Membrane interaction of the portal protein gp20 of bacteriophage T4.

Assembly of the bacteriophage T4 head structure occurs at the cytoplasmic face of the inner membrane of Escherichia coli with the formation of proheads. The proheads contain an internal scaffolding core that determines the size and the structure of the capsid. In a mutant where the major shell protein gp23 was compromised, core structures without a shell had been detected. Such core structures were also found in the mutant T4am20am23. Since the mutation in gene 20 is at the N terminus of gp20, it was assumed that these core structures assemble in the absence of gp20. However, sequencing showed that the mutation introduces a new ribosome binding site that leads to a restart at codon 15. Although the mutant protein gp20s lacks the very N-terminal sequence, we found that it still binds to the membrane of the host cell and can initiate prohead assembly. This explains its activity to allow the assembly of core structures and proheads at the membrane surface. With a cross-linking approach, we show here that gp20 and gp20s are escorted by the chaperones DnaK, trigger factor, and GroEL and dock on the membrane at the membrane protein YidC.