AbstractProper cell division at the mid-site of Gram-negative bacteria reflects stringent regulation by the min system (MinC, MinD and MinE). Herein we report crystal structure of the C-terminal domain of MinC from Escherichia coli (EcMinC_CTD). The MinC_CTD beta helical domain is engaged in a tight homodimer, similar to Thermotoga maritima MinC_CTD (TmMinC_CTD). However, both EcMinC_CTD and TmMinC_CTD lack an α-helix (helix3) at their C-terminal tail, in comparison to Aquifex aerolicu MinC_CTD (AaMinC_CTD) which forms an extra interaction interface with MinD. To understand the role of this extra binding element in MinC/MinD interactions, we fused this helix (Aahelix3) to the C-terminus of EcMinC and examined its effect on cell morphology and cell growth. Our results revealed that Aahelix3 impaired normal cell division in vivo. Furthermore, results of a co-pelleting assay and binding free energy calculation suggested that Aahelix3 plays an essential role in AaMinCD complex formation, under the circumstance of lacking MinE in A. aerolicu. Combining these results with sequence analysis of MinC and MinD in different organisms, we propose an evolutionary relationship to rationalize different mechanisms in cell division positioning in various organisms.