Interdomain interactions within the gene 3 protein of filamentous phage

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Abstract
Infection of Escherichia coli by filamentous phage fd is mediated by the phage gene 3 protein (g3p). The g3p consists of three domains (g3p‐D1, D2 and D3) linked by flexible glycine‐rich linkers. All three domains are indispensable for phage infectivity; the g3p‐D1 domain binds to the TolA receptor presumably at the inner face of the outer membrane, the g3p‐D2 domain to the F‐pilus and the g3p‐D3 domain anchors g3p to the phage coat. The N‐terminal domains g3p‐D1 and D2 interact with each other; this interaction is abrogated by binding of g3p‐D2 to the F‐pilus leading to the release of g3p‐D1 to bind to TolA. Here, using phages with deletions in g3p, we have discovered a specific interaction between the two N‐terminal domains and g3p‐D3, the C‐terminal domain of g3p. We propose that these interdomain interactions within g3p lead to a compact and stable organisation when displayed on the phage tip, but that during infection, this compact state must be unraveled.