Fig. 1.

Model dynamics of microcompartment assembly. (A) Cargo monomers (green spheres) in our molecular model occupy sites of an FCC lattice, experiencing short-ranged attraction to their nearest neighbors and also to protein monomers composing the shell. Each shell monomer corresponds to a triangle in a discretized shell (gray) that resists bending and stretching according to an elastic Hamiltonian. Closure of the shell requires the presence of topological defects in the triangulated surface, vertices that are connected to only five neighbors. Red spheres in A and B highlight the locations of these defects. (B) A fully assembled microcompartment includes at least 12 of these defects. In B and E, we show the boundary of the cargo droplet as a translucent green surface. (C) At early time $t$ in the assembly process, high curvature of the cargo droplet limits shell growth to an area that is relatively flat while maintaining contact with the cargo. (D) Droplet growth reduces curvature until encapsulation becomes thermodynamically favorable and kinetically facile (Movie S1). (E) The nearly closed shell effectively halts cargo aggregation, but the approach to a simply connected envelope proceeds slowly as defects reposition and combine to heal grain boundaries.