A rapid flow cytometry assay for the relative quantification of protein encapsulation into bacterial microcompartments

Bacterial microcompartments (MCPs) are protein-based organelles that have been suggested as scaffolds for creating in vivo nanobioreactors. One of the key steps towards engineering MCPs is to understand and maximize the encapsulation of enzymes into the lumen of the MCP. However, there are currently no high-throughput methods for investigating protein encapsulation. Here, we describe the development of a rapid in vivo assay for quantifying the relative amount of protein encapsulated within MCPs based on fluorescence. In this assay, we fuse a degradation peptide to a MCP-targeted fluorescence reporter and use flow cytometry to measure overall fluorescence from the encapsulated, protected reporter protein. Using this assay, we characterized various MCP-targeting signal sequence mutants for their ability to encapsulate proteins and identified mutants that encapsulate a greater amount of protein than the wild type signal sequence. This assay is a powerful tool for reporting protein encapsulation and is an important step towards encapsulating metabolic enzymes into MCPs for synthetic biochemical pathways.