The gene I protein (pI) of the filamentous bacteriophage f1 is required for the assembly of this virus. Antibodies specific to either the amino or carboxyl terminus of this protein were used to determine the location and topology of the gene I protein in f1-infected bacteria. pI is anchored in the inner membrane of Escherichia coli cells via a 20-amino-acid hydrophobic stretch, with its carboxyl-terminal 75 residues located in the periplasm and its amino-terminal 253 amino acids residing in the cytoplasm. By using the carboxyl-terminal pI antibody, a smaller protein, pI*, is also detected in f1-infected cells at a ratio of one to two molecules per molecule of pI. Analysis of proteins produced from a gene I amber mutant plasmid or bacteriophage suggests that pI* is most likely the result of an in-frame internal translational initiation event at methionine 241 of the 348-amino-acid pI. pI* is shown to be an integral inner membrane protein inserted in the same orientation as pI. The relation of the cellular locations of pI and pI* to some of the proposed functions of pI is discussed
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