Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 104, Issue 40, Pages 15929-15934Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.0705406104
Keywords
chromaffin cells; exocytosis; granule motion; total internal reflection fluorescence microscopy
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Funding
- NINDS NIH HHS [R01 NS38129, R01 NS038129] Funding Source: Medline
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The state of secretory granules immediately before fusion with the plasma membrane is unknown, although the granules are generally assumed to be stably bound (docked). We had previously developed methods using total internal reflection fluorescence microscopy and image analysis to determine the position of chromaffin granules immediately adjacent to the plasma membrane with high precision, often to within approximate to 10 nm, or <5% of the granule diameter (300 nm). These distances are of the dimensions of large proteins and are comparable with the unitary step sizes of molecular motors. Here we demonstrate with quantitative measures of granule travel in the plane parallel to the plasma membrane that secretory granules change position within several hundred milliseconds of nicotinic agonist-induced fusion. Furthermore, just before fusion, granules frequently move to areas that they have rarely visited. The movement of granules to new areas is most evident for granules that fuse later during the stimulus. The movement may increase the probability of productive interactions of the granule with the plasma membrane or may reflect the pull of molecular interactions between the granule and the plasma membrane that are part of the fusion process. Thus, instead of being stably docked before exocytosis, granules undergo molecular-scale motions and travel immediately preceding the fusion event.
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