Many pathogenic bacteria use proteinaceous ethanolamine-utilization microcompartments (Eut BMCs) to facilitate the catabolism of ethanolamine, an abundant nutrient in the mammalian gut. The ability to metabolize ethanolamine gives pathogens a competitive edge over commensal microbiota which can drive virulence in the inflamed gut. Despite their critical functions, the molecular mechanisms underlying the synthesis of Eut BMCs in bacterial cells remain elusive. Here, we report a systematic study for dissecting the molecular basis underlying Eut BMC assembly in Salmonella. We determined the functions of individual building proteins in the structure and function of Eut BMCs and demonstrated that EutQ plays an essential role in both cargo encapsulation and Eut BMC formation through specific association with the shell and cargo enzymes. Furthermore, our data reveal that Eut proteins can self-assemble to form cargo and shell aggregates independently in vivo, and that the biogenesis of Eut BMCs follows a unique Shell-first pathway. Cargo enzymes exhibit dynamic liquid-like organization within the Eut BMC. These discoveries provide mechanistic insights into the structure and assembly of the Eut BMC, which serves as a paradigm for membrane-less organelles. It opens up new possibilities for therapeutic interventions for infectious diseases.
