Various signals elicit exocytosis. The best-studied example is regulated secretion, in which a specific cargo is delivered outside the cell. But regulated exocytosis can occur in nonsecretory cells, begging the question of which organelle in these cells responds to the stimulus. Recently, lysosomes were shown to fuse to the PM in response to damage-induced calcium signals, but they no longer appear to be the only organelle in the arsenal.
By comparing the PM of living cells before and after stimulation, Meldolesi's group has now identified desmoyokin (dA), a marker for regulated exocytosis that appears on the PM in response to increases in intracellular calcium. dA was found in a set of small vesicles within 0.5 μm of the PM that fused rapidly in secretory and nonsecretory cells. The vesicles lacked markers for known organelles, including endosomes, trans-Golgi, and lysosomes, and are thus a distinct class.
The group named the new organelles enlargosomes. “The name we gave them shows what we have in mind [for their function],” says Meldolesi. He thinks the organelles may provide cells with a means to increase PM surface area during wound healing without releasing hydrolytic enzymes. More enlargosomes were found in differentiating cells, which are also expected to be expanding. Enlargosomes may favor a lasting increase in cell size, as they were recycled more slowly than vesicles involved in regulated secretion. ▪