Structure of the Reovirus Membrane-Penetration Protein, μ1, in a Complex with Its Protector Protein, σ3

by an outer protein coat (Figure 1A) (Nibert and Schiff, 2001). The major core protein, known as ␭1, forms an icosahedral shell, stabilized by surface binding of a second core protein, ␴2 (Reinisch et al., 2000). Twelve turret-like pentamers of a third core protein, ␭2, also project Harvard Medical School RNA transcripts, produced by the viral polymerase com-320 Longwood Avenue plexes (␭3 and ␮2) inside the shell, exit through the ␭2 Boston, Massachusetts 02115 turrets, which cap the 5Ј ends. In the virion, the cores are 2 Department of Microbiology coated in equimolar ratio by a layer of the two proteins and Molecular Genetics studied here, ␮1 and ␴3. These proteins form a T ϭ 13l Harvard Medical School icosahedral lattice, interrupted at the 5-fold axes by the Boston, Massachusetts 02115 ␭2 turrets (Dryden et al., 1993). A trimer of one further 3 Department of Biological Sciences protein, ␴1, associates with each turret (Furlong et al., Like most animal viruses, reoviruses are activated for infection by proteolytic cleavages. Exposure of virions to proteases in the intestinal lumen (reoviruses infect intestinal cells) or in endocytic vacuoles leads to forma-Summary tion of " infectious subviral particles " (ISVPs) (Chang and Zweerink, 1971; Silverstein et al., 1970). These particles Cell entry by nonenveloped animal viruses requires lack ␴3, and their ␴1 proteins have undergone a confor-membrane penetration without membrane fusion. The mational change that results in fiber-like spikes pro-reovirus penetration agent is the outer-capsid protein, jecting from each 5-fold vertex (Dryden et al., 1993). ␮1. The structure of ␮1, complexed with its " protector " Moreover, at some time between assembly and activa-protein, ␴3, and the fit of this ␮1 3 ␴3 3 heterohexameric tion, ␮1 undergoes a cleavage, believed to be autolytic, complex into the cryoEM image of an intact virion, into a small, N-terminal fragment (␮1N, 4 kDa) and a reveal molecular events essential for viral penetration. large, C-terminal fragment (␮1C, 72 kDa) (Nibert et al., Autolytic cleavage divides ␮1 into myristoylated ␮1N 1991). This cleavage depends on the presence of ␴3, and ␮1C. A long hydrophobic pocket can receive the on the N-terminal, myristoyl group of ␮1, and on the myristoyl group. Dissociation of ␮1N, linked to a major ␮1 residues 42 and 43 (Asn-Pro) at the cleavage site conformational change of the entire ␮1 trimer, must (Tillotson and Shatkin, 1992). precede myristoyl-group insertion into the cellular The first step …