Bacterial microcompartment organization by cryo electron tomography and high-resolution subtomogram averaging

Roughly 20% of bacteria employ bacterial microcompartments (BMCs) to sequester dangerous or inefficient enzymatic processes. In these structures, a cargo of enzymes and accessory proteins is encased within a semi-permeable protein shell that permits passage of substrates and products but restricts movement of intermediates. In addition to their importance as a component of many bacterial species' metabolisms, BMCs have become a target of protein engineering due to their potential for alternative pharmacological and biotechnological applications. Despite the importance of BMCs in prokaryotic biology and bioengineering, structural heterogeneity has prevented a complete understanding of the architecture, ultrastructure, and spatial organization of both the shell proteins and the cargo. Here, we employ cryo electron tomography to image intact alpha-carboxysomes, a model BMC responsible for carbon fixation in cyanobacteria and chemoautotrophs. We have identified novel arrangements of the enzymatic cargo, RuBisCO, characterized states of known protein-protein interactions, and solved a high-resolution subtomogram average of the enzyme in situ. Efforts are underway to identify and resolve additional cargo components and the lattice-like arrangement of the shell proteins to build a complete model of the entire assembly. Taken together, these data offer new insights into conserved BMC organization and heterogeneity, and characterize arrangements that were unresolved in previous in vitro and low-resolution in situ studies.