Streptococcus mutans antigen I/II (AgI/II) protein was one of the first cell wall-anchored adhesins identified in Gram-positive bacteria. It mediates attachment of S. mutans to tooth surfaces and has been a focus for immunization studies against dental caries. The AgI/II family polypeptides recognize salivary glycoproteins, and are also involved in biofilm formation, platelet aggregation, tissue invasion and immune modulation. The genes encoding AgI/II family polypeptides are found among Streptococcus species indigenous to the human mouth, as well as in Streptococcus pyogenes, S. agalactiae and S. suis. Evidence of functionalities for different regions of the AgI/II proteins has emerged. A sequence motif within the C-terminal portion of Streptococcus gordonii SspB (AgI/II) is bound by Porphyromonas gingivalis, thus promoting oral colonization by this anaerobic pathogen. The significance of other epitopes is now clearer following resolution of regional crystal structures. A new picture emerges of the central V (variable) region, predicted to contain a carbohydrate-binding trench, being projected from the cell surface by a stalk formed by an unusual association between an N-terminal α-helix and a C-terminal polyproline helix. This presentation mode might be important in determining functional conformations of other Gram-positive surface proteins that have adhesin domains flanked by α-helical and proline-rich regions.
Ever since dental caries (tooth decay) was first shown to be caused by bacteria, there has been continued interest in developing vaccine or passive immunization protocols for its control or prevention (Lehner et al., 1980). Although dental caries is not fatal, and in developed countries caries is now considered to be largely avoidable through controlled diet and good oral hygiene, there remain significant problems with childhood disease, especially among indigent populations. Consequently, caries is one of the most common worldwide infectious diseases. Therefore, research continues towards employing vaccine formulations comprised of peptide components derived from surface proteins of Streptococcus mutans, a major agent associated with dental caries (Lehner et al., 1975). One of the most promising strategies seems to be delivery of peptides, derived from glucan-binding protein B (GbpB) and antigen I/II (AgI/II) protein, via a mucosal (nasal) route. The GbpB polypeptide binds extracellular glucans, thus promoting co-adhesion of S. mutans cells in the development of dental plaque (Taubman and Nash, 2006). The AgI/II protein (also named P1, SpaP, AgB or PAc) is a major surface protein that functions as an adhesin, attaching S. mutans to the saliva-coated tooth enamel surface (Koga et al., 1990; Kelly et al., 1995). Antibodies against SpaP and GbpB block adherence and co-adhesion, respectively, thus disrupting colonization of the oral cavity by S. mutans (Ma et al., 1990; 1998; Taubman and Nash, 2006).
The terminology AgI/II derives from the identification of two major cell wall antigens I and II in S. mutans by Russell et al. (1980), and the subsequent recognition that AgII was a component of AgI. Following the discovery of AgI/II, it became apparent that genes encoding orthologous proteins were widely dispersed among the streptococci (Jenkinson and Demuth, 1997). The viridans Streptococcus AgI/II adhesins range in composition from 1310 to 1653 amino acid (aa) residues, while the Streptococcus agalactiae AgI/II proteins are smaller (826–932 aa residues) (Tettelin et al., 2005). The widespread distribution of these AgI/II protein genes across the streptococci is perhaps not surprising, given the complex streptococcal communities that exist on surfaces of the oro- and naso-pharynx and within the bacterial soup of saliva. It is interesting, though, that the AgI/II family polypeptide genes have not yet been discovered in Streptococcus pneumoniae, which might be by the fact that S. pneumoniae forms a distinct evolutionary cluster (Kilian et al., 2008).
Wiley , 2010. Vol. 77, no 2, 276-286 p.