3QE5: Complete Structure Of Streptococcus Mutans Antigen III CARBOXY- Terminus

The Streptococcus mutans antigen I/II (AgI/II) is a cell surface-localized protein that adheres to salivary components and extracellular matrix molecules. Here we report the 2.5 A resolution crystal structure of the complete C-terminal region of AgI/II. The C-terminal region is comprised of three major domains: C(1), C(2), and C(3). Each domain adopts a DE-variant IgG fold, with two beta-sheets whose A and F strands are linked through an intramolecular isopeptide bond. The adherence of the C-terminal AgI/II fragments to the putative tooth surface receptor salivary agglutinin (SAG), as monitored by surface plasmon resonance, indicated that the minimal region of binding was contained within the first and second DE-variant-IgG domains (C(1) and C(2)) of the C terminus. The minimal C-terminal region that could inhibit S. mutans adherence to SAG was also confirmed to be within the C(1) and C(2) domains. Competition experiments demonstrated that the C- and N-terminal regions of AgI/II adhere to distinct sites on SAG. A cleft formed at the intersection between these C(1) and C(2) domains bound glucose molecules from the cryo-protectant solution, revealing a putative binding site for its highly glycosylated receptor SAG. Finally, electron microscopy images confirmed the elongated structure of AgI/II and enabled building a composite tertiary model that encompasses its two distinct binding regions.
PDB ID: 3QE5Download
MMDB ID: 90026
PDB Deposition Date: 2011/1/19
Updated in MMDB: 2011/05
Experimental Method:
x-ray diffraction
Resolution: 2.5  Å
Source Organism:
Similar Structures:
Biological Unit for 3QE5: monomeric; determined by author and by software (PISA)
Molecular Components in 3QE5
Label Count Molecule
Protein (1 molecule)
Major Cell-surface Adhesin PAC
Molecule annotation
Chemicals (9 molecules)
* Click molecule labels to explore molecular sequence information.

Citing MMDB