In vitro assembly of bacteriophage P4 procapsids from purified capsid and scaffolding proteins

Bacteriophage P4 is a satellite virus of bacteriophage P2, which has acquir ed the ability to utilize the structural gene products of P2 to assemble it s own capsid. The normal P2 capsid has a T = 7 icosahedral structure compri sed of the gpN-derived capsid protein, whereas the capsid produced under th e control of P4 has a smaller, T = 4 structure. The protein responsible for this size determination is the P4-coded gene product Sid, which forms an e xternal scaffold on the P4 procapsid. Using an in vitro assembly system, we show that gpN and Sid can coassemble into procapsid-like particles, indist inguishable from those produced in vivo, in the absence of any other gene p roducts. The fidelity of the assembly reaction is enhanced by the inclusion of PEG and has a pH optimum between 8.0 and 8.5. Analysis of the assembly properties of truncated versions of Sid and gpN suggests that the amino-ter minal part of Sid is involved in gpN binding, while the carboxyl-terminal p ar? forms trimeric Sid-Sid interactions, and that the first 31 amino acids of gpN are required for binding to Sid as well as for size determination, ( C) 2000 Academic Press.