Mechanical elasticity as a physical signature of conformational dynamics in a virus particle

Abstract

In this study we test the hypothesis that mechanically elastic regions in a virus particle (or large biomolecular complex) must coincide with conformationally dynamic regions, because both properties are intrinsically correlated. Hypothesis-derived predictions were subjected to verification by using 19 variants of the minute virus of mice capsid. The structural modifications in these variants reduced, preserved, or restored the conformational dynamism of regions surrounding capsid pores that are involved in molecular translocation events required for virus infectivity. The mechanical elasticity of the modified capsids was analyzed by atomic force microscopy, and the results corroborated every prediction tested: Any mutation (or chemical cross-linking) that impaired a conformational rearrangement of the pore regions increased their mechanical stiffness. On the contrary, any mutation that preserved the dynamics of the pore regions also preserved their elasticity. Moreover, any pseudo-reversion that restored the dynamics of the pore regions (lost through previous mutation) also restored their elasticity. Finally, no correlation was observed between dynamics of the pore regions and mechanical elasticity of other capsid regions. This study (i) corroborates the hypothesis that local mechanical elasticity and conformational dynamics in a viral particle are intrinsically correlated; (ii) proposes that determination by atomic force microscopy of local mechanical elasticity, combined with mutational analysis, may be used to identify and study conformationally dynamic regions in virus particles and large biomolecular complexes; (iii) supports a connection between mechanical properties and biological function in a virus; (iv) shows that viral capsids can be greatly stiffened by protein engineering for nanotechnological applications.

Fig. 1.

MVM capsid structure and mutations tested. (A) Icosahedral model and AFM images of capsids, viewed along a S5, S3, or S2 symmetry axis. The circle around the labelled S5 axis delimits the region represented as a ribbon model of the crystallographic structure in B (front view) and C (lateral view), with a S5 pore at the center. The residues around the base of the pore or other mutated residues are, respectively, represented as yellow or purple spacefilling models and labeled in C. The program PyMol (DeLano Scientific) and the Protein Data Bank coordinates 1Z1C (29) were used.

Fig. 2.

Heat-induced conformational transition in some variant MVM capsid-like particles followed by the variation in tryptophan fluorescence as a function of temperature. (A) Closed circles, WT capsid; squares, mutant L172A; triangles, D58A; inverted triangles, N183A. In the virion, mutation L172A led to a drastically reduced infectivity; mutations D58A and N183A had no severe effect on the infectivity. (B) Closed circles, WT capsid; open circles, cross-linked WT capsid.

Fig. 3.

Comparison of the mechanical stiffness at the S5 regions between the WT and mutant MVM capsids. Each histogram depicts the number of determinations relative to the k_s obtained for a capsid variant. In each graph, the histogram in purple corresponds to the WT capsid, and the one in a different color to a mutant capsid (labeled). (A) Capsids that carry a mutation that impairs both S5 dynamics and virus infectivity relative to the WT (Table 1). (B) Representative examples of capsids that carry a mutation that neither impair S5 dynamics nor virus infectivity relative to the WT. For statistical analysis see Table 1.

Fig. 4.

Comparison of the mechanical stiffness at S5 regions between the WT and variant MVM capsids. Each histogram depicts the number of determinations relative to the k_s obtained for a capsid variant. Left: Comparison between unmodified capsid (WT) (that undergoes the conformational transition) and the cross-linked WT capsid (that lost the conformational transition). Center: comparison between mutant capsids L172A (that lost the conformational transition) and A172I pseudo-revertant (that recovered the conformational transition). Right: Comparison between D263A (that lost the conformational transition) and A263N pseudo-revertant (that recovered the conformational transition). For statistical analysis see Table 2.