Identification of an extracellular matrix protein adhesin, EmaA, which mediates the adhesion of Actinobacillus actinomycetemicomitans to collagen

Actinobacillus actinomycetemcomitans is an aetiologic agent in the development of periodontal and some systemic diseases in humans. This pathogen localizes to the underlying connective tissue of the oral cavity in individuals with periodontal disease. The adhesion of A. actinomycetemcomitans to extracellular matrix components of the connective tissue prompted this study to identify gene products mediating the interaction of A. actinomycetemcomitans to these molecules. A transposon mutagenesis system was optimized for use in A. actinomycetemcomitans and used to generate an insertional mutant library. A total of 2300 individual insertion transposon mutants were screened for changes in the adhesion to collagen and fibronectin. Mutants were identified which exhibited the following phenotypes: a decrease in collagen binding; a decrease in fibronectin binding; a decrease in binding to both proteins; and an increase in binding to both collagen and fibronectin. The identification of mutants defective in adhesion to the individual proteins indicates that distinct adhesins are expressed by this organism. Molecular analysis of these mutants implicated 11 independent loci in protein adhesion. One gene, emaA, is likely to encode a direct mediator of collagen adhesion, based on predicted protein features homologous to the collagen-binding protein YadA of Yersinia enterocolitica. EmaA was localized to the outer membrane, as expected for an adhesin. Reduction in fibronectin adhesion appeared to be influenced by abrogation of proteins involved in molybdenum-cofactor biosynthesis. Several other loci identified as reducing or increasing adhesion to both collagen and fibronectin are suggested to be involved in regulatory cascades that promote or repress expression of collagen and fibronectin adhesins. Collectively, the results support the hypothesis that A. actinomycetemcomitans host colonization involves afimbrial adhesins for extracellular matrix proteins, and that the expression of adhesion is modulated by global regulatory mechanisms.