Fast and slow modes of fusion and retrieval visualized by IRM. (A) Histogram showing the distribution of D1/2 (time for the IRM signal to increase to half maximum) for 311 fusion events. This population includes both synchronous and asynchronous events. The curve is the sum of two Gaussians, one with a mean of 61 ms and an SD of 41 ms and the second with a mean of 180 ms and an SD of 92 ms. The dashed line shows a value of 120 ms, which was used as a cut-off between fast and slow events. (B) The distribution of D1/2 for 29 asynchronous events described by a single Gaussian with a mean of 61 ms and an SD of 41 ms. The dashed line shows the cut-off value between fast and slow events. (C) The distribution of D1/2 for 282 synchronous events described by the same two Gaussians as in A (black dashed line). 130 of these events were defined as fast and were described by the Gaussian fit used in B (red dashed line). The dashed line shows the cut-off value between fast and slow events. (D) Relation between D1/2 and the time for the IRM signal to recover half maximum from the peak (C1/2). Synchronous events in black (n = 282) and asynchronous in red (n = 29). Line shows a linear fit (r = 0.94) through the binned results (solid circles). Events obtained in cells dialyzed with amphiphysinΔSH3 are shown in green. Error bars indicate SEM. Note the narrower distribution of asynchronous events. (E) Relationship between the peak amplitude of the IRM signal and D1/2 for synchronous and asynchronous fusions. Bins (average of 18 events) were well described by a single exponential function with a rate constant of 49.5 s−1 (mean ± SEM).
