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Huang, Rick K.; Khayat, Reza; Lee, Kelly K.; Gertsman, Ilya; Duda, Robert L.; Hendrix, Roger W.; Johnson, John E.
Journal of Molecular Biology/Journal of molecular biology, 05/2011, Volume: 408, Issue: 3Journal Article
Virus capsid assembly requires recruiting and organizing multiple copies of protein subunits to form a closed shell for genome packaging that leads to infectivity. Many viruses encode scaffolding proteins to shift the equilibrium toward particle formation by promoting intersubunit interactions and stabilizing assembly intermediates. Bacteriophage HK97 lacks an explicit scaffolding protein, but the capsid protein (gp5) contains a scaffold-like N-terminal segment termed the delta domain. When gp5 is expressed in Escherichia coli, the delta domain guides 420 copies of the subunit into a procapsid with T = 7 laevo icosahedral symmetry named Prohead-I. Prohead-I can be disassembled and reassembled under mild conditions and it cannot mature further. When the virally encoded protease (gp4) is coexpressed with gp5, it is incorporated into the capsid and digests the delta domain followed by autoproteolysis to produce the metastable Prohead-II. Prohead-I +P was isolated by coexpressing gp5 and an inactive mutant of gp4. Prohead-I and Prohead-I +P were compared by biochemical methods, revealing that the inactive protease stabilized the capsid against disassembly by chemical or physical stress. The crystal structure of Prohead-I +P was determined at 5.2 Å resolution, and distortions were observed in the subunit tertiary structures similar to those observed previously in Prohead-II. Prohead-I +P differed from Prohead-II due to the presence of the delta domain and the resulting repositioning of the N-arms, explaining why Prohead-I can be reversibly dissociated and cannot mature. Low-resolution X-ray data enhanced the density of the relatively dynamic delta domains, revealing their quaternary arrangement and suggesting how they drive proper assembly. Display omitted ► The X-ray crystal structure of Prohead-I +P was determined at 5.2 Å resolution. ► Prohead-I +P particles were stabilized by encapsidated, virally encoded proteases. ► Pores at the Class-2 contacts were derived from tertiary-structure distortions. ► Density of the scaffolding domain and repositioning of the N-arm were observed. ► Weak threefold contacts and the ordered N-arms regulate assembly and maturation.
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