Physiological regulation of Munc18/nSec1 phosphorylation on serine-313

Increased protein phosphorylation enhances exocytosis in most secretory cell types, including neurones. However, the molecular mechanisms by which this occurs and the specific protein targets remain unclear. Munc18-1/nSec1 is essential for exocytosis in neurones, and is known to be phosphorylated by protein kinase C (PKC) in vitro at Ser-313. This phosphorylation has been shown to decrease its affinity for syntaxin, and to alter the kinetics of exocytosis in chromaffin cells. However, there are no data on the physiological regulation of Ser-313 phosphorylation. Using phospho-Ser-313-specific antisera, we demonstrate here that Ser-313 is phosphorylated in intact and permeabilized chromaffin cells in response to histamine and Ca2+ respectively. Furthermore, Ser-313 is rapidly and transiently phosphorylated in intact synaptosomes in response to depolarization by KCl treatment or by 4-aminopyridine, and by the metabotropic glutamate receptor agonist dihydroxyphenylglycine. PKC was identified as the kinase, and PP1 and PP2B as the phosphatases responsible for regulating Ser-313 phosphorylation. As phosphorylation of nSec1 on Ser-313 affects the rate of transmitter release in chromaffin cells, the demonstration here that this phosphorylation event occurs in neurones suggests that synaptic neurotransmitter release may be similarly regulated by nSec1 phosphorylation. Furthermore, such changes in release kinetics are associated with long-term potentiation and depression, thus implicating nSec1 phosphorylation as a potential regulatory mechanism underlying presynaptic plasticity.     

PKA-catalyzed phosphorylation of tomosyn and its implication in Ca2+-dependent exocytosis of neurotransmitter

Neurotransmitter is released from nerve terminals by Ca2+-dependent exocytosis through many steps. SNARE proteins are key components at the priming and fusion steps, and the priming step is modulated by cAMP-dependent protein kinase (PKA), which causes synaptic plasticity. We show that the SNARE regulatory protein tomosyn is directly phosphorylated by PKA, which reduces its interaction with syntaxin-1 (a component of SNAREs) and enhances the formation of the SNARE complex. Electrophysiological studies using cultured superior cervical ganglion (SCG) neurons revealed that this enhanced formation of the SNARE complex by the PKA-catalyzed phosphorylation of tomosyn increased the fusion-competent readily releasable pool of synaptic vesicles and, thereby, enhanced neurotransmitter release. This mechanism was indeed involved in the facilitation of neurotransmitter release that was induced by a potent biological mediator, the pituitary adenylate cyclase-activating polypeptide, in SCG neurons. We describe the roles and modes of action of PKA and tomosyn in Ca2+-dependent neurotransmitter release.     

Phosphorylation of Munc18 by protein kinase C regulates the kinetics of exocytosis

Protein phosphorylation by protein kinase C (PKC) has been implicated in the control of neurotransmitter release and various forms of synaptic plasticity. The PKC substrates responsible for phosphorylation-dependent changes in regulated exocytosis in vivo have not been identified. Munc18a is essential for neurotransmitter release by exocytosis and can be phosphorylated by PKC in vitro on Ser-306 and Ser-313. We demonstrate that it is phosphorylated on Ser-313 in response to phorbol ester treatment in adrenal chromaffin cells. Mutation of both phosphorylation sites to glutamate reduces its affinity for syntaxin and so acts as a phosphomimetic mutation. Unlike phorbol ester treatment, expression of Munc18 with this phosphomimetic mutation in PKC phosphorylation sites did not affect the number of exocytotic events. The mutant did, however, produce changes in single vesicle release kinetics, assayed by amperometry, which were identical to those caused by phorbol ester treatment. Furthermore, the effects of phorbol ester treatment on release kinetics were occluded in cells expressing phosphomimetic Munc18. These results suggest that the dynamics of vesicle release events during exocytosis are controlled by PKC directly through phosphorylation of Munc18 on Ser-313. Phosphorylation of Munc18 by PKC may provide a mechanism for the control of exocytosis and thereby synaptic plasticity.     

