Mouse SNAP Associated Protein (Snapap)

FUNCTION

Ontology

(1) Process: synaptic vesicle exocytosis.
(2) Component of synaptosome.
(3) SNARE binding.
(4) May play a role in intracellular vesicle trafficking.
(2) Protein interaction in BLOC-1.

Location

May be cytoplasmic and peripheral membrane bound or anchored to the vesicular membrane through an N- terminal signal anchor. Ilardi, et al found that snapin is exclusively associated with the snaptic vesicle fraction and absent from the cytosolic and plasma membrane fractions. They concluded snapin behaves as an integral, vesicle-associated, membrane protein. Buxton, et al reported that 70% of total cellular snapin is present in the cytosol.

Interaction

Snapin is a subunit of the biogenesis of lysosome-related organelles complex 1 (BLOC-1), which contains the products of seven other HPS genes, sdy, mu, pa, cno, rp, Blos1, Blos2 (Ciciotte, et al; Falcon-Perez , et al; Li, et al; Moriyama, et al; Starcevic, et al). It interacts with dysbindin, cno, Blos1, and Blos2 within the complex ( Starcevic, et al) (view diagram of BLOC-1 complex here). The same 69-residue region of dysbindin that is sufficient for dystrobrevin binding in vitro also contains the binding sites for pallidin and snapin, and at least part of the muted-binding interface (Nazarian, et al). Feng, et al further narrowed down the interaction domain to dysbindin 90-119 peptide. Snapin is a binding partner of dysbindin-1 in vitro and in the brain. Tissue fractionation of whole mouse brains and human hippocampal formations revealed that both dysbindin-1 and snapin are concentrated in tissue enriched in synaptic vesicle membranes and less commonly in postsynaptic densities. It is not detected in presynaptic tissue fractions lacking synaptic vesicles (Talbot, et al). Loss of dysbindin reduces 1/4 of snapin in hippocampal tissues which may affect neuronal transmission efficacy that leads to the development of schizophrenic symptoms (Feng, et al).

Snapin may associate with the SNARE complex. The interactions with SNAP23 and SNAP25 are controversial. SNAP23 is a ubiquitously expressed SNAP25-related protein. Ilardi, et al firstly identified the snapin by using SNAP25 as a bait to screen a human brain cDNA library. Snapin bound to GST-SNAP25 at a stoichiometry of 1:1. No binding was detectable to SNAP23, VAMP2, and syntaxin 1A. The C-terminal half of snapin was sufficient for the interaction with SNAP25. Snapin was a SNAP25 binding protein that forms a complex by association with syntaxin, SNAP25 and VAMP2. Snapin was required for high-affinity synaptotagmin-SNARE interation. PKA-phosphorylation of Ser50 increased its interaction with SNAP25 (Chheda, et al). Overexpression of Snapin S50D, a mutant mimicking the phosphorylated state, resulted in a decreased number of readily releasable vesicles (Thakur, et al). Expression of Snapin-C66A, a dimerization-defective mutant with impaired interactions with SNAP-25 and Synaptotagmin, reduces the readily releasable pool size but exhibits less effect on synchronized fusion (Pan, et al).

In vitro and in vivo protein-protein interaction assays showed that snapin interacted with SNAP23 and the C-terminal helical domain of snapin contained the SNAP-23-binding site. Snapin formed a complex with SNAP23 and syntaxin 4, but snapin did not affect the affinity of syntaxin 4 and SNAP23 (Buxton, et al). In pull-down studies, snapin detected AQP2, syntaxin-3, syntaxin-4, and SNAP23 from the inner medullary collecting duct. Snapin plays an important role as a linker between the water channel and the t-SNARE complex, the association of AQP2 with both syntaxin-3 and syntaxin-4 is highly enhanced by the presence of snapin (Mistry, et al (2009)). Vites, et al were unable to detect any specific interaction between snapin and SNAP25 (or SNAP23) by pulldown experiments, co-IP, CD- and fluorescence anisotropy spectroscopy. Recombinant snapin forms a stable dimmer with a predominantly alpha-helical secondary structure. Snapin is required to maintain a proper balance of the late endocytic protein LAMP-1 and late endosomal SNARE proteins syntaxin 8 and Vti1b. Deleting the snapin gene in mice selectively led to the accumulation of these proteins in late endocytic organelles (Lu, et al). Snapin regulates the recruitment of late endosomes to the dynein motor complex for retrograde trafficking along microtubules and maturation of lysosomes (Cai, et al).

