Essential cell division protein that activates septal peptidoglycan synthesis and constriction of the cell. Acts on both sides of the membrane, via interaction with FtsA in the cytoplasm and interaction with the FtsQBL complex in the periplasm. These interactions may induce a conformational switch in both FtsA and FtsQBL, leading to septal peptidoglycan synthesis by FtsI and associated synthases (Probable) (PubMed:25496160). Required for full FtsI activity (PubMed:25496160). Required for recruitment of AmiC to the septal ring (PubMed:12787347). Interacts with FtsA via its N-terminal cytoplasmic domain (PubMed:22328664, PubMed:24750258, PubMed:25496160, PubMed:25496259). Interacts with ZapA, FtsQ, FtsW and FtsI (PubMed:17185541, PubMed:20497333). Localizes to the septum (PubMed:9282742, PubMed:19684127, PubMed:24750258). Localizes to the midcell via interaction with the early cell division protein FtsA and via the periplasmic SPOR domain (PubMed:9282742, PubMed:19684127, PubMed:24750258). FtsA-dependent localization precedes SPOR-dependent localization, and both are needed for efficient localization (PubMed:24750258). Localization depends upon FtsZ, FtsA, ZipA, FtsQ and FtsI (PubMed:9282742, PubMed:11948172). The cytoplasmic region is required for interaction with FtsA (PubMed:24750258). The periplasmic region is composed of a membrane-proximal region containing three short partially formed helices (H1, H2 and H3), followed by an unstructured glutamine-rich linker, and a C-terminal globular SPOR domain (PubMed:15101973, PubMed:19684127). Essential function of FtsN is accomplished by a small region of at most 35 residues that is centered about the H2 helix (PubMed:19684127). The SPOR domain, which exhibits a ribonucleoprotein (RNP) fold, binds peptidoglycan and is a strong septal localization determinant, but it seems not essential for cell division (PubMed:15466024, PubMed:19684127, PubMed:24056104). Depletion does not affect localization of FtsZ, FtsA, ZipA, FtsQ, FtsL and FtsI to the division site (PubMed:11703663). Cells containing low levels of FtsN stop dividing while their mean cell length increases (PubMed:20345660). Absence of FtsN is followed by an inverse sequential disassembly of already assembled divisome compounds (PubMed:20345660). Belongs to the FtsN family.