Possible implication of the Golgi apparatus casein kinase in the phosphorylation of vesicle docking protein p115 Ser-940: a study with peptide substrates

Phosphorylation of human vescicle docking protein p115 at Ser-942 (homologous to Ser-940 in rat p115) promotes its dissociation from the Golgi membrane. Here we show that a peptide encompassing the 934--950 sequence of p115 is unaffected or poorly phosphorylated by a variety of Ser/Thr protein kinases with the notable exception of the Golgi apparatus casein kinase (G-CK) which phosphorylates it with an efficiency comparable to that of its optimal peptide substrates. In contrast phosphorylation of the p115 peptide by protein kinase CK2 is negligible compared to that of the specific peptide substrates of this kinase. Phosphorylation by G-CK is abolished if a conserved cluster of acidic residues at position between n + 4 and n + 9 (EDDDDE) is replaced by a neutral stretch (GAGAGA). These data strongly support the view that G-CK but not the other two classes of ubiquitous "casein kinases" (CK1 and CK2) is the natural phosphorylating agent of p115.     

Synaptotagmin-12, a synaptic vesicle phosphoprotein that modulates spontaneous neurotransmitter release

Central synapses exhibit spontaneous neurotransmitter release that is selectively regulated by cAMP-dependent protein kinase A (PKA). We now show that synaptic vesicles contain synaptotagmin-12, a synaptotagmin isoform that differs from classical synaptotagmins in that it does not bind Ca(2+). In synaptic vesicles, synaptotagmin-12 forms a complex with synaptotagmin-1 that prevents synaptotagmin-1 from interacting with SNARE complexes. We demonstrate that synaptotagmin-12 is phosphorylated by cAMP-dependent PKA on serine(97), and show that expression of synaptotagmin-12 in neurons increases spontaneous neurotransmitter release by approximately threefold, but has no effect on evoked release. Replacing serine(97) by alanine abolishes synaptotagmin-12 phosphorylation and blocks its effect on spontaneous release. Our data suggest that spontaneous synaptic-vesicle exocytosis is selectively modulated by a Ca(2+)-independent synaptotagmin isoform, synaptotagmin-12, which is controlled by cAMP-dependent phosphorylation.     

Cooperative phosphorylation of the tumor suppressor phosphatase and tensin homologue (PTEN) by casein kinases and glycogen synthase kinase 3beta

The phosphatase and tensin homologue (PTEN) tumor suppressor is a phosphatidylinositol D3-phosphatase that counteracts the effects of phosphatidylinositol 3-kinase and negatively regulates cell growth and survival. PTEN is itself regulated by phosphorylation on multiple serine and threonine residues in its C terminus. Previous work has implicated casein kinase 2 (CK2) as the kinase responsible for this phosphorylation. Here we showed that CK2 does not phosphorylate all sites in PTEN and that glycogen synthase kinase 3beta (GSK3beta) also participates in PTEN phosphorylation. Although CK2 mainly phosphorylated PTEN at Ser-370 and Ser-385, GSK3beta phosphorylated Ser-362 and Thr-366. More importantly, prior phosphorylation of PTEN at Ser-370 by CK2 strongly increased its phosphorylation at Thr-366 by GSK3beta, suggesting that the two may synergize. Using RNA interference, we showed that GSK3 phosphorylates PTEN in intact cells. Finally, PTEN phosphorylation was affected by insulin-like growth factor in intact cells. We concluded that multiple kinases, including CK2 and GSK3beta, participate in PTEN phosphorylation and that GSK3beta may provide feedback regulation of PTEN.     

Casein kinase 1-dependent phosphorylation of familial advanced sleep phase syndrome-associated residues controls PERIOD 2 stability

The mammalian circadian clock component PERIOD2 (PER2) plays a critical role in circadian rhythm entrainment. Recently, a missense mutation at a putative phosphorylation site in hPER2, Ser-662, was identified in patients that suffer from familial advanced sleep phase syndrome (FASPS). Patients with FASPS display abnormal sleep-wake patterns characterized by a lifelong pattern of sleep onset in the early evening and offset in the early morning. Although the phosphorylation of PER2 is strongly implied from functional studies, it has not been possible to study the site-specific phosphorylation of PER2 on Ser-662, and the biochemical functions of this residue are unclear. Here, we used phospho-specific antibodies to show that PER2 is phosphorylated on Ser-662 and flanking casein kinase (CK) sites in vivo. The phosphorylation of PER2 was carried out by the combined activities of casein kinase 1δ (CK1 δ) and casein kinase 1ε (CK1ε) and was antagonized by protein phosphatase 1. PER2 phosphorylation was rapidly induced in response to circadian entrainment of mammalian cell lines and occurred in both cytosolic and nuclear compartments. Importantly, we found that the pool of Ser-662-phosphorylated PER2 proteins was more stable than the pool of total PER2 molecules, implying that the FASPS phosphorylation cluster antagonizes PER2 degradation. Consistent with this idea, a Ser-662→Ala mutation that abrogated PER2 phosphorylation significantly reduced its half-life, whereas a phosphomimetic Ser-662→Asp substitution led to an elevation in half-life. Our combined findings provide new insights into PER2 regulation and the biochemical basis of FASPS.     

