Table 1.
Results of sedimentation equilibrium studies on synaptotagmin using analytical ultracentrifugation (actual molecular mass of synaptotagmin is 37,187 Da)
| Single ideal species model
| ||
|---|---|---|
| Sample | Molecular mass, Da | Standard deviation of fit |
| apo-syt | 46,922 | 5.09 × 10−3 |
| syt + 110 μM Ca2+ | 44,537 | 5.30 × 10−3 |
| syt + 550 μM Ca2+ | 46,617 | 4.79 × 10−3 |
| syt + 1.1 mM Ca2+ | 49,243 | 4.86 × 10−3 |
| Single non-ideal species model
| ||
|---|---|---|
| Sample | Molecular mass, Da | Standard deviation of fit |
| apo-syt | 36,113 | 3.46 × 10−3 |
| syt + 110 μM Ca2+ | 30,504 | 3.52 × 10−3 |
| syt + 550 μM Ca2+ | 33,897 | 3.04 × 10−3 |
| syt + 1.1 mM Ca2+ | 38,897 | 3.93 × 10−3 |
| Monomer-dimer ideal associative model
| |||
|---|---|---|---|
| Sample | Molecular mass, Da | Kd | Standard deviation of fit |
| apo-syt | 34,009 | >100 mM | 4.09 × 10−3 |
| syt + 110 μM Ca2+ | 31,975 | >100 mM | 4.34 × 10−3 |
| syt + 550 μM Ca2+ | 34,347 | >100 mM | 4.08 × 10−3 |
| syt + 1.1 mM Ca2+ | 33,015 | ≈100 mM | 4.41 × 10−3 |
The best fit to the data is obtained by using a single non-ideal species model. Use of an associative model does not improve the quality of the fit and yields a Kd for synaptotagmin-synaptotagmin interactions (≥100 mM) that is too high to be physiologically relevant.