FT-IR SPECTROSCOPY OF THE MAJOR COAT PROTEIN OF M13 AND PF1 IN THE PHAGE AND RECONSTITUTED INTO PHOSPHOLIPID SYSTEMS

FT-IR spectroscopy has been applied to study the secondary structure o f the major coat protein of Pf1 and M13 as present in the phage and re constituted in DOPG and mixed DOPC/DOPG (4/1) bilayers. Infrared absor bance spectra of the samples were examined in dehydrated films and in suspensions of D2O and H2O. The secondary structure of the coat protei n is investigated by second-derivative analysis, Fourier seIf-deconvol ution, and curve fitting of the infrared bands in the amide I region ( 1600-1700 cm(-1)). It is found that, in dehydrated films of Pf1 and M1 3 phage, the amide I region contains three bands located at about 1633 , 1657, and 1683 cm(-1), that are assigned to hydrogen-bonded turn, al pha-helix/random coil, and non-hydrogen-bonded turn, respectively. Fro m a comparison of the infrared spectra in dehydrated film with those i n aqueous suspension, the percentages of secondary structure were foun d with an accuracy of about +/-5%. For the coat protein of Pf1 phage, the FT-IR quantification gives 69% alpha-helix conformation, 19% turn structure, and 12% random coil structure. For Pf1 coat protein in the membrane-embedded state, the amount of alpha-helix is 57%, whereas 42% is in a turn structure and 1% in a random coil structure. The same as signment strategy was used for the analysis of the data obtained for M 13 coat protein reconstitution into phospholipid systems. For M13 coat protein in the phage, this gives 75% alpha-helix conformation, 21% tu rn structure, and 4% random coil structure. For reconstituted M13 coat protein, it can be calculated that the amount of alpha-helix is 50% a nd the amount of random coil structure is 4%. The remaining structure (46%) is a turnlike structure. These results show that for both the Pf 1 and M13 coat protein in the phage environment contains a high amount of alpha-helical structure, which decreases in a lipidic environment in favor of turnlike structures. It is concluded that, when embedded i n a membrane, the C- and N-terminal regions of the coat proteins conta in a large amount of secondary structure, resembling a partly ''broken '' and discontinuous helix structure. This suggests that these regions probably undergo an increased molecular motion.