Phosphorylation of the N-ethylmaleimide-sensitive factor is associated with depolarization-dependent neurotransmitter release from synaptosomes

Critical to SNARE protein function in neurotransmission are the accessory proteins, soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP), and NSF, that play a role in activation of the SNAREs for membrane fusion. In this report, we demonstrate the depolarization-induced, calcium-dependent phosphorylation of NSF in rat synaptosomes. Phosphorylation of NSF is coincident with neurotransmitter release and requires an influx of external calcium. Phosphoamino acid analysis of the radiolabeled NSF indicates a role for a serine/threonine-specific kinase. Synaptosomal phosphorylation of NSF is stimulated by phorbol esters and is inhibited by staurosporine, chelerythrine, bisindolylmaleimide I, calphostin C, and Ro31-8220 but not the calmodulin kinase II inhibitor, Kn-93, suggesting a role for protein kinase C (PKC). Indeed, NSF is phosphorylated by PKC in vitro at Ser-237 of the catalytic D1 domain. Mutation of this residue to glutamic acid or to alanine eliminates in vitro phosphorylation. Molecular modeling studies suggest that Ser-237 is adjacent to an inter-subunit interface at a position where its phosphorylation could affect NSF activity. Consistently, mutation of Ser-237 to Glu, to mimic phosphorylation, results in a hexameric form of NSF that does not bind to SNAP-SNARE complexes, whereas the S237A mutant does form complex. These data suggest a negative regulatory role for PKC phosphorylation of NSF.     

CAPS Activity in Priming Vesicle Exocytosis Requires CK2 Phosphorylation

CAPS (Ca²⁺-dependent activator protein for secretion) functions in priming Ca²⁺-dependent vesicle exocytosis, but the regulation of CAPS activity has not been characterized. Here we show that phosphorylation by protein kinase CK2 is required for CAPS activity. Dephosphorylation eliminated CAPS activity in reconstituting Ca²⁺-dependent vesicle exocytosis in permeable and intact PC12 cells. Ser-5, -6, and -7 and Ser-1281 were identified by mass spectrometry as the major phosphorylation sites in the 1289 residue protein. Ser-5, -6, and -7 but not Ser-1281 to Ala substitutions abolished CAPS activity. Protein kinase CK2 phosphorylated CAPS in vitro at these sites and restored the activity of dephosphorylated CAPS. CK2 is the likely in vivo CAPS protein kinase based on inhibition of phosphorylation by tetrabromo-2-benzotriazole in PC12 cells and by the identity of in vivo and in vitro phosphorylation sites. CAPS phosphorylation by CK2 was constitutive, but the elevation of Ca²⁺ in synaptosomes increased CAPS Ser-5 and -6 dephosphorylation, which terminates CAPS activity. These results identify a functionally important N-terminal phosphorylation site that regulates CAPS activity in priming vesicle exocytosis.Regulated neurotransmitter secretion is central to intercellular communication in the nervous system. Two types of secretory vesicles mediate neurotransmitter release; that is, synaptic vesicles that release transmitters such as glutamate at synapses and dense-core vesicles that release modulatory transmitters and neuropeptides at non-synaptic sites. Both types of secretory vesicles are recruited to docking sites on the plasma membrane where they are primed to a ready release state to undergo fusion in response to Ca²⁺ elevations. Many of the proteins that mediate the targeting, docking, priming, and Ca²⁺-dependent fusion of vesicles with the plasma membrane function in both synaptic vesicle and dense-core vesicle pathways (1). CAPS-1² (also known as Cadps1) is a 1289-residue protein that reconstitutes Ca²⁺-triggered dense-core vesicle exocytosis in permeable neuroendocrine cells at a priming step (2–4). CAPS is required for secretion of a subset of transmitters in Caenorhabditis elegans (5) and Drosophila melanogaster (6) and for priming dense-core vesicle exocytosis in neuroendocrine cells (7) and synaptic vesicle exocytosis in neurons (8). Vesicle priming reactions are extensively modulated during physiological demand (9), but mechanisms that regulate CAPS function remain to be identified.Reversible protein phosphorylation is a major mechanism for the regulation of cellular processes including vesicle exocytosis. Many proteins that function in evoked vesicle exocytosis are phosphoproteins (10, 11). The neuronal SNARE proteins syntaxin 1A, VAMP-2, and SNAP-25 are phosphorylated by several protein kinases in vitro (12–14). Protein kinase C and protein kinase A sites on SNAP-25 affect refilling rates and size, respectively, of the primed pool of vesicles in chromaffin cells (15, 16). Several SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor)-binding proteins such as munc18, RIM1, and rabphilin undergo regulated phosphorylation, but it is not known whether phosphorylation affects function (10, 11, 17).Because the function of CAPS at a priming step in vesicle exocytosis may be regulated, we determined whether CAPS is phosphorylated. We show that CAPS is a phosphoprotein with functionally essential N-terminal phosphorylated Ser residues. Ser-5, -6, and -7 in CAPS were substrates for protein kinase CK2 in vitro and in vivo as well as for a Ca²⁺-dependent dephosphorylation mechanism. The results indicate that phosphorylation by protein kinase CK2 is necessary for CAPS activity in priming vesicle exocytosis and that regulated dephosphorylation may constitute a mechanism for terminating CAPS activity.     

