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FEBS Lett. 2002 Apr 10;516(1-3):93-6.

NMR measurement of the off rate from the first calcium-binding site of the synaptotagmin I C2A domain.

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Departament de Química Orgànica, Universitat de Barcelona, Martí i Franquès 1-11, 08028, Barcelona, Spain.


The off rate from the first calcium-binding site of the C2A domain of synaptotagmin I, a putative calcium receptor in neurotransmitter release, has been determined by 15N-nuclear magnetic resonance relaxation dispersion measurements. The exchange rate was obtained by fitting the dependence of the transverse relaxation rates on the interval between 180 degrees pulses in relaxation-compensated CPMG experiments at 3.2 microM calcium concentration. The measured k(ex) is 2.0x10(3) x s(-1). The calcium on rate of 3.5+/-1x10(7) x s(-1), determined from the measured off rate and the dissociation constant (5.3x10(-5) M), is close to the diffusion limit. These results are consistent with the proposed role of synaptotagmin I as a calcium sensor in release, but suggest that additional factors may help to accelerate the diffusion of Ca2+ to the sensor.

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