  Cyanobacteria sequester photosynthetic enzymes into microcompartments which
facilitate the conversion of carbon dioxide into sugars. Geometric similarities
between these structures and self-assembling viral capsids have inspired models
that posit microcompartments as stable equilibrium arrangements of the
constituent proteins. Here we describe a different mechanism for
microcompartment assembly, one that is fundamentally nonequilibrium and yet
highly reliable. This pathway is revealed by simulations of a molecular model
resolving the size and shape of a cargo droplet, and the extent and topography
of an elastic shell. The resulting metastable microcompartment structures
closely resemble those of carboxysomes, with a narrow size distribution and
faceted shells. The essence of their assembly dynamics can be understood from a
simpler mathematical model that combines elements of classical nucleation
theory with continuum elasticity. These results highlight important control
variables for achieving nanoscale encapsulation in general, and for modulating
the size and shape of carboxysomes in particular.
