Ryutaro Shirakawa

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Munc13-4 is a widely expressed member of the CAPS/Munc13 protein family proposed to function in priming secretory granules for exocytosis. Munc13-4 contains N- and C-terminal C2 domains (C2A and C2B) predicted to bind Ca(2+), but Ca(2+)-dependent regulation of Munc13-4 activity has not been described. The C2 domains bracket a predicted SNARE-binding domain,(More)
Weibel-Palade bodies (WPBs) are endothelial-cell-specific organelles that, upon fusion with the plasma membrane, release cargo molecules that are essential in blood vessel abnormalities, such as thrombosis and inflammation, as well as in angiogenesis. Despite the importance of WPBs, the basic mechanisms that mediate their secretion are only poorly(More)
Upon activation, platelets release many active substances stored in alpha- and dense-core granules. However, the molecular mechanisms governing regulated exocytosis are not yet fully understood. Here, we have established an assay system using permeabilized platelets to analyze the Ca(2+)-induced exocytosis of both types of granules, focusing on RabGTPases.(More)
The small GTPases RalA and RalB are multifunctional proteins regulating a variety of cellular processes. Like other GTPases, the activity of Ral is regulated by the opposing effects of guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Although several RalGEFs have been identified and characterized, the molecular identity of(More)
Platelets play critical roles in hemostasis and thrombosis through their aggregation following activation of integrin alphaIIbbeta3. However, the molecular mechanism of the integrin activation inside platelets remains largely unknown. Pharmacological experiments have demonstrated that protein kinase C (PKC) plays an important role in platelet aggregation.(More)
Upon activation, platelets release many active substances. Here, we have analyzed the mechanism governing Ca(2+)-induced secretion of von Willebrand factor stored in alpha-granules and 5-hydroxytryptamine in dense-core granules in permeabilized human platelets. Both secretions were dependent on ATP and cytosol. An essential factor for both granule(More)
Mutations of the perforin (PRF1) and MUNC13-4 genes distinguish 2 forms of familial hemophagocytic lymphohistiocytosis (FHL2 and FHL3, respectively), but the clinical and biologic correlates of these genotypes remain in question. We studied the presenting features and cytotoxic T lymphocyte/natural killer (CTL/NK) cell functions of 35 patients for their(More)
Platelets store self-agonists such as ADP and serotonin in dense core granules. Although exocytosis of these granules is crucial for hemostasis and thrombosis, the underlying mechanism is not fully understood. Here, we show that incubation of permeabilized platelets with unprenylated active mutant Rab27A-Q78L, wild type Rab27A, and Rab27B inhibited the(More)
The diaphanous-related formins are actin nucleating and elongating factors. They are kept in an inactive state by an intramolecular interaction between the diaphanous inhibitory domain (DID) and the diaphanous-autoregulatory domain (DAD). It is considered that the dissociation of this autoinhibitory interaction upon binding of GTP-bound Rho to the GTPase(More)
Non-hydrolyzable GTP analogues, such as guanosine 5'-(beta, gamma-imido)triphosphate (GppNHp), induce granule secretion from permeabilized platelets in the absence of increased intracellular Ca(2+). Here, we show that the GppNHp-induced dense granule secretion from permeabilized platelets occurred concomitantly with the activation of small GTPase Ral. This(More)