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Proteins on exocytic vesicles mediate calcium-triggered fusion
1Laboratory of Theoretical and Physical Biology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892.
Summary
Sea urchin egg secretory granules fuse when exposed to calcium ions (Ca2+). This calcium-triggered fusion involves protein components on the granules themselves, suggesting localized fusion machinery.
Area of Science:
- Cell Biology
- Biochemistry
- Exocytosis
Background:
- Intracellular calcium ions (Ca2+) are critical regulators of exocytosis in many cell types.
- Secretory granules store and release molecules via membrane fusion.
Purpose of the Study:
- To investigate the mechanism of calcium-triggered fusion of isolated secretory granules.
- To determine if secretory granules possess intrinsic machinery for calcium-induced fusion.
Main Methods:
- Isolated secretory granules from sea urchin eggs were perfused with calcium ions (Ca2+).
- Fusion was assessed by monitoring membrane dye transfer and granule content mixing using differential interference microscopy.
- A light scattering technique was developed to quantify fusion.
- The effects of protein modifiers (trypsin, N-ethylmaleimide) on fusion were examined.
Main Results:
- Aggregates of secretory granules fused at Ca2+ concentrations ≥ 10 microM.
- Membrane components mixed, and granule contents melded, confirming fusion.
- Trypsin and N-ethylmaleimide inhibited granule-granule fusion, similar to their effects on granule-plasma membrane fusion.
Conclusions:
- Purified secretory granules contain molecular machinery sufficient for Ca2+-triggered fusion.
- Proteinaceous components on the granule surface are likely essential for this fusion process.