Browsing by Subject "Virion assembly"

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Interaktion des Portalring-Proteins gp20 des Bakteriophagen T4 mit Wirtsproteinen von Escherichia coli
(2012) Quinten, Tobias; Kuhn, Andreas
Bacteriophage T4 is composed of the three structural subunits i. e. capsid, tail and tail fibres. Because of its contractible tail T4 is a member of the Myoviridae. A characteristic feature of its morphogenesis is the membrane-associated assembly of the head structure during a wild-type infection of E. coli. The portal protein, gp20, and the phage-chaperone gp40 are used to form a membrane-bound complex. Most likely, also proteins from the phage-host E. coli are involved in this process. This complex is the starting point for the assembly of the head-related core and scaffold structure. The mature head detaches from the membrane and is filled with DNA thereafter. In the context of this doctoral thesis the role and function of the portal protein gp20 and its interactions with cellular proteins was analyzed. A His-tag was fused to gp20 and it was purified by nickel-affinity chromatography. Also, interactions with the cellular chaperones DnaK, GroEL, Tig and YidC and the phage-chaperone gp40 were detected after formaldehyde crosslinking. Further studies of the cellular localization showed that gp20 and the fusion protein gp20-GFP are membrane-bound. The importance of YidC and DnaK for this membrane-association was demonstrated by fluorescence microscopy. Phage propagation was not affected by YidC depletion, whereas the loss of DnaK led to a reduced propagation. Prohead-structures, that are an intermediate stage of the capsid assembly, were isolated in YidC-free E. coli-cells membrane-unbound when infected with a phage mutant. Previous studies had led to the isolation of different amber mutants. Mutant amber20E481 was used in this thesis to analyze the assembly process in more detail. Here, under non-permissive conditions a 14 amino acid shortened protein, gp20s, is synthesized. Despite the fact that the capsid assembly is blocked in the non-suppressor strain, the localization and expression of the truncated protein was comparable to the wild type gp20. Overexpressed and His-tagged gp20s was found to crosslink with YidC, GroEL and gp40. Structural studies with a transmission electron microscope showed, that mature proheads were found and these were not filled with phage DNA. Most probably, a malfunction of gp20s during DNA packaging accounts for this.