Calcium increases the appetite

Calcium increases the appetite eutrophils are gluttonous little cells—they will engulf just about any small particle that comes their way. But eating up an entire bacterium takes significant effort. On page 181, Dewitt and Hallett report that neutrophils need integrins and a Ca 2 ϩ boost to build a mouth that is up to the task. Phagocytosis around a particle occurs in a series of steps that includes pseudo-podial formation of a phagocytic cup and cup closure/retraction. When microbes present complement components, which are the targets for integrin binding, the phagocytic cups close around the bugs more rapidly. The authors found that this speediness N Integrin (yellow) freed by calpain (left) aggregates around the closing phagocytic cup. he need for Ran and GTP in nuclear import reactions depends on the size of the imported cargo, according to new results by Lyman et al. (page 55), who show that large proteins in particular are susceptible to stalling mid-import when the small GTPase Ran is missing. Imported proteins, in association with cargo receptors such as importin ␤ or transportin, pass from one nucleoporin T Large cargo is not imported into the nucleus without GTP (bottom). protein to another within the nuclear pore complex (NPC) on their way to the nucleoplasm. Ran was already known to help cargo detach from its receptor in the nucleus. The new results give Ran a second unexpected function within the center of the channel. This function stems from the ability of RanGTP to bind to cargo receptors. When Ran or GTP was absent from importin ␣ / ␤ and transportin import reactions, or correlated with a sequence of Ca 2 ϩ changes during both cup formation and closure. At cup formation, complement binding to integrin initiated the release of local Ca 2 ϩ stores near the contact site. Local increase caused an unknown PI3K-dependent signal to launch a cell-wide influx of Ca 2 ϩ just before particle enclosure. Although Hallett speculates that a locally activated phospholipid could stimulate Ca 2 ϩ channels as it diffuses around the plasma membrane, he admits that diffusion of the PI3K product, PIP 3 , is probably too slow to account for the rapid Ca 2 ϩ influx. Global Ca 2 ϩ changes effected the rapid engulfment of tagged particles by releasing integrin receptors throughout the cell membrane. These receptors were then free to diffuse to the phagocytic site, where they could …