Calcium-dependent interaction of N-type calcium channels with the synaptic core complex

Abstract

Neurotransmitter release is initiated by influx of Ca2+ through voltage-gated Ca2+ channels, within 200 microseconds of the action potential arriving at the synaptic terminal, as the Ca2+ concentration increases from 100 nM to > 200 microM. Exocytosis requires high Ca2+ concentration, with a threshold of 20-50 microM and half-maximal activation at 190 microM. The synaptic membrane proteins syntaxin, 25K synaptosome-associated protein (SNAP25), and vesicle-associated membrane protein (VAMP)/synaptobrevin, are thought to form a synaptic core complex which mediates vesicle docking and membrane fusion. Synaptotagmin may be the low-affinity Ca(2+)-sensor, but other Ca(2+)-sensors are involved as residual neurotransmission persists in synaptotagmin-null mutants. Syntaxin binds to N-type Ca2+ channels at a site in the intracellular loop connecting domains II and III. Here we describe Ca(2+)-dependent interaction of this site with syntaxin and SNAP25 which has a biphasic dependence on Ca2+, with maximal binding at 20 microM free Ca2+, near the threshold for transmitter release. Ca(2+)-dependent interaction of Ca2+ channels with the synaptic core complex may be important for Ca(2+)-dependent docking and fusion of synaptic vesicles.

Old proteins, developing roles: The regulation of calcium channels by synaptic proteins

“…2). The C-terminal also directly interacts with the II-III linker regions of N-type 17,58 and P/Q-type 51 channels, while early evidence points against its association with R-type channels. 22 Three SNAP-25 splice variants exist, the mature, and two developmentally regulated isoforms: SNAP-25a, found in immature terminals, and SNAP-25b, predominant during synaptogenesis.…”

“…2) 21,51,59 and ensures that the synaptic core complex is within 50 nm of calcium entry. 17,60 It may also serve as an auto-inhibitory domain. 61 In addition to anchoring to the synprint site, another region of the H3 domain binds elsewhere on the channel to regulate its biophysical properties.…”

“…15 Calcium may rise by 50-100 μM for a few hundred microseconds before it diffuses away or is sequestered by calcium binding proteins, so the exocytotic machinery needs to be in close apposition to the source of calcium entry, in a small microdomain called the active zone. [16][17][18] Neurotransmitter exocytosis is a highly regulated, multi-step process that may occur through a host of molecular mechanisms. One well studied mechanism is the formation of a soluble NSF attachment receptor (SNARE) complex that tethers a neurotransmitter-laden vesicle to the presynaptic membrane close to the site of calcium influx.…”

Calcium-dependent interactions of the human norepinephrine transporter with syntaxin 1A

“…2). Ca 2+ is known to trigger SNARE complex formation between SYN1A, VAMP, and SNAP-25 for membrane fusion (Chen et al, 1999) and directly influences SYN1A interactions with other presynaptic proteins such as synaptotagmin, syncollin, and Ca 2+ channels (Chapman et al, 1995;Edwardson et al, 1997;Sheng et al, 1996). Our in vitro studies suggest, however, that over a wide range of concentrations spanning physiological dynamics of presynaptic Ca 2+ , the cation does not appear to directly influence NET/SYN1A physical associations (Fig.…”

Molecular biology of neuronal voltage-gated calcium channels

“…P/Q-type VGCCs have also been shown to co-immunoprecipitate with syntaxincontaining SNARE complexes in rat cerebellar synaptosomes (Martin et al, 1996). A peptide motif on N-or P/Q-type VGCC (termed 'synprint' site) that binds to syntaxin in a calcium-dependent manner was found (Sheng et al, 1995). A functional consequence of this molecular coupling was subsequently demonstrated in a study where injection of N-type calcium channel synprint peptide into superior cervical ganglion neurons resulted in a dramatic reduction in fast neurotransmitter release (Mochida et al, 1996).…”