Baseplate protein that is located next to the tail tube (inner baseplate). Involved in sheath assembly. The gp25-(gp6)2-gp7 module is involved in sheath contraction (PubMed:27193680). Homodimer. Interacts with gp53 and with the (gp6)2-gp7 heterotrimeric molecule; The gp25-(gp6)2-gp7 module is involved in sheath contraction (PubMed:27193680). 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 6 copies in the baseplate. Expressed in the late phase of the viral replicative cycle. Belongs to the GpW/Gp25 family.