Orthogonality of shell proteins across BMC subclasses in cyanobacteria

Bacterial microcompartments (BMC) are protein-based organelles broadly distributed across all bacterial phyla and subclassified into ≥60 functional variants. Despite their evolutionary and metabolic diversity, shell proteins that structurally compose the BMC surface are closely related across BMC classes. Herein, we sought to identify molecular and physiological features that could promote independent operation of more than one BMC type within the same cell by reducing inter-organelle cross-talk of shell proteins. We heterologously expressed shell proteins from the structurally well-defined BMC of Haliangium ochraceum (HO) within Synechococcus elongatus PCC 7942, a model cyanobacterium containing the β-carboxysome. We find considerable cross-reactivity of the HO hexameric shell protein (HO BMC-H) with components of the β-carboxysome HO BMC-H can integrate into carboxysomes, disrupt its ultrastructural organization, and impair its associated CO2 fixation reactions. S. elongatus is unable to maintain the integrity of the β-carboxysome over time when HO BMC-H is expressed in the absence of one or more of three broad strategies that act to increase the orthogonality between HO and carboxysome BMC shell proteins: i) reduced expression of promiscuous shell proteins; ii) sequestration of free HO BMC-H proteins via co-expression of other members of the same HO shell protein class, or; iii) heterologous expression of BMC positional system proteins McdAB (Maintenance of carboxysome distribution AB), revealing a putative moonlighting function of the McdAB protein family. Our results have implications for bacteria that encode more than one BMC within their genome and may have translational implications for the use of engineered BMCs for biotechnological applications.