Figure 3. Network interactions in the MCP floor and function of MCP N-lasso.

a, b, Part of the MCP network viewed from outside (a) or inside (b) the capsid. Pen1 and Pen2 are two of the five MCP subunits in the penton that have the same structure owing to five-fold symmetry of the penton. c–f, Three types of network interaction among hexon MCPs; e and f represent decomposition of structures in d. Type I interactions are intracapsomeric augmentations of β-strands from adjacent MCPs (P2 and P3 in c). Type II and type III interactions are intercapsomeric interactions among two pairs of MCPs (P2–P3 and C5–C6 in d), diagonally across the local two-fold axis (black ovals in d–f). Type III interactions build upon and fortify type I interactions (C5 N-lasso in c). The two dimerization domains that are joined in a type II interaction (e) also sit on top of (from the perspective of inside of the capsid) a pair of type III interactions (d) and prevent the two N-lassoes (C5 and P2 N-lassoes in d and f) from unwinding. g, h, Interactions between penton MCPs and the P1 and P6 hexon MCPs are different from the canonical hexon MCP network interactions (shown in c and d). An elbow-like helix–turn–helix structure in the buttress domain of hexon MCP is folded into a single long helix in penton MCP (see c and g). i, Design of serial-truncation mutageneses in the MCP N-lasso region. j, Comparison of viral titres of wild type and MCP-truncated mutants. Data are mean ± s.e.m. (n = 3 biologically independent samples). UD, undetectable. k, l, Transmission electron microscopy images of ultrathin sections of cells replicating the wildtype virus (k) or the MCPΔ N40 mutant (l). Experiments were repeated independently twice with similar results.