The degranulation two-step

Degranulation of mast cells involves a two-step process, report Nishida et al. on page 115. First, antigen stimulation triggers microtubule polymerization and granule translocation to the cell surface in a calcium-independent process. Second, the granules fuse with the plasma membrane in a well-characterized calcium-dependent process.

Mast cells are so full of granules that degranulation was thought to occur through granule-to-membrane fusion and granule-to-granule fusion, without the need for granule transport. However, inhibition of microtubule polymerization blocked degranulation.

In response to antigen stimulation, Nishida et al. found that tubulin staining increased and fluorescently tagged granules translocated to the cell surface before exocytosis. Removal of calcium from the culture medium prevented granule fusion to the membrane but had no effect on microtubule polymerization or granule movement, suggesting that the steps are distinct.

When the team...