Spontaneous insertion of gene 9 minor coat protein of bacteriophage M13 in model membranes

Gene 9 minor coat protein from bacteriophage M13 is known to be located in the inner membrane after phage infection of Escherichia coli. The way of insertion of this small protein (32 amino acids) into membranes is still unknown. Here we show that the protein is able to insert in monolayers. The limiting surface pressure of 35 mN/m for 1,2-dioleoyl-sn-glycero-3-phosphocholine and 1,2-dioleoyl-sn-glycero-3-phosphoglycerol lipid systems indicates that this spontaneous insertion can also occur in vivo. By carboxyfluorescein leakage experiments of vesicles it is demonstrated that protein monomers, or at least small aggregates, are more effective in releasing carboxyfluorescein than highly aggregated protein. The final orientation of the protein in the bilayer after insertion was addressed by proteinase K digestion, thereby making use of the unique C-terminal location of the antigenic binding site. After insertion the C-terminus is still available for the enzymatic digestion, while the N-terminus is not. This leads to the overall conclusion that the protein is able to insert spontaneously into membranes without the need of any machinery or transmembrane gradient, with the positively charged C-terminus remaining on the outside. The orientation after insertion of gene 9 protein is in agreement with the 'positive inside rule'.