The C2B domain of synaptotagmin I is a Ca2+-binding module

Area of Science:

• Neuroscience
• Molecular Biology
• Biochemistry

Background:

• Synaptotagmin I is a key Ca(2+) sensor in neurotransmitter release.
• The Ca(2+) binding properties of its C(2)B domain are not fully understood.
• Previous studies suggested Ca(2+)-dependent C(2)B domain multimerization in fusion pore formation.

Purpose of the Study:

• To clarify the Ca(2+)-binding and dimerization behavior of the synaptotagmin I C(2)B domain.
• To investigate potential contaminants affecting previous studies.
• To determine the role of the C(2)B domain in synaptotagmin I multimerization.

Main Methods:

• Biophysical techniques, including NMR spectroscopy.
• Expression and purification of recombinant C(2)B domain as a GST fusion protein.
• Analysis of native synaptotagmin I fragments from brain homogenates.

Main Results:

• The pure recombinant C(2)B domain binds Ca(2+) but remains monomeric.
• Bacterial contaminants, binding to a polybasic region, were identified in impure preparations.
• Native synaptotagmin I fragments form Ca(2+)-dependent high molecular weight complexes.

Conclusions:

• The recombinant C(2)B domain is a monomeric, autonomously folded Ca(2+)-binding module.
• Direct C(2)B domain interactions are unlikely to mediate synaptotagmin I multimerization in fusion pore formation.
• Synaptotagmin I multimerization may involve other domains or post-translational modifications.