Plays essential roles both in the penetration of the viral genome into the bacterial host via pilus retraction and in the extrusion process. During the initial step of infection, G3P mediates adsorption of the phage to its primary receptor, the tip of host F-pilus. Subsequent interaction with the host entry receptor tolA induces penetration of the viral DNA into the host cytoplasm. In the extrusion process, G3P mediates the release of the membrane-anchored virion from the cell via its C-terminal domain. Interacts with G6P; this interaction is required for proper integration of G3P and G6P into the virion. Interacts with G8P (By similarity). Interacts with host tolA. Prior to assembly, G3P is found associated with the bacterial host inner membrane. There are about five copies of this protein per mature phage that are located on the head side of the filamentous virion. Consists of three domains (N1, N2, and CT). The N2 domain interacts with the F pilus, whereas the N1 domain forms a complex with the C-terminal domain of tolA at later stages of the infection process. N1 is connected to N2 by a flexible glycine-rich linker on the phage. The C-terminal domain is required for release of viral particles from the host bacterial membrane and proper integration of G3P and G6P proteins in the virion. Belongs to the inovirus G3P protein family.