Peripheral baseplate protein that is part of the tail fiber network (PubMed:10545330, PubMed:27193680). Connects the long tail fibers to the baseplate and, after virus attachment to a host cell, probably changes its conformation to trigger the signal for tail contraction (PubMed:10545330). Involved in the tail assembly (PubMed:21129200). Homotrimer (PubMed:27193680). The gp9 trimer interacts with the long tail fiber (LTF) that comprises gp34 trimer, gp35, gp36 and a gp37 trimer. Part of the baseplate macromolecular complex which consists of gp5, gp5.4, gp27 (central spike complex); gp6, gp25, gp53 (inner baseplate); gp7, gp8 (intermediate baseplate); gp9, gp10, gp11, gp12 (peripheral); gp48 and gp54 (proximal region of the tail tube). Present in 18 copies in the baseplate. Expressed in the late phase of the viral replicative cycle. The C-terminus is bound to the baseplate, and the N-terminus coiled-coil domain is associated with the long tail fibers.