UCLA

Fusobacterium nucleatum is a Gram-negative bacterium that colonizes the oral cavity as well as other sites in the human body. It is known for its ability to adhere to a large number of different bacterial species and serving as a "bridging" organism. It plays an important role in the formation and maintenance of mature oral biofilms. At the same time it has been implicated in a number of important oral and systemic diseases and is considered an opportunistic pathogen. Very little is known about the outer membrane proteins of F. nucleatum and the molecular mechanisms of its adhesion. There is a large number of predicted membrane proteins that are expected to localize to either of the two membranes surrounding the cell. A number of lipoproteins are predicted from sequence searches, yet none are currently characterized on a molecular level.

We have identified and characterized a novel hypothetical protein in F. nucleatum encoded by the ORF FN1253. The gene was shown to play a role in interspecies interactions with oral streptococci and was named aid1 (Adhesion Inducing Determinant 1). Aid1 is important in modulation of adhesion to oral streptococci, as aberrant expression of the aid1 gene leads to an uncharacteristic development of the dual species biofilm, which ultimately could lead to changes in the overall multispecies biofilm formation. The functional domains of the protein were also analyzed, and we showed that this small lipoprotein localizes to the inner leaflet of the outer membrane and requires the presence of its signal sequence as well as the anchoring cysteine residue for proper processing and maturation of the final protein product. Aberrant processing of the protein led to rapid turnover inside the cytosol, as the accumulation of the unprocessed protein appeared to be toxic to the cells. Expression of the aid1 gene and the Aid1 protein production is tightly regulated, particularly in the presence of oral streptococci. We showed that the gene can be transcribed as two messenger RNA species, and only the longer polycistronic messenger is used for transcription.

To the best of our knowledge, this is the first molecular characterization of a fusobacterial lipoprotein, and the first hypothetical protein that is implicated in interspecies interactions within the oral cavity. This work also describes an interesting translational and transcriptional regulatory mechanism utilized by F. nucleatum, and is the first study to describe the regulation of a lipoprotein in this organism.