The bacteriophage T4 DNA packaging apparatus targets the unexpanded prohead

During the morphogenesis of the bacteriophage T4 capsid, a conformational change of the major head shell protein, gene product (gp) 23, causes a 50% increase in capsid volume. This expansion is required to accept the full length chromosome and, therefore, must precede the completion of packaging. The expanded shell is thinner and more stable than its precursor, and can bind accessory proteins which further stabilize it. In phages lambda, T3, T7 and P22, expansion occurs during DNA packaging. However, in T4, expanded capsids can package DNA in vitro and expansion occurs in cells infected with packaging-defective mutants, raising the possibility that expansion and packaging are not coupled. Proteolytically mature gp23 (gp23∗) in unexpanded proheads is sensitive to chymotrypsin cleavage at Phe154-Ser155, creating a 38 kDa peptide, while gp23∗ in expanded capsids is refractory to the protease. We used an expansion assay based on this protease sensitivity to determine the expansion status of capsids isolated from various packaging-defective mutants with the goal of determining whether packaging and expansion are normally linked. In infections at 20°C, mutants in the packaging enzymes gp16 and gp17 fail to expand. However, in gene 49 − mutants, which initiate packaging but fail to complete it, expansion is complete. Thus, packaging drives expansion, and the unexpanded prohead is the substrate for the packaging reaction. We also show that expansion observed in 16 − and 17 − infections at 37°C is linked to aberrant packaging. Capsids produced at 15 minutes, when no packaging can be detected, never expand. However, by 35 minutes when aberrant packaging begins, so does expansion of freshly made capsids. Thus in all cases now examined, expansion is only observed in vivo when DNA packaging is also occurring, indicating that these two processes are coupled.