1.B.53 The Filamentous Phage g3p Channel-Forming Protein (FP-g3p) Family
The Gene 3 products, g3p proteins of filamentous phages f1, fd and M13, are members of the phage coat A superfamily. They form pores in the outer membranes of the host bacteria. The pores have similar conformations as the porins, OmpF and OmpC. The C-terminal domain of TolA may interact with the N-terminus of g3p to transport the DNA of the filamentous phage into the cytoplasm of the host cell. The high resolution (1.9 Å) structures of the first two domains have been solved (Holliger et al. 1999).
g3p proteins contain a pentapeptide repeat, GGGSE, tandemly repeated 3 times. These proteins consist of 3 domains, N1, N2 and CT. N2 interacts with the F pilus while N1 (connected to N2 by a flexible glycine-rich linker as cited above) forms a complex with the C-terminal domain of TolA during infection. It has a single putative C-terminal TMS.
The filamentous phages are very different from all non filamentous phage and show no constituents with sequence similarities between these two groups of phage.
g3p brings the DNA to the cytoplasmic membrane and then forms the pore in the cytoplasmic membrane for DNA transport. Thus, the pmf probably plays a role in DNA uptake. An unfolding reaction activates the gene-3-protein, g3p, of the filamentous phage fd for its function during the infection of Escherichia coli (Eckert and Schmid, 2007). Before infection, the gene-3-protein is in a fully folded locked form, in which the binding site for the phage receptor TolA is buried at the domain interface. To expose this binding site, the gene-3-protein must be activated. Cis-to-trans isomerization at Pro213 in the hinge region between the two domains of gp3 is a key step of activation. Pro213 isomerization destabilizes the protein and leads to a loss of folded structure, presumably in the hinge region. The partially unfolded form of the gene-3-protein is metastable, and trans-Pro213 arrests the protein in this activated form for an extended time, long enough to find the receptor TolA. The partial unfolding and its timing by prolyl isomerization are essential for the biological function (Eckert and Schmid, 2007).
The generalized reactions catalyzed by filamentous phage g3p proteins are:
g3p (phage coat) + DNA (phage particles) ↔ g3p (bacterial envelope) + DNA (bacterial cytoplasm)