Forms the phage's tail tube composed of 32 hexameric disks. When it encounters the appropriate initiation complex gpM and gpL, it assembles in hexameric rings that stack on top of each others. Multimerization ceases when the correct tail length is achieved through a mechanism dependent on tail terminator protein. Multimerizes into a structure formed by 32 hexameric rings stacked on phage baseplate gpM and gpL. Does not multimerize in solution without the tail initiation complex. Soluble major tail protein interacts with tail assembly protein GT during tail assembly.