To infect and replicate, bacteriophage P22 injects its 43 kbp genome across the cell wall of Salmonella enterica serovar Typhimurium. The attachment of phage P22 to the host cell as well as the injection of the viral DNA into the host is mediated by the virion's tail complex. This 2.8 MDa molecular machine is formed by five proteins, which include the portal protein gp1, the adhesion tailspike protein gp9, and three tail accessory factors: gp4, gp10, gp26. We have isolated the tail accessory factor gp4 and characterized its structure and binding interactions with portal protein. Interestingly, gp4 exists in solution as a monomer, which displays an exceedingly low structural stability (Tm 34 degrees C). Unfolded gp4 is prone to aggregation within a narrow range of temperatures both in vitro and in Salmonella extracts. In the virion the thermal unfolding of gp4 is prevented by the interaction with the dodecameric portal protein, which stabilizes the structure of gp4 and suppresses unfolded gp4 from irreversibly aggregating in the Salmonella milieu. The structural stabilization of gp4 is accompanied by the concomitant oligomerization of the protein to form a ring of 12 subunits bound to the lower end of the portal ring. The interaction of gp4 with portal protein is complex and likely involves the distinct binding of two non-equivalent sets of six gp4 proteins. Binding of the first set of six gp4 equivalents to dodecameric portal protein yields a gp(1)12:gp(4)6 assembly intermediate, which is stably populated at 30 degrees C and can be resolved by native gel electrophoresis. The final product of the assembly reaction is a bi-dodecameric gp(1)12:gp(4)12 complex, which appears hollow by electron microscopy, suggesting that gp4 does not physically plug the DNA entry/exit channel, but acts as a structural adaptor for the other tail accessory factors: gp10 and gp26.