Sequencing and characterization of the 185 kDa cell surface antigen of Streptococcus mutans

The gene spa P (formerly designated as spa P1) encoding the M_r 185,000 surface antigen I/II of Streptococcus mutans, serotype c (strain NG5) has been sequenced. The deduced amino acid sequence of antigen I/II (1561 residues) includes a putative signal peptide (residues 1-38), as well as a transmembrane region (residues 1537-1556). The N-terminal part of the protein (residues 39-550) is particularly rich in alanine and includes three tandem repeats of a sequence of 82 residues. This region is predicted to be α-helical, adopting a coiled-coil formation, and may account for the cell surface hydrophobicity associated with expression of antigen I/II. In contrast the C-terminal region (residues 800-1549) is proline-rich, favouring an extended conformation. Comparison with the sequence determined from Strep. mutans strain MT8148 showed that antigen I/II is highly conserved with the exception of a short central region (residues 750-805). N-terminal sequencing of purified antigens I and II components indicated that antigen I extends from the amino-terminus of the intact M_r 185,000 surface antigen while antigen II extends from residue 996.