Baseplate protein that is located next to the tail tube (inner baseplate). Involved in the tail assembly. Involved in the tail assembly. 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). Interacts with gp25 and with the (gp6)2-gp7 heterotrimeric molecule. Present in 6 copies in the baseplate. Expressed in the late phase of the viral replicative cycle.