Structure and Mechanisms of a Protein-Based Organelle in Escherichia coli

Bacterial Compartmentalization In diverse bacteria, reactions that involve toxic or volatile metabolites are carried out by enzymes inside proteinaceous microcompartments. Tanaka et al. (p. 81; see the Perspective by Kang and Douglas) now report high-resolution crystal structures for four homologous proteins that are constituents of the shell that sequesters the metabolism of ethanolamine in bacteria. While the structures have similar overall folds, they have distinctive structural features that provide insight into how they build the shell and participate in microcompartment function. Structures of the shell proteins from a bacterial organelle help to explain how it functions in metabolizing ethanolamine. Many bacterial cells contain proteinaceous microcompartments that act as simple organelles by sequestering specific metabolic processes involving volatile or toxic metabolites. Here we report the three-dimensional (3D) crystal structures, with resolutions between 1.65 and 2.5 angstroms, of the four homologous proteins (EutS, EutL, EutK, and EutM) that are thought to be the major shell constituents of a functionally complex ethanolamine utilization (Eut) microcompartment. The Eut microcompartment is used to sequester the metabolism of ethanolamine in bacteria such as Escherichia coli and Salmonella enterica. The four Eut shell proteins share an overall similar 3D fold, but they have distinguishing structural features that help explain the specific roles they play in the microcompartment. For example, EutL undergoes a conformational change that is probably involved in gating molecular transport through shell protein pores, whereas structural evidence suggests that EutK might bind a nucleic acid component. Together these structures give mechanistic insight into bacterial microcompartments.