Essential and not essential interactions in interactome networks: the case of FtsQ, an E. coli division protein.

A key step of bacterial cytochinesis is the dynamic assembly of the division proteins into a machinery called divisome. We described the division interactome of Escherichia coli and Streptococcus pneumoniae. Our results showed that the protein-protein interaction (PPI) web is extremely complex: generally, each protein undergoes multiple interactions with both itself and other partners to form multimeric complexes. One of the main problems in the PPI studies is to determine the biological role of each interaction and the behaviour of interaction-defective mutants can furnish an answer to this question. To establish whether an interaction has a biological role, the mutated gene is inserted in a bacterial strain where the same gene, deleted in the chromosome, is cloned in a plasmid under the control of an inducible promoter. If the interaction is essential for the bacterial viability, the mutated gene will not be able to sustain the bacterial growth. The ability of the mutated gene to recruit to the septum the proteins with which it loses the interaction ability was also analyzed. This analysis allowed the identification of which interactions between FtsQ and its partner proteins are essential, in fact, this protein, beside its ability to homodimerize, interacts with many division partners, as FtsI, FtsL, FtsK, FtsW, FtsB, e FtsN. We selected and studied mutants impaired in the interaction ability with each of the FtsQ partner. Moreover, since two separates sites for both FtsQ homodimerization and FtsI and FtsN interactions were identified, double mutants were selected and studied as described above.