Serine 727 phosphorylation and activation of cytosolic phospholipase A2 by MNK1-related protein kinases

We have previously reported that in thrombin-stimulated human platelets, cytosolic phospholipase A(2) (cPLA2) is phosphorylated on Ser-505 by p38 protein kinase and on Ser-727 by an unknown kinase. Pharmacological inhibition of p38 leads to inhibition of cPLA2 phosphorylation at both Ser-505 and Ser-727 suggesting that the kinase responsible for phosphorylation on Ser-727 is activated in a p38-dependent pathway. By using Chinese hamster ovary, HeLa, and HEK293 cells stably transfected with wild type and phosphorylation site mutant forms of cPLA2, we show that phosphorylation of cPLA2 at both Ser-505 and Ser-727 and elevation of Ca(2+) leads to its activation in agonist-stimulated cells. The p38-activated protein kinases MNK1, MSK1, and PRAK1 phosphorylate cPLA2 in vitro uniquely on Ser-727 as shown by mass spectrometry. Furthermore, MNK1 and PRAK1, but not MSK1, is present in platelets and undergo modest activation in response to thrombin. Expression of a dominant negative form of MNK1 in HEK293 cells leads to significant inhibition of cPLA2-mediated arachidonate release. The results suggest that MNK1 or a closely related kinase is responsible for in vivo phosphorylation of cPLA2 on Ser-727.     

Phosphorylation by CK2 and MAPK enhances calnexin association with ribosomes.

Calnexin was initially identified as an endoplasmic reticulum (ER) type I integral membrane protein, phosphorylated on its cytosolic domain by ER-associated protein kinases. Although the role of the ER luminal domain of calnexin has been established as a constituent of the molecular chaperone machinery of the ER, less is known about the role of the cytosolic phosphorylation of calnexin. Analysis by two-dimensional phosphopeptide maps revealed that calnexin was in vitro phosphorylated in isolated microsomes by casein kinase 2 (CK2) and extracellular-signal regulated kinase-1 (ERK-1) at sites corresponding to those for in vivo phosphorylation. In canine pancreatic microsomes, synergistic phosphorylation by CK2 and ERK-1 led to increased association of calnexin with membrane-bound ribosomes. In vivo, calnexin-associated ERK-1 activity was identified by co-immunoprecipitation. This activity was abolished in cells expressing a dominant-negative MEK-1. Activation of ERK-1 in cells by addition of serum led to a 4-fold increase in ribosome-associated calnexin over unstimulated cells. Taken together with studies revealing calnexin association with CK2 and ERK-1, a model is proposed whereby phosphorylation of calnexin leads to a potential increase in glycoprotein folding close to the translocon.     

