P15 and P3, the tail completion proteins of bacteriophage T4, both form hexameric rings.

Two proteins, gp15 and gp3 (gp for gene product), are required to complete the assembly of the T4 tail. gp15 forms the connector which enables the tail to bind to the head, whereas gp3 is involved in terminating the elongation of the tail tube. In this work, genes 15 and 3 were cloned and overexpressed, and the purified gene products were studied by analytical ultracentrifugation, electron microscopy, and circular dichroism. Determination of oligomerization state by sedimentation equilibrium revealed that both gp15 and gp3 are hexamers of the respective polypeptide chains. Electron microscopy of the negatively stained P15 and P3 (P denotes the oligomeric state of the gene product) revealed that both proteins form hexameric rings, the diameter of which is close to that of the tail tube. The differential roles between gp15 and gp3 upon completion of the tail are discussed.