During assembly scaffolding proteins interact with capsid proteins and drive the protein polymerization in a defined pathway. Most scaffolding proteins are elongated molecules with high a-helical content (Dokland 1999). These spirals presumably result from inappropriate relative positioning of the hexameric and penatmeric capsomeres required to build a capsid with >60 subunits (Berger et al. In the absence of scaffolding protein, aberrant spirals dominate the assembly reactions. Second, they ensure proper form determination by positioning the capsid protein subunits relative to one another in the icosahedral shells (King et al. of polymerization, resulting in a decreased critical concentration. Its dynamic nature might play a role in the assembly and release mechanism. The cooperative opening rate was increased in the procapsid-bound form, suggesting this region might interact with the capsid protein. At low temperature where high energy motions were damped, or in a mutant in which the helices were tethered through the introduction of a disulfide bond, this region displayed restricted cooperative opening motions as demonstrated by a switch in the exchange kinetics from correlated EX1 exchange to uncorrelated EX2 exchange. The motions can be promoted by destabilizing the hydrophobic contact between two helices. The H/D exchange experiments revealed highly dynamic and cooperative opening motions of scaffolding molecules in the N-terminal helix-loop-helix (H-L-H) region.
#SCAFFOLD PROTEIN FREE#
To identify the switch controlling scaffolding protein association and release, hydrogen deuterium exchange was applied to Bacillus subtilis phage Ø29 scaffolding protein gp7 in both free and procapsid-bound forms. Removal of scaffolding protein is achieved either by proteolysis or alternatively by some form of conformational switch that allows it to dissociate from the capsid. The interactions between the scaffolding and capsid proteins are transient and are subsequently disrupted during DNA packaging. High assembly fidelity requires the assistance of scaffolding protein molecules, which interact with the capsid proteins to insure proper geometrical incorporation of subunits into the growing icosahedral lattices. Abstract In the double-stranded DNA containing bacteriophages, hundreds of copies of capsid protein subunits polymerize to form icosahedral shells, called procapsids, into which the viral genome is subsequently packaged to form infectious virions.
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