Sequence analysis of the cloned streptococcal cell surface antigen I/II

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Abstract
The gene spa P (formerly designated as spa P1) encoding the M r 185,000 surface antigen (I/II) of Streptococcus mutons, serotype c (NG5), has been sequenced. The gene (4683 bp) encodes a protein of 1561 amino acid residues including putative signal peptide (residues 1–38) and transmembrane (residues 1537–1556) sequences. The N‐terminal region (60–550) has alanine‐rich repeats and is predicted to be α‐helical. However, the C‐ternunal region (800–1540) is proline‐rich and favours an extended structure. Except for a short central variable region the sequences appear to be highly conserved for S. mutans serotype c. N‐Tenninal sequencing of separated antigen I and antigen II polypeptides suggests that the former represents the N‐terminal and the latter the C‐terminal portions of the intact antigen.