Understanding the Asymmetric MipZ Gradient in Caulobacter Crescentus

Abstract

Caulobacter crescentus is a freshwater α-proteobacterium that divides into two asymmetric daughter cells. These curved, rod-shaped cells differ in their chromosome replication fates and in their cell length at division. C. crescentus divides into a replication-competent ‘stalked’ cell and a smaller, replication-quiescent ‘swarmer’ cell. The site of cell division is dictated by the polymerization of the eukaryotic tubulin homologue, FtsZ. MipZ is an essential ATPase and inhibitor of FtsZ polymerization. MipZ forms an intracellular bipolar gradient that directs FtsZ to the lowest concentration of MipZ, which is found slightly towards the new pole as opposed to the exact mid-cell. This localization bias leads to two differently sized daughter cells. To better understand how MipZ directs FtsZ to this off-center location, we employed single-particle tracking photoactivated localization microscopy (sptPALM) to characterize the diffusive states and transition state kinetics of MipZ. We then used deterministic and stochastic simulations with these biophysical parameters to recreate the asymmetric MipZ gradient in silico. These simulations produce a distribution of cell lengths after cell division similar to that observed in vivo. These studies provide more insight into the asymmetric division of C. crescentus, but further questions remain regarding the nature of the asymmetric division.