Shell Proteins' Roles in Bacterial Microcompartment Assembly*

Bacterial microcompartments are nanometer-scale proteinaceous containers that house biochemical pathways in bacteria, protecting the organism from toxic intermediates and co-localizing enzymes. Recently, there has been much interest in re-engineering these systems for use as "metabolic modules" through the engineering of shell proteins. However, this can be a daunting task, since these shells can be composed of up to eight unique shell proteins whose roles in the assembly are not yet well understood. Here, we use atomistic simulations to measure a vast range of interaction strengths including a multitude of different bending interactions between pairs of Pdu shell proteins. These interactions inform a coarse grain model that shows how varying these interactions in a system with multiple types of shell proteins leads to different assembly paths and shells with different properties. Specifically, we show an assembly where one shell protein forms a nucleus in the bulk, which is then recruited to the enzymatic core by a second type. Once this nucleus binds the cargo a third type of shell protein helps to close the shell. These shells have, on average, weak interactions with the enzyme core leading the shell to be larger and partially empty. Computational results are experimentally validated throughout.