Protein secretion systems in Fusobacterium nucleatum: genomic identification of Type 4 piliation and complete Type V pathways brings new insight in mechanisms of pathogenesis

Recent genomic analyses of the two sequenced strains F nucleatum subsp. nucleatum ATCC 25586 and F nucleatum subsp. vincentii ATCC 49256 suggested that the major protein secretion systems were absent. However, such a paucity of protein secretion systems is incongruous with F nucleatum pathogenesis. Moreover, the presence of one or more such systems has been described for every other Gram-negative organism sequenced to date. In this investigation, the question of protein secretion in F nucleatum was revisited. In the current study, the absence in F. nucleatum of a twin-arginine translocation system (TC #2.A.64.), a Type III secretion system (TC #3.A.6.), a Type IV secretion system (TC #3.A.7.) and a chaperone/usher pathway (TC #1.13.11.) was confirmed. However, contrary to previous findings, our investigations indicated that a Type I protein secretion system was also absent from F nucleatum. In contrast, members of the holin family (TC #1.E) and the machinery required for a Type 4 piliation/fimbriation system (TC #3.A.15.2.) were identified using a variety of bioinformatic tools. Furthermore, a complete range of proteins resembling members of the Type V secretion pathway, i.e., the Type Va (autotransporter; TC #1.B.12.), Type Vb (two-partner secretion system; TIC #1.B.20.) and Type Vc (YadA-like trimeric autotransporter; TIC #1.B.42.), was found. This work provides new insight into the protein secretion and virulence mechanisms of F nucleatum. (c) 2005 Elsevier B.V. All rights reserved.