1TJX: Crystallographic Identification of Ca2+ Coordination Sites in Synaptotagmin I C2B Domain

Synaptotagmin I has two tandem Ca(2+)-binding C(2) domains, which are essential for fast synchronous synaptic transmission in the central nervous system. We have solved four crystal structures of the C(2)B domain, one of them in the cation-free form at 1.50 A resolution, two in the Ca(2+)-bound form at 1.04 A (two bound Ca(2+) ions) and 1.65 A (three bound Ca(2+) ions) resolution and one in the Sr(2+)-bound form at 1.18 A (one bound Sr(2+) ion) resolution. The side chains of four highly conserved aspartic acids (D303, D309, D363, and D365) and two main chain oxygens (M302:O and Y364:O), together with water molecules, are in direct contact with two bound Ca(2+) ions (sites 1 and 2). At higher Ca(2+) concentrations, the side chain of N333 rotates and cooperates with D309 to generate a third Ca(2+) coordination site (site 3). Divalent cation binding sites 1 and 2 in the C(2)B domain were previously identified from NMR NOE patterns and titration studies, supplemented by site-directed mutation analysis. One difference between the crystal and NMR studies involves D371, which is not involved in coordination with any of the identified Ca(2+) sites in the crystal structures, while it is coordinated to Ca(2+) in site 2 in the NMR structure. In the presence of Sr(2+), which is also capable of triggering exocytosis, but with lower efficiency, only one cation binding site (site 1) was occupied in the crystallographic structure.
PDB ID: 1TJXDownload
MMDB ID: 30317
PDB Deposition Date: 2004/6/7
Updated in MMDB: 2007/10
Experimental Method:
x-ray diffraction
Resolution: 1.04  Å
Source Organism:
Similar Structures:
Biological Unit for 1TJX: monomeric; determined by author
Molecular Components in 1TJX
Label Count Molecule
Protein (1 molecule)
Similar to Synaptotagmini/p65(Gene symbol: Syt1)
Molecule annotation
Chemicals (4 molecules)
* Click molecule labels to explore molecular sequence information.

Citing MMDB