Secretory Vesicle Priming by CAPS Is Independent of Its SNARE-Binding MUN Domain

Abstract

Priming of secretory vesicles is a prerequisite for their Ca(2+)-dependent fusion with the plasma membrane. The key vesicle priming proteins, Munc13s and CAPSs, are thought to mediate vesicle priming by regulating the conformation of the t-SNARE syntaxin, thereby facilitating SNARE complex assembly. Munc13s execute their priming function through their MUN domain. Given that the MUN domain of Ca(2+)-dependent activator protein for secretion (CAPS) also binds syntaxin, it was assumed that CAPSs prime vesicles through the same mechanism as Munc13s. We studied naturally occurring splice variants of CAPS2 in CAPS1/CAPS2-deficient cells and found that CAPS2 primes vesicles independently of its MUN domain. Instead, the pleckstrin homology domain of CAPS2 seemingly is essential for its priming function. Our findings indicate a priming mode for secretory vesicles. This process apparently requires membrane phospholipids, does not involve the binding or direct conformational regulation of syntaxin by MUN domains of CAPSs, and is therefore not redundant with Munc13 action.

CAPS-1 promotes fusion competence of stationary dense-core vesicles in presynaptic terminals of mammalian neurons

“…A recent paper indeed showed that CAPS-2 and Munc13 use different mechanisms to prime vesicles, whereas Munc13-dependent priming requires its MUN domain this domain in CAPS-2 is dispensable for priming. Instead CAPS-2 appears to require its PIP2 binding pleckstrin homology domain (Nguyen Truong et al, 2014). We found that both proteins play important stimulatory roles in DCV release from mammalian neurons.…”

“…CAPS-1 binds to PI 4,5-P2 (PIP2) (Loyet et al, 1998) and localizes to PIP2 clusters in the plasma membrane via its PIP2-binding pleckstrin homology (PH) domain (James et al, 2008). The PH domain is also required to prime secretory vesicles (Kabachinski et al, 2014; Nguyen Truong et al, 2014). Robust Ca2+ influx in our experiments likely activates phospholipase C, which hydrolysis PIP2 and may trigger CAPS-1 dispersion from synapses.…”

“…Robust Ca2+ influx in our experiments likely activates phospholipase C (PLC) which hydrolysis PIP2 and may trigger CAPS1 dispersion from synapses. Recent work on CAPS2 shows that the PH-domain is required to prime vesicles (Nguyen Truong et al, 2014). Calcium-dependent PIP2 hydrolysis may therefore act as negative feedback mechanism reducing CAPS1 availability at the synapse upon robust activity.…”

CAPS1 RNA Editing Promotes Dense Core Vesicle Exocytosis

“…Subsequently, the C-terminal region that encompasses MDH1 (Munc13-homology domain) and DCVBD has been reported to bind to SNARE proteins and to promote SNARE-mediated vesicle fusion (Hammarlund et al, 2008; James et al, 2009; Parsaud et al, 2013). In contrast, a recent study demonstrated that the pleckstrin homology (PH) domain located in the middle of CAPS1, but not the C-terminal region, is essential for the priming of secretory vesicles by CAPS1 (Nguyen Truong et al, 2014). However, the present study demonstrated that a single point mutation in the DCVBD, leading to the exclusive expression of edited CAPS1, induced leanness and hyperactivity in mice.…”

CAPS1 Negatively Regulates Hepatocellular Carcinoma Development through Alteration of Exocytosis-Associated Tumor Microenvironment

“…The protein is a dimer of 145 kDa subunits, exhibits Ca 2+ -dependent interaction with hydrophobic matrix, and binds to phospholipid vesicles, suggesting a membrane-associated function. CAPS1 exists in a variety of species; in vertebrates, CAPS1 has a closely related isoform called CAPS2 [5,6,7]. The CAPS family regulates secretory granule exocytosis, including monoamines and neuropeptides, as well as Golgi trafficking.…”