The membrane domain of a bacteriophage assembly protein. Membrane insertion and growth inhibition

The gene I protein (pI) of the f1 filamentous bacteriophage is a non-capsid protein that is required for the assembly of the bacteriophage. It spans the Escherichia coli inner membrane once with its amino terminus in the cytoplasm and its carboxyl-terminal portion in the periplasm. The presence of moderate amounts of this protein in the membrane results in rapid inhibition of cell growth, probably from a loss of membrane potential. Previous observations defined a 55-amino-acid sequence within pI required for its membrane insertion which includes a 20-residue hydrophobic stretch preceded by a 13-residue positively charged amphiphilic helix. To define the minimal sequence required for membrane translocation and for growth inhibition, a deletion analysis was performed on a tripartite fusion construct containing the 55-residue pI sequence flanked upstream by the amino-terminal portion of EcoRI endonuclease and downstream by the enzymatic portion of alkaline phosphatase. Only the 20-residue hydrophobic stretch immediately preceded by 1 arginine residue is required for membrane insertion of the fusion proteins. This region also sufficed to inhibit cell growth provided it contained protein domains exposed in both the cytoplasm and periplasm. It was not possible to separate the domains required for membrane insertion and cell growth inhibition. No requirement for the positively charged amphiphilic helix was detected either for membrane insertion or growth inhibition, suggesting that it plays a role in phage assembly and not membrane insertion.