Regulation of intra-S phase checkpoint by ionizing radiation (IR)-dependent and IR-independent phosphorylation of SMC3

Structure maintenance of chromosome 1 (SMC1) is phosphorylated by ataxia telangiectasia-mutated (ATM) in response to ionizing radiation (IR) to activate intra-S phase checkpoint. A role of CK2 in DNA damage response has been implicated in many previous works, but the molecular mechanism for its activation is not clear. In the present work, we report that SMC3 is phosphorylated at Ser-1067 and Ser-1083 in vivo. Ser-1083 phosphorylation is IR-inducible, depends on ATM and Nijmegen breakage syndrome 1 (NBS1), and is required for intra-S phase checkpoint. Interestingly, Ser-1067 phosphorylation is constitutive and is not induced by IR but also affects intra-S phase checkpoint. Phosphorylation of Ser-1083 is weakened in cells expressing S1067A mutant, suggesting interplay between Ser-1067 and Ser-1083 phosphorylation in DNA damage response. Consistently, small interfering RNA knockdown of CK2 leads to attenuated phosphorylation of Ser-1067 as well as intra-S phase checkpoint defect. Our data provide evidence that phosphorylation of a core cohesin subunit SMC3 by ATM plays an important role in DNA damage response and suggest that a constitutive phosphorylation by CK2 may affect intra-S phase checkpoint by modulating SMC3 phosphorylation by ATM.     

Determination of the phosphorylation sites of calmodulin catalyzed by casein kinase 2

Calmodulin is specifically phosphorylated by casein kinase 2 (CK 2), but not by casein kinase 1, A kinase, or C kinase. In the present report, the stoichiometry of the phosphorylation of calmodulin by CK 2 in the presence and absence of polylysine and its phosphorylation sites were examined. In the absence of polylysine, the radioactive phosphate incorporated into calmodulin by CK 2 was only 0.01 mol/mol and the phosphorylation occurred at Ser-101. In the presence of polylysine, 1.2 mol of radioactive phosphate was incorporated into 1 mol of calmodulin. In this case, Thr-79 in addition to Ser-101 was phosphorylated, but Ser-81 was not. The sequence around the phosphorylated Thr is Asp-Thr(P)-Asp-Ser-Glu-Glu-Glu-.     

Unique features of HIV-1 Rev protein phosphorylation by protein kinase CK2 ('casein kinase-2')

The HIV-1 Rev transactivator is phosphorylated in vitro by protein kinase CK2 at two residues, Ser-5 and Ser-8; these sites are also phosphorylated in vivo. Here we show that the mechanism by which CK2 phosphorylates Rev is unique in several respects, notably: (i) it is fully dependent on the regulatory, beta-subunit of CK2; (ii) it relies on the integrity of an acidic stretch of CK2 beta which down-regulates the phosphorylation of other substrates; (iii) it is inhibited in a dose-dependent manner by polyamines and other polycationic effectors that normally stimulate CK2 activity. In contrast, a peptide corresponding to the amino-terminal 26 amino acids of Rev, including the phosphoacceptor site, is readily phosphorylated by the catalytic subunit of CK2 even in the absence of the beta-subunit. These data, in conjunction with the observation that two functionally inactive derivatives of Rev with mutations in its helix-loop-helix motif are refractory to phosphorylation, indicate the phosphorylation of Rev by CK2 relies on conformational features of distinct regions that are also required for the transactivator's biological activity.     

Regulation of ribosomal protein S6 phosphorylation by casein kinase 1 and protein phosphatase 1

Ribosomal protein S6 (rpS6) is a critical component of the 40 S ribosomal subunit that mediates translation initiation at the 5'-m(7)GpppG cap of mRNA. In response to mitogenic stimuli, rpS6 undergoes ordered C-terminal phosphorylation by p70 S6 kinases and p90 ribosomal S6 kinases on four conserved Ser residues (Ser-235, Ser-236, Ser-240, and Ser-244) whose modification potentiates rpS6 cap binding activity. A fifth site, Ser-247, is also known to be phosphorylated, but its function and regulation are not well characterized. In this study, we employed phospho-specific antibodies to show that Ser-247 is a target of the casein kinase 1 (CK1) family of protein kinases. CK1-dependent phosphorylation of Ser-247 was induced by mitogenic stimuli and required prior phosphorylation of upstream S6 kinase/ribosomal S6 kinase residues. CK1-mediated phosphorylation of Ser-247 also enhanced the phosphorylation of upstream sites, which implies that bidirectional synergy between C-terminal phospho-residues is required to sustain rpS6 phosphorylation. Consistent with this idea, CK1-dependent phosphorylation of rpS6 promotes its association with the mRNA cap-binding complex in vitro. Additionally, we show that protein phosphatase 1 (PP1) antagonizes rpS6 C terminus phosphorylation and cap binding in intact cells. These findings further our understanding of rpS6 phospho-regulation and define a direct link between CK1 and translation initiation.