Except for the interaction between snapin and SNARE complex, several snapin binding partners have been found.
(1) Hunt, et al identified that snapin interacts with the N-terminus of regulator of G protein signaling 7 (RGS7) by using yeast two-hybrid analysis to screen a human whole brain cDNA library. The interaction is mediated primarily by amino acid residues 1-69 of RGS7 (which contains the proximal portion of the DEP domain).

(2) Snapin interacts with type VI adenylyl cyclase (ACVI) via amino acids 1 approximately 86 of ACVI and 33-51 of Snapin. Phosphorylation of Snapin by PKC or PKA might not be crucial for Snapin action on ACVI (Chou, et al).

(3) Receptor tyrosine kinase MET is a putative interacting protein of snapin (Schaaf, et al).

(4) The estrogen receptor-binding fragment-associated gene9 (EBAG9) gene product was recently identified as a modulator of tumor-associated O-linked glycan expression in nonneuronal cells. EBAG9 interacts with Snapin, which is likely to be a modulator of Synaptotagmin-associated regulated exocytosis. EBAG9 decreased phosphorylation of Snapin, which has a regulatory function in exocytosis (Ruder, et al).

(5) The vanilloid receptor-1 (TRPV1) plays a key role in the perception of peripheral thermal and inflammatory pain. Snapin and synaptotagmin IX (Syt IX), strongly interact in vitro and in vivo with the TRPV1 N-terminal domain. PKC signaling promotes at least in part the SNARE-dependent exocytosis of TRPV1 to the cell surface (Morenilla-Palao, et al).

(6) Using a yeast two-hybrid screen of a rat brain cDNA library with cypin lacking the carboxyl terminal eight amino acids as bait, snapin was identified as a cypin binding partner. The carboxyl-terminal coiled-coil domain (H2) of snapin is required for cypin binding. Snapin competes with tubulin for binding to cypin through cypin's CRMP homology domain, resulting in decreased microtubule assembly, therefore snapin regulates dendrite number in developing neurons (Chen, et al).

(7) Collectrin binds to SNARE complexes by interacting with snapin and facilitated SNARE complex formation. Therefore, collectrin is a regulator of SNARE complex function, which thereby controls insulin exocytosis (Fukui, et al).

(8) The interaction of granulocyte colony-stimulating factor (GCSF-R) and Snapin was found by yeast two-hybrid experiment by screen a mouse liver library. This interactionwas further confirmed by GST pull-down experiment, mammalian two-hybrid experiment and co-immunoprecipitation study. The immuno-fluorescence assay was shown that the two proteins of GCSF-R with Snapin were co-localized in the cytoplasm and plasma membrane. The region of C-terminal GCSF-R between box2 and box3, including the residue Tyr703, was responsible for the interaction with Snapin (Yuan, et al).

(9) Snapin binds to a 170-residue predicted ryanodine receptor (RyR) cytosolic loop (RyR2 residues 4596-4765), containing a hydrophobic segment required for snapin interaction. Deletion analysis indicates that the ryanodine receptor interacts with the snapin C-terminus, the same region as the SNAP25-binding site. Competition experiments with native ryanodine receptor and SNAP25 suggest that these two proteins share an overlapping binding site on snapin. The snapin-RyR1 association appears to sensitise the channel to Ca(2+) activation in [(3)H]ryanodine-binding studies (Zissimopoulos, et al).

(10) TRPM7, a member of the transient receptor potential (TRP) ion channel family, resides in the membrane of synaptic vesicles of sympathetic neurons, forms molecular complexes with the synaptic vesicle proteins synapsin I and synaptotagmin I, and directly interacts with synaptic vesicular snapin. Changes in TRPM7 levels and channel activity alter acetylcholine release, as measured by EPSP amplitudes and decay times in postsynaptic neurons. TRPM7 affects EPSP quantal size, and targeted peptide interference of TRPM7's interaction with snapin affects the amplitudes and kinetics of postsynaptic EPSPs (Krapivinsky, et al).

(11) Wolff, et al identified snapin as an interaction partner of the CK1 isoform delta (CK1delta) in the yeast two-hybrid system and confirmed the interaction by co-immunoprecipitation. Snapin was phosphorylated by CK1delta in vitro. Both proteins localized in close proximity in the perinuclear region, wherein snapin was found to associate with membranes of the Golgi apparatus.

