CONVENTIONAL AND SATURATION-TRANSFER EPR OF SPIN-LABELED MUTANT BACTERIOPHAGE-M13 COAT PROTEIN IN PHOSPHOLIPID-BILAYERS

A mutant of bacteriophage M13 was prepared in which a cysteine residue was introduced at position 25 of the major coat protein. The mutant c oat protein was spin-labeled with a nitroxide derivative of maleimide and incorporated at different lipid-to-protein (L/P) ratios in DOPC or DOPG. The rotational dynamics of the reconstituted mutant coat protei n was studied using EPR and saturation transfer (ST) EPR techniques. T he spectra are indicative for an anisotropic motion of the maleimide s pin label with a high order parameter (S = 0.94). This is interpreted as a wobbling motion of the spin label with a correlation time of abou t 10(-6) to 10(-5) s within a cone, and a rotation of the spin label a bout its long molecular axis with a correlation time of about 10(-7) s . The wobbling motion is found to correspond generally to the overall rotational motion of a coat protein monomer about the normal to the bi layer. This motion is found to be sensitive to the temperature and L/P ratio. The high value of the order parameter implies that the spin la bel experiences a strong squeezing effect by its local environment, th at reduces the amplitude of the wobbling motion. This squeezing effect . is suggested to arise from a turn structure in the coat protein from Gly23 to Glu20. (C) 1997 Elsevier Science B.V.