the N- and C-termini. In the simulations of PLB in methanol and water, the helices were almost perpendicular, with average interhelix angles of 54 +/- 13 degrees and 63 +/-15 degrees , respectively. In the lipid bilayer trajectory, both the interhelix angle and its fluctuations were larger, with an average of 130 +/- 19 degrees and with the transmembrane C-terminal approximately perpendicular to the bilayer plane. The internal dynamics of phospholamban is characterized by large amplitude collective motions of the two helical domains: hinge bending, twisting of both N- and C-terminal helices, and flexing of the C-terminal helix. The central linker of PLB is highly flexible, due mostly to elastic deformations of this region. The simulation results are in good agreement with NMR data on PLB secondary structure and helix orientations in solution, micelles, and lipid bilayers, as well as fluorescence measurements of interdomain distances. Our most interesting findings involve the details of the PLB dynamics, which are difficult to obtain by experimental approaches. Two kinds of motions of the helical domains found in the simulations can clearly have functional roles. The population of conformations with relatively open interdomain angles, as well as large fluctuations of this coordinate in the bilayer, allows the N-terminal helix to come into contact with the PLB binding site on the calcium ATPase, while the presence of twisting motions around its axis enables the helix to orient the correct face to the binding site.