Membrane affinity difference between MinD monomer and dimer is not crucial for MinD gradient formation in <i>Bacillus subtilis</i>.

Abstract

Proteins can diffuse micrometers in seconds, yet bacterial cells are able to maintain stable protein gradients. The best-studied bacterial protein gradient is the Min system of <i>Escherichia coli</i>. In rod-shaped bacteria, the MinCD proteins prevent formation of minicells by inhibiting FtsZ polymerization close to the cell poles. In <i>E. coli</i>, these proteins oscillate between cell poles within a minute, resulting in an increased MinCD concentration at the poles. This oscillation is caused by the interaction between MinD and the protein MinE, which form an ATP-driven reaction-diffusion system, whereby the ATPase MinD cycles between a monomeric cytosolic and a dimeric membrane-attached state. <i>Bacillus subtilis</i> also has MinCD, but lacks MinE. In this case, MinCD forms a static gradient that requires the transmembrane protein MinJ, located at cell poles and cell division sites. A recent reaction-diffusion model was successful in recreating the MinD gradient in <i>B. subtilis</i>, assuming that MinD cycles between cytosol and membrane, like in <i>E. coli</i>. Here, we show that the monomeric and dimeric states of <i>B. subtilis</i> MinD have comparable membrane affinities, that MinD interacts with MinJ as a dimer, and that MinJ is not required for membrane localization of MinD. Based on these new findings, we tested different models, using kinetic Monte Carlo simulations, and found that a difference in diffusion rate between the monomer and dimer, rather than a difference in membrane affinity, is important for <i>B. subtilis</i> MinCD gradient formation.