Elongated fibrillar structure of a streptococcal adhesin assembled by the high-affinity association of α- and PPII-helices

Academic Article


  • Streptococcus mutans antigen I/II (AgI/II) is a cell surface-localized protein adhesin that interacts with salivary components within the salivary pellicle. AgI/II contributes to virulence and has been studied as an immunological and structural target, but a fundamental understanding of its underlying architecture has been lacking. Here we report a high-resolution (1.8 Å) crystal structure of the A3VP1 fragment of S. mutans AgI/II that demonstrates a unique fibrillar form (155 Å) through the interaction of two noncontiguous regions in the primary sequence. The A 3 repeat of the alanine-rich domain adopts an extended α-helix that intertwines with the P1 repeat polyproline type II (PPII) helix to form a highly extended stalk-like structure heretofore unseen in prokaryotic or eukaryotic protein structures. Velocity sedimentation studies indicate that full-length AgI/II that contains three A/P repeats extends over 50 nanometers in length. Isothermal titration calorimetry revealed that the high-affinity association between the A3 and P1 helices is enthalpically driven. Two distinct binding sites on AgI/II to the host receptor salivary agglutinin (SAG) were identified by surface plasmon resonance (SPR). The current crystal structure reveals that AgI/II family proteins are extended fibrillar structures with the number of alanine-and proline-rich repeats determining their length.
  • Digital Object Identifier (doi)

    Author List

  • Larson MR; Rajashankar KR; Patel MH; Robinette RA; Crowley PJ; Michalek S; Brady LJ; Deivanayagam C
  • Start Page

  • 5983
  • End Page

  • 5988
  • Volume

  • 107
  • Issue

  • 13