Rab27a and Rab27b have recently been recognized to play versatile roles in regulating the exocytosis of secretory granules and lysosome-related organelles by using multiple effector proteins. However, the precise roles of these effector proteins in particular cell types largely remain uncharacterized, except for those in pancreatic ␤ cells and in melanocytes. Here, we showed that one of the Rab27a/b effectors, exophilin4/Slp2-a, is specifically expressed in pancreatic ␣ cells, in contrast to another effector, granuphilin, in ␤ cells. Like granuphilin toward insulin granules, exophilin4 promotes the targeting of glucagon granules to the plasma membrane. Although the interaction of granuphilin with syntaxin-1a is critical for the targeting activity, exophilin4 does this primarily through the affinity of its C2A domain toward the plasma membrane phospholipids phosphatidylserine and phosphatidylinositol-4,5-bisphosphate. Notably, the binding activity to phosphatidylserine is inhibited by a physiological range of the Ca 2؉ concentration attained after secretagogue stimulation, which presents a striking contrast to the Ca 2؉ -stimulatory activity of the C2A domain of synaptotagmin I. Analyses of the mutant suggested that this novel Ca 2؉ -inhibitory phospholipid-binding activity not only mediates docking but also modulates the subsequent fusion of the secretory granules.

A number of studies show that environmental stress conditions increase abscisic acid (ABA) and hydrogen peroxide (H 2 O 2 ) levels in plant cells. Despite this central role of ABA in altering stomatal aperture by regulating guard cell ion transport, little is known concerning the relationship between ABA and H 2 O 2 in signal transduction leading to stomatal movement. Epidermal strip bioassay illustrated that ABA-inhibited stomatal opening and ABA-induced stomatal closure were abolished partly by externally added catalase (CAT) or diphenylene iodonium (DPI), which are a H 2 O 2 scavenger and a NADPH oxidase inhibitor respectively. In contrast, internally added CAT or DPI nearly completely or partly reversed ABA-induced closure in half-stoma. Consistent with these results, whole-cell patch-clamp analysis showed that intracellular application of CAT or DPI partly abolished ABA-inhibited inward K + current across the plasma membrane of guard cells. H 2 O 2 mimicked ABA to inhibit inward K + current, an effect which was reversed by the addition of ascorbic acid (Vc) in patch clamping micropipettes. These results suggested that H 2 O 2 mediated ABA-induced stomatal movement by targeting inward K + channels at plasma membrane.

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