Bacterial microcompartments are primitive organelles composed entirely of protein subunits. Genomic sequence databases reveal the widespread occurrence of microcompartments across diverse microbes. The prototypical bacterial microcompartment is the carboxysome, a protein shell for sequestering carbon fixation reactions. We report three-dimensional crystal structures of multiple carboxysome shell proteins, revealing a hexameric unit as the basic microcompartment building block and showing how these hexamers assemble to form flat facets of the polyhedral shell. The structures suggest how molecular transport across the shell may be controlled and how structural variations might govern the assembly and architecture of these subcellular compartments.

Type III secretion systems (T3SSs) are bacterial membrane-embedded secretion nanomachines designed to export specifically targeted sets of proteins from the bacterial cytoplasm. Secretion through T3SS is governed by a subset of inner membrane proteins termed the ‘export apparatus’. We show that a key member of the Shigella flexneri export apparatus, MxiA, assembles into a ring essential for secretion in vivo. The ring forming interfaces are well conserved in both non-flagellar and flagellar homologues, implying that the ring is an evolutionary conserved feature in these systems. Electron cryo-tomography reveals a T3SS-associated cytoplasmic torus of size and shape corresponding to the MxiA ring aligned to the secretion channel located between the secretion pore and the ATPase complex. This defines the molecular architecture of the dominant component of the export apparatus and allows us to propose a model for the molecular mechanisms controlling secretion.