The tubulin-like GTPase protein FtsZ, which forms a discontinuous cytokinetic ring at mid-cell, is a central player to recruit the division machinery to orchestrate cell division. To guarantee the production of two identical daughter cells, the assembly of FtsZ, namely Z-ring, and its precise positioning should be finely regulated. In Streptococcus pneumoniae, the positioning of Z-ring at the division site is mediated by a bitopic membrane protein MapZ (mid-cell-anchored protein Z) through direct interactions between the intracellular domain (termed MapZ-N (the intracellular domain of MapZ)) and FtsZ. Using nuclear magnetic resonance titration experiments, we clearly assigned the key residues involved in the interactions. In the presence of MapZ-N, FtsZ gains a shortened activation delay, a lower critical concentration for polymerization and a higher cooperativity towards GTP hydrolysis. On the other hand, MapZ-N antagonizes the lateral interactions of single-stranded filaments of FtsZ, thus slows down the formation of highly bundled FtsZ polymers and eventually maintains FtsZ at a dynamic state. Altogether, we conclude that MapZ is not only an accelerator to trigger the polymerization of FtsZ, but also a brake to tune the velocity to form the end-product, FtsZ bundles. These findings suggest that MapZ is a multi-functional regulator towards FtsZ that controls both the precise positioning and proper timing of FtsZ polymerization.