The McdAB Carboxysome Positioning System is Widespread Among β-cyanobacteria

Carboxysomes are protein-based organelles that are essential for allowing cyanobacteria to fix CO2. Previously we identified a two-component system, McdAB, responsible for equidistantly positioning carboxysomes in the model cyanobacterium Synechococcus elongatus PCC 7942. McdA, a ParA-type ATPase, non-specifically binds the nucleoid in the presence of ATP. McdB, a novel factor that directly binds carboxysomes, displaces McdA from the nucleoid. Removal of McdA from the nucleoid in the vicinity of carboxysomes by McdB causes a global break in McdA symmetry, and carboxysome motion occurs via a Brownian-ratchet based mechanism towards the highest concentration of McdA. Despite the importance for cyanobacteria to properly position their carboxysomes, whether the McdAB system is widespread among cyanobacteria remains an open question. Here, we used neighborhood analysis to show that the McdAB system is widespread among {beta}-cyanobacteria and often clusters near carboxysome-related components. Moreover, we show that two distinct McdAB systems exist in {beta}-cyanobacteria, with Type 2 systems being the most abundant (>98% of {beta}-cyanobacteria) and Type 1 systems, like that of S. elongatus, possibly being acquired more recently. Surprisingly, our analysis suggests that the McdAB system is completely absent in -cyanobacteria. Lastly, all McdB proteins we identified share the sequence signatures of a protein capable of undergoing Liquid-Liquid Phase Separation (LLPS). Indeed, we find that S. elongatus McdB undergoes LLPS in vitro, the first example of a ParA-type ATPase partner protein exhibiting this behavior. This is an intriguing finding given the recent demonstration of LLPS activity by {beta}-carboxysome core components. Our results have broader implications for understanding carboxysome biogenesis and positioning across all {beta}-cyanobacteria.\n\nIn BriefWe found that the McdAB carboxysome positioning system is widespread among {beta}-cyanobacteria, absent in -cyanobacteria, exists in two distinct forms, and that S. elongatus McdB undergoes liquid-liquid phase separation.