Configurations of the N-terminal amphipathic domain of the membrane-bound M13 major coat protein

The M13 major coat protein has been extensively studied in detergent-based and phospholipid model systems to elucidate its structure. This resulted in an L-shaped model structure of the protein in membranes. An amphipathic al pha -helical N-terminal arm, which is parallel to the surface of the membra ne, is connected via a flexible linker to an alpha -helical transmembrane d omain. In the present stud,? a fluorescence polarity probe or ESR spin prob e is attached to the SH group of a series of N-terminal single cysteine mut ants, which were reconstituted into DOPC model membranes. With ESR spectros copy, we measured the local mobility of N-terminal positions of the protein in the membrane. This is supplemented with relative depth measurements at these positions by fluorescence spectroscopy via the wavelength of maximum emission and fluorescence quenching. Results show the existence of at least two possible configurations of the M13 amphipathic N-terminal arm on the E SR time scale. The arm is bound either to the membrane surface or in the wa ter phase, The removal or addition of a hydrophobic membrane-anchor by site -specific mutagenisis changes the ratio between the membrane-bound and the water phase fraction.