Three functional subdomains of the Escherichia coli FtsQ protein are involved in its interaction with the other division proteins.

FtsQ, an essential protein for the Escherichia coli divisome assembly, is able to interact with various division proteins, namely FtsI, FtsL, FtsN, FtsB and FtsW. In this paper, the FtsQ domains involved in these interactions were identified by two-hybrid assays and co-immunoprecipitations. Progressive deletions of the ftsQ gene suggested that the FtsQ self-interaction and its interactions with the other proteins are localized in three periplasmic subdomains: (i) residues 50-135 constitute one of the sites involved in FtsQ, FtsI and FtsN interaction, and this site is also responsible for FtsW interaction; (ii) the FtsB interaction is localized between residues 136 and 202; and (iii) the FtsL interaction is localized at the very C-terminal extremity. In this third region, the interaction site for FtsK and also the second site for FtsQ, FtsI, FtsN interactions are located. As far as FtsW is concerned, this protein interacts with the fragment of the FtsQ periplasmic domain that spans residues 67-75. In addition, two protein subdomains, one constituted by residues 1-135 and the other from 136 to the end, are both able to complement an ftsQ null mutant. Finally, the unexpected finding that an E. coli ftsQ null mutant can be complemented, at least transiently, by the Streptococcus pneumoniae divIB/ftsQ gene product suggests a new strategy for investigating the biological significance of protein-protein interactions.