Shape and DNA packaging activity of bacteriophage SPP1 procapsid: Protein components and interactions during assembly

The procapsid of the Bacillus sutbilis bacteriophage SPP1 is formed by the major capsid protein gp13, the scaffolding protein gp11, the portal protein gp6, and the accessory protein gp7. The protein stoichiometry suggests a T = 7 symmetry for the SPP1 procapsid. Overexpression of SPP1 procapsid prot eins in Escherichia coil leads to formation of biologically active procapsi ds, procapsid-like, and aberrant structures. Co-production of gp11, gp13 an d gp6 is essential for assembly of procapsids competent for DNA packaging i n vitro. Presence of gp7 in the procapsid increases the yield of viable pha ges assembled during the reaction in vitro five- to tenfold. Formation of c losed procapsid-like structures requires uniquely the presence of the major head protein and the scaffolding protein. The two proteins interact only w hen co-produced but not when mixed in vitro after separate synthesis. Gp11 controls the polymerization of gp13 into normal (T = 7) and small sized (T = 4?) procapsids. Predominant formation of T = 7 procapsids requires presen ce of the portal protein. This implies that the portal protein has to be in tegrated at an initial stage of the capsid assembly process. Its presence, however, does not have a detectable effect on the rate of procapsid assembl y during SPP1 infection. A stable interaction between gp6 and the two major procapsid proteins was only detected when the three proteins are co-produc ed. Efficient incorporation of a single portal protein in the procapsid app ears to require a structural context created by gp11 and gp13 early during assembly, rather than strong interactions with any of those proteins. Gp7, which binds directly to gp6 both in vivo and in vitro, is not necessary for incorporation of the portal protein in the procapsid structure. (C) 2000 A cademic Press.