C2-domains are widespread protein modules with diverse Ca2+-regulatory functions. Although multiple Ca2+ ions are known to bind at the tip of several C2-domains, the exact number of Ca2+-binding sites and their functional relevance are unknown. The first C2-domain of synaptotagmin I is believed to play a key role in neurotransmitter release via its Ca2+-dependent interactions with syntaxin and phospholipids. We have studied the Ca2+-binding mode of this C2-domain as a prototypical C2-domain using NMR spectroscopy and site-directed mutagenesis. The C2-domain is an elliptical module composed of a β-sandwich with a long axis of 50 Å. Our results reveal that the C2-domain binds three Ca2+ ions in a tight cluster spanning only 6 Å at the tip of the module. The Ca2+-binding region is formed by two loops whose conformation is stabilized by Ca2+ binding. Binding involves one serine and five aspartate residues that are conserved in numerous C2-domains. All three Ca2+ ions are required for the interactions of the C2-domain with syntaxin and phospholipids. These results support an electrostatic switch model for C2-domain function whereby the β-sheets of the domain provide a fixed scaffold for the Ca2+ binding loops, and whereby interactions with target molecules are triggered by a Ca2+-induced switch in electrostatic potential.
- Ca binding
- Protein NMR
ASJC Scopus subject areas
- Molecular Biology
- Biochemistry, Genetics and Molecular Biology(all)
- Immunology and Microbiology(all)