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.