Figure 1

Assembly pathway of tailed bacteriophages. In the tailed phages, capsid assembly starts with the construction of an icosahedral protein lattice called procapsid, essentially composed of a major capsid protein (noted MCP in brown in Figure 1). At a specialized vertex of the procapsid, the dodecameric portal protein (Portal in blue) forms a channel which is the docking point for an ATPase complex called terminase. This complex normally contains multiple copies of a large subunit with ATPase and endonuclease activities (TermL in orange), and a small DNA binding subunit that recognizes the cognate viral DNA Sun et al.[20]. It translocates viral dsDNA into the procapsid cavity through the portal channel. When DNA packaging is completed, the terminase motor disassembles and the portal dodecamer recruits head-completion proteins to prevent leakage of the viral DNA. One such protein directly binds to the portal: it is called the adaptor protein (Ad in magenta); it can also be supplemented with the so-called head-closure protein (Hc in green) [18, 21–23]. Altogether the head-completion proteins provide a platform for completion of short tail assembly in Podoviridae[24, 25] as well as for docking of pre-assembled long tails in Sipho- and Myoviridae[26–28]. Located at one end of the Sipho- and Myoviridae long tails, the tail-completion protein (Tc in red) allows for the tail attachment to the head. Head- and tail-completion proteins form the head-to-tail connection and, together with the portal protein, constitute the virion’s neck. The major tail protein (MTP in kaki) is the main component of the tail tube structure. In Myoviridae, the surrounding tail sheath protein (Sheath in cyan) contracts upon host infection, initiating viral DNA injection in the host cell.