Bacteriophage P2 and P4 morphogenesis: structure and function of the connector.

The connector, the structure located between the bacteriophage capsid and tail, is interesting from several points of view. The connector is in many cases involved in the initiation of the capsid assembly process, functions as a gate for DNA transport in and out of the capsid, and is, as implied by the name, the structure connecting a tail to the capsid. Occupying a position on a 5-fold axis in the capsid and connected to a coaxial 6-fold tail, it mediates a symmetry mismatch between the two. To understand how the connector is capable of all these interactions its structure needs to be worked out. We have focused on the bacteriophage P2/P4 connector, and here we report an image reconstruction based on 2D crystalline layers of connector protein expressed from a plasmid in the absence of other phage proteins. The overall design of the connector complies well with that of other phage connectors, being a toroid structure having a conspicuous central channel. Our data suggests a 12-fold symmetry, i.e., 12 protrusions emerge from the more compact central part of the structure. However, rotational analysis of single particles suggests that there are both 12- and 13-mers present in the crude sample. The connectors used in this image reconstruction work differ from connectors in virions by having retained the amino-terminal 26 amino acids normally cleaved off during the morphogenetic process. We have used different late gene mutants to demonstrate that this processing occurs during DNA packaging, since only mutants in gene P, coding for the large terminase subunit, accumulate uncleaved connector protein. The suggestion that the cleavage might be intimately involved in the DNA packaging process is substantiated by the fact that the fragment cleaved off is highly basic and is homologous to known DNA binding sequences.