Abstract
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.
Highlights
Cyanobacteria sequester photosynthetic enzymes into microcompartments which facilitate the conversion of carbon dioxide into sugars
Spatial segregation is an ubiquitous strategy in biology for organizing the crowded, active viscera of the cell [1,2,3,4]. Viral capsids exemplify this organization at very small scales, sequestering genetic material from the cytosol and recapturing it for delivery to new hosts
Bacterial microcompartments serve a very different biomolecular purpose from viruses but have striking structural similarities, namely, a quasi-icosahedral protein shell that assembles around a fluctuating cargo [2, 12,13,14]. This comparison raises the question, Do the same assembly principles, based on a balance between equilibrium stability and kinetic accessibility, apply to microcompartments as well? Here we focus on a paradigmatic example of a microcompartment, the carboxysome
Summary
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. 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. Bacterial microcompartments serve a very different biomolecular purpose from viruses but have striking structural similarities, namely, a quasi-icosahedral protein shell that assembles around a fluctuating cargo [2, 12,13,14] This comparison raises the question, Do the same assembly principles, based on a balance between equilibrium stability and kinetic accessibility, apply to microcompartments as well? Recent atomic force microscopy measurements have demonstrated that these constituents form flat monolayers
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