Assembling filamentous phage occlude pIV channels

Filamentous phage f1 is exported from its Escherichia coli host without kil ling the bacterial cell. Phage-encoded protein pIV, which is required for p hage assembly and secretion, forms large highly conductive channels in the outer membrane of E. coli. It has been proposed that the phage are extruded across the bacterial outer membrane through pIV channels. To test this pre diction, we developed an in vivo assay by using a mutant pIV that functions in phage export but whose channel opens in the absence of phage extrusion. In E. coli lacking its native maltooligosacharride transporter Lama, this pIV variant allowed oligosaccharide transport across the outer membrane. Th is entry of oligosaccharide was decreased by phage production and still fur ther decreased by production of phage that cannot be released from the cell surface. Thus, exiting phage block the pIV-dependent entry of oligosacchar ide, suggesting that phage occupy the lumen of pIV channels. This study pro vides the first evidence, to our knowledge, for viral exit through a large aqueous channel.