3.A.13 The Filamentous Phage Exporter (FPhE) Family
Filamentous bacteriophage f1, fd, M13, If1, I2-2, Ike and fs2 are made by a concerted assembly/export process in which the assembling viruses are secreted across both membranes of the Gram-negative bacterial envelope without causing cell lysis. The filament contains a single stranded circular DNA (ssDNA) coated with the major coat protein (pVIII; ~2700 copies) as well as two minor coat proteins (pVII and pIX; ~5 copies each) present at the end of the filament where assembly begins, and two other minor coat proteins (pIII and pVI; ~5 copies each) at the other end where release from the cell occurs. Before assembly, all 5 coat proteins are in the membrane while the nucleo-protein complex (DNA + pV, a ssDNA binding protein) is in the cytoplasm.
Four other proteins are required for f1 assembly/export (pIV, a secretin in the outer membrane (TC #1.B.22.5.1); pI and pXI, transmembrane proteins in the inner membrane, and host thioredoxin in the cytoplasm). pI (348 residues) is an integral membrane protein with a large, N-terminal, cytoplasmic containing Walker-type nucleotide-binding motif domain, a single TMS, and a short periplasmic segment that is thought to interact directly with the secretin, pIV. The pIV secretin of phage f1 forms large aqueous channels in the outer membranes of enterobacteria (Marciano et al., 1999).
f1 assembly and export require ATP hydrolysis and a pmf. pI is the ATP-dependent transporter; pXI (108 aas) is an internal in-frame translation product of gene I with only the TMS and the periplasmic domain of pI. Both are required for f1 assembly/export. Interaction of the inner membrane components with the outer membrane component may cause the pIV channel to open.