(12) Suzuki, et al identified snapin as a protein interacting with the C terminus of the alpha(1A)-AR (alpha(1A)-adrenoceptor), and found that snapin co-immunoprecipitated with both TRPC6 (transient receptor potential canonical channel 6) and alpha(1A)-AR, and these interactions were augmented upon alpha(1A)-AR activation, increasing the recruitment of TRPC6 to the cell surface. In receptor-expressing PC12 cells, co-transfection of Snapin augmented alpha(1A)-AR-stimulated sustained increases in intracellular Ca(2+) ([Ca(2+)](i)) via ROC channels.

(13) The C-terminal coiled-coil domain (H2) of snapin is required for UT-A1 (urea transporter A1) interaction. Snapin binds to the intracellular loop of UT-A1 but not to the N- or C-terminal fragments. GST-pulldown experiments and co-immunoprecipitation studies verified that snapin interacts with native UT-A1, SNAP23, and syntaxin-4 in rat kidney inner medulla. Confocal analysis shows UT-A1 and snapin co-localised in both the cytoplasm and in the plasma membrane (Mistry, et al (2007)).

(14) The interaction between Exo70 and Snapin is mediated via an N-terminal coil-coil domain in Exo70 and a C-terminal helical region in Snapin. Exo70 competes with SNAP23 for Snapin binding, suggesting that Snapin does not provide a direct link between the exocyst and the SNARE complex but, rather, mediates cross-talk between the two complexes by sequential interactions. Depletion of Snapin in adipocytes using RNA interference inhibits insulin-stimulated glucose uptake, suggesting Snapin interacts with the exocyst and plays a modulatory role in GLUT4 vesicle trafficking (Bao, et al). Snapin mediates incretin action and augments glucose-dependent insulin secretion (Song, et al).

(15) An AAA-ATPase torsinA (TA) is an interacting partner of Snapin. Snapin co-localised with endogenous torsinA on dense core granules in PC12 cells in regulating exocytosis (Granata, et al (2008)). TA binds CSN4 and snapin in neuroblastoma cells and in brain synaptosomes. CSN4 and TA are required for the stability of both snapin and the synaptotagmin-specific endocytic adaptor stonin 2. Snapin is phosphorylated by the CSN-associated kinase protein kinase D (PKD) and its expression is decreased upon PKD inhibition. (Granata, et al (2011))

(16) PUMILIO2 forms a complex with NANOS1 in human male germ cells. In addtion, both interacts with snapin (Ginter-Matuszewska, et al).

(17) Yeast two hybrid screening experiments showed that EHD1 interacts with snapin. EHD1 binds to the C terminus of snapin via its C terminus EH domain. It negatively affects the binding of a SNARE complex protein, SNAP-25, to snapin, probably due to the competition for overlapping binding sites on the C terminus of snapin. EHD1 affects the coupling of synaptotagmin-1 to the SNARE complex, and could be a negative regulator of exocytosis. (Wei, et al).

(18) TorsinA (TA) binds CSN4 and snapin in neuroblastoma cells and in brain synaptosomes. CSN4 and TA are required for the stability of both snapin and the synaptotagmin-specific endocytic adaptor stonin 2. Snapin is phosphorylated by the CSN-associated kinase protein kinase D (PKD) and its expression is decreased upon PKD inhibition. (Granata, et al).

(19) Ca(2+) entry via the alpha(1A)-AR-Snapin-TRPC6-pathway plays an important role in physiological regulation of cardiac contractility. This pathway involves alpha(1A)-AR-activated translocation of Snapin and the transient receptor potential canonical 6 (TRPC6) channel to the plasma membrane (Mohl, et al

Snapin drosophila homolog CG32951 interaction information in CuraGen interaction database.

Pathway

Snapin is a component of the SNARE complex of proteins that is required for synaptic vesicle docking and fusion. It may modulate a step between vesicle priming, fusion and calcium-dependent neurotransmitter release by potentiating the interaction of synaptotagmins with the SNAREs and the plasma- membrane-associated protein SNAP25 (Ilardi, et al). It may also have a role in the mechanisms of SNARE-mediated membrane fusion in non-neuronal cells.

Snapin may be involved in the development of lysosome-related organelles, such as melanosomes and platelet-dense granules (view diagram of BLOC-1 pathway here).