Proteolysis of bacteriophage phi X174 prohead protein gpB by a protease located in the Escherichia coli outer membrane.

The gene B protein (gpB) of bacteriophage phi X174 is required for prohead assembly and is removed from prohead during phage maturation. Protease activity was observed in isolated prohead which specifically cleaved gpB. Cleavage of gpB produced two fragments that had apparent molecular weights of 12,300 and 3,700 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Amino-terminal sequencing of the fragments confirmed that they resulted from the cleavage of gpB and identified the cleavage site as an Arg-Arg at amino acids 76 to 77 of the 120-amino-acid protein. gpB-specific protease activity was observed in both phi X174-infected and uninfected Escherichia coli extracts. This protease activity was localized to the outer-membrane fraction of uninfected cells. Protease activities present in the outer membrane and in isolated prohead produced identical fragments and had the same protease inhibition profile. The gpB-specific activity in uninfected cells was induced by growth at 42 degrees C and was inhibited by the protease inhibitors, 1,10-phenanthroline, EDTA, and N-ethylmaleimide.