Encapsulation of cargo by protein shells with and without spontaneous curvature

Bacterial microcompartments are large, roughly icosahedral protein shells that encase certain metabolic pathways in bacteria. Microcompartments form by assembling around a dense complex containing thousands of copies of enzymes and other components. How the formation of the microcompartment depends on the interplay between protein-protein interactions and the cargo is poorly understood. In this talk we present dynamical simulations and an equilibrium continuum theory exploring how microcompartment size and degree of shell completion depend on control parameters including interaction strengths, subunit and cargo stoichiometries, and the shell spontaneous curvature. Depending on these parameters, we find that the presence of a cargo can either increase or decrease the size of a shell, relative to the intrinsic protein spontaneous curvature. For example, even proteins without spontaneous curvature (which preferentially form flat sheets) exhibit a well-defined size distribution when assembling around stoichiometrically limiting cargo. These results may shed light on recent experimental observations – in some BMC systems, empty shells are smaller and more monodisperse than full shells, whereas in other systems empty shells are larger than full ones.