J Neurochem. modulation of presynaptic neurotransmitter release and/or postsynaptic receptor business and requires the intricate regulation of membrane trafficking events. Crucial actions in Simeprevir the dynamics of the plasmalemma are the addition of new Simeprevir membrane through the process of vesicle fusion and the retrieval of membrane by endocytosis. Several genes or gene families have been implicated in the membrane fusion process and are therefore likely targets for controlling the dynamics of the neuronal plasma membrane. The products of three of these genes, VAMP (also called synaptobrevin), SNAP-25, and syntaxin, are collectively referred to as SNAREs (Sdhof, 1995). These proteins form a core fusion complex that is composed of a four-helical bundle spanning the vesicle and target membranes (Poirier et al., 1998a; Sutton et al., 1998). Formation of this complex is a late step in the membrane fusion process and perhaps actually drives fusion of the lipid bilayers (Hanson et al., 1997; Lin and Scheller, 1997). Given the central role of membrane fusion in the functions of various membrane compartments, it is critical to understand how the SNAREs may be regulated. In particular, insight into the regulation of neuronal SNAREs will most likely be crucial in understanding mechanisms of synaptic development and plasticity. The three neuronal SNAREs, VAMP2, syntaxin1, and SNAP-25, have been shown to be phosphorylated by different kinases: VAMP2 by Ca2+- and calmodulin-dependent protein kinase II (Hirling and Scheller, 1996) and casein kinase II (CKII) (Nielander et al., 1995), syntaxin 1 by casein kinase II (Bennett et al., 1993b; Hirling and Scheller, 1996; Risinger and Bennett, 1999), and SNAP-25 by protein kinase A (Risinger and Bennett, 1999) and protein kinase C (Shimazaki et al., 1996). The occurrence, functional significance, cell or developmental specificity, and the stimuli that control the phosphorylation of these SNAREs remain largely unknown. In this report we investigate the phosphorylation on serine-14 of syntaxin 1 by casein kinase II using phosphosyntaxin-specific antibodies. We show Simeprevir that this phosphorylation occurs and that phosphosyntaxin levels increase during development. Specific domains along subsets of axons are marked by phosphosyntaxin, and immunoprecipitation experiments show that phosphorylated syntaxin is usually enriched in complexes with SNAP-25. Although the phosphosyntaxin occurs in regions of brain that are actively undergoing synaptogenesis, the labeled axonal domains do not correspond to synaptic sites. The data suggest a role for casein kinase II and phosphosyntaxin 1 in defining specific subdomains in the axonal plasma membrane that are segregated from the synaptic active zones. These subdomains are likely enriched in the binary SNAP-25/syntaxin complex and therefore may be primed for the exocytosis of a novel class of vesicles. MATERIALS AND METHODS The mouse monoclonal antibodies used in this study were anti-MAP2 (Transduction Laboratories, Lexington, KY), anti-tau and anti-synaptophysin (Boehringer Mannheim, Indianapolis, IN), anti-HPC-1 (described in Barnstable et al., 1985), anti-calbindin (Swant, Bellinzona, Mouse monoclonal to BLNK Switzerland), and anti-SNAP-25 (Sternberger Monoclonals, Lutherville, MD). The affinity-purified polyclonal anti-VAMP2 antibody was described previously (Pevsner et al., 1994). The nuclear marker TOTO-3 was purchased from Molecular Probes (Eugene, OR). Secondary antibodies for immunohistochemistry were from Jackson ImmunoResearch Laboratories (West Grove, PA) and included fluorescein isothiocyanate (FITC)-conjugated AffiniPure Simeprevir goat anti-rabbit IgG and Texas Red (TxR)-conjugated AffiniPure goat anti-mouse IgG. Secondary antibodies for quantitative Western Blot analysis were bought from Amersham Pharmacia Biotech (Arlington, IL) and included anti-rabbit Ig from donkey, 125I-labeled F(ab)2 fragment and anti-mouse Ig from sheep,125I-labeled F(ab)2fragment. Paraformaldehyde was purchased from Electron Microscopy Sciences (Fort Washington, PA), and casein kinase II (human, recombinant from A peptide corresponding to amino acids 9C19 of syntaxin 1A (RTAKDSDDDDD) (Bennett et al., 1992) was synthesized with a phosphoserine at position 14 and an additional cysteine residue at the C terminus (introduced for coupling purposes). The peptide was coupled to Imject maleimide-activated keyhole limpet hemocyanin (KLH) (Pierce, Rockford, IL) and used as immunogen in rabbit. The polyclonal antiserum was affinity-purified as follows. A peptide with unrelated sequence, a peptide with the same sequence but with unphosphorylated serine (related.