FtsA is crucial for assembly of the E. coli divisome, as it dynamically links cytoplasmic FtsZ filaments with transmembrane cell division proteins. FtsA allegedly initiates cell division by switching from an inactive polymeric to an active monomeric confirmation, which recruits downstream proteins and stabilizes FtsZ filaments. Here, we use biochemical reconstitution experiments combined with quantitative fluorescence microscopy to study divisome activation in vitro. We compare wildtype-FtsA with FtsA-R286W, a constantly active gain-of-function mutant and find that R286W outperforms the wildtype protein in replicating FtsZ treadmilling dynamics, stabilizing FtsZ filaments and recruiting FtsN. We attribute these differences to a faster membrane exchange of FtsA-R286W and its higher packing density below FtsZ filaments. Using FRET microscopy, we find that FtsN binding does not compete with, but promotes FtsA self-interaction. Our findings suggest a model where FtsA always forms dynamic polymers on the membrane, which re-organize during assembly and activation of the divisome.