A trans-envelope protein complex needed for filamentous phage assembly andexport

Assembly and export of filamentous phage requires four non-capsid proteins: the outer membrane protein, pIV; the inner membrane proteins, pI and pXI; and a cytoplasmic host factor, thioredoxin. Chemical cross-linking of intac t cells demonstrates a trans-membrane complex containing pI and pIV. Format ion of the complex protects pI from proteolytic cleavage by an endogenous p rotease. This protection also requires pXI, which is identical to the C-ter minal portion of pI. This indicates that pXI, which is required for phage a ssembly in its own right, is also part of the complex. This complex forms i n the absence of any other phage proteins or the DNA substrate; hence, it r epresents the first preinitiation step of phage morphogenesis. On the basis of protease protection data, we propose that the preinitiation complex is converted to an initiation complex by binding phage DNA, thioredoxin and th e initiating minor coat protein(s).