Filamentous phage infection:: crystal structure of g3p in complex with its coreceptor, the C-terminal domain of TolA

Background: Infection of male Escherichia coli cells by filamentous Ff bacteriophages (M13, fd, and f1) involves interaction of the phage minor coat gene 3 protein (g3p) with the bacterial F pilus (primary receptor), and subsequently with the integral membrane protein TolA (coreceptor). G3p consists of three domains (N1, N2, and CT). The N2 domain interacts with the F pilus, whereas the N1 domain - connected to N2 by a flexible glycine-rich linker and tightly interacting with it on the phage - farms a complex with the C-terminal domain of TolA at later stages of the infection process. Results: The crystal structure of the complex between g3p N1 and TolA D3 was obtained by fusing these domains with a long flexible linker, which was not visible in the structure, indicating its very high disorder and presumably a lack of interference with the formation of the complex. The interface between both domains, corresponding to similar to 1768 Angstrom(2) of buried molecular surface, is clearly defined. Despite the lack of topological similarity between TolA D3 and g3p N2, both domains interact with the same region of the g3p N1 domain. The fold of TolA D3 is not similar to any previously known protein motifs. Conclusions: The structure of the fusion protein presented here clearly shows that, during the infection process, the g3p N2 domain is displaced by the TolA D3 domain. The folds of g3p N2 and TolA D3 are entirely different, leading to distinctive interdomain contacts observed in their complexes with g3p N1. We can now also explain how the interactions between the g3p N2 domain and the F pilus enable the g3p N1 domain to form a complex with TolA.