Secondary structure of M13 coat protein in phospholipids studied by circular dichroism, Raman, and Fourier transform infrared spectroscopy.

There is considerable uncertainty about the precise secondary structure adopted by the M13 coat protein when embedded in a phospholipid bilayer. Circular dichroism (CD) spectroscopy suggests that a major change in the structure of the coat protein occurs upon membrane insertion. It is reported that the structure of the protein in the membrane has only about 50% alpha-helix, the rest being mainly in a beta-sheet conformation, whereas the protein is almost completely alpha-helical when intact in the phage. In this study we have undertaken a spectroscopic analysis using Fourier transform infrared, Raman, and CD spectroscopy to characterize the secondary structure of M13 coat protein when present in membranes consisting of dioleoylphosphatidylglycerol and dimyristoylphosphatidylglycerol. In sharp contrast to earlier CD studies, our results indicate that the coat protein in its membrane-embedded state has a very high alpha-helical content with virtually no beta-sheet structures present. This result indicates that the structures of the coat protein when intact in the phage or when embedded in the membrane are similar. Although our results differ from earlier CD studies, they are consistent with a recent NMR study, which showed that the M13 coat protein in sodium dodecyl sulfate micelles is primarily alpha-helical with no evidence for beta-sheet structure [Henry, G. D., & Sykes, B.D. (1992) Biochemistry 31, 5284-5297]. These results lead to the conclusion that the M13 coat protein can insert from the membrane-bound state into a virus particle with a similar secondary structure, without large energy implications.(ABSTRACT TRUNCATED AT 250 WORDS)