Regulation of releasable vesicle pool sizes by protein kinase A-dependent phosphorylation of SNAP-25

Protein kinase A (PKA) is a key regulator of neurosecretion, but the molecular targets remain elusive. We combined pharmacological manipulations of kinase and phosphatase activities with mutational studies on the exocytotic machinery driving fusion of catecholamine-containing vesicles from chromaffin cells. We found that constitutive PKA activity was necessary to maintain a large number of vesicles in the release-ready, so-called primed, state, whereas calcineurin (protein phosphatase 2B) activity antagonized this effect. Overexpression of the SNARE protein SNAP-25a mutated in a PKA phosphorylation site (Thr-138) eliminated the effect of PKA inhibitors on the vesicle priming process. Another, unidentified, PKA target regulated the relative size of two different primed vesicle pools that are distinguished by their release kinetics. Overexpression of the SNAP-25b isoform increased the size of both primed vesicle pools by a factor of two, and mutations in the conserved Thr-138 site had similar effects as in the a isoform.     

Modulation of N-type calcium channel activity by G-proteins and protein kinase C

N-type voltage-gated calcium channel activity in rat superior cervical ganglion neurons is modulated by a variety of pathways. Activation of heterotrimeric G-proteins reduces whole-cell current amplitude, whereas phosphorylation by protein kinase C leads to an increase in current amplitude. It has been proposed that these two distinct pathways converge on the channel's pore-forming alpha(1B) subunit, such that the actions of one pathway can preclude those of the other. In this study, we have characterized further the actions of PKC on whole-cell barium currents in neonatal rat superior cervical ganglion neurons. We first examined whether the effects of G-protein-mediated inhibition and phosphorylation by PKC are mutually exclusive. G-proteins were activated by including 0.4 mM GTP or 0.1 mM GTP-gamma-S in the pipette, and PKC was activated by bath application of 500 nM phorbol 12-myristate 13-acetate (PMA). We found that activated PKC was unable to reverse GTP-gamma-S-induced inhibition unless prepulses were applied, indicating that reversal of inhibition by phosphorylation appears to occur only after dissociation of the G-protein from the channel. Once inhibition was relieved, activation of PKC was sufficient to prevent reinhibition of current by G-proteins, indicating that under phosphorylating conditions, channels are resistant to G-protein-mediated modulation. We then examined what effect, if any, phosphorylation by PKC has on N-type barium currents beyond antagonizing G-protein-mediated inhibition. We found that, although G-protein activation significantly affected peak current amplitude, fast inactivation, holding-potential-dependent inactivation, and voltage-dependent activation, when G-protein activation was minimized by dialysis of the cytoplasm with 0.1 mM GDP-beta-S, these parameters were not affected by bath application of PMA. These results indicate that, under our recording conditions, phosphorylation by PKC has no effect on whole-cell N-type currents, other than preventing inhibition by G-proteins.     

Mucolipin 1 channel activity is regulated by protein kinase A-mediated phosphorylation

Mucolipins constitute a family of cation channels with homology with the transient receptor potential family. Mutations in MCOLN1 (mucolipin 1) have been linked to mucolipidosis type IV, a recessive lysosomal storage disease characterized by severe neurological and ophthalmologic abnormalities. At present, little is known about the mechanisms that regulate MCOLN1 activity. In the present paper, we addressed whether MCOLN1 activity is regulated by phosphorylation. We identified two PKA (protein kinase A) consensus motifs in the C-terminal tail of MCOLN1, containing Ser(557) and Ser(559). Ser(557) was the principal phosphorylation site, as mutation of this residue to alanine caused a greater than 75% reduction in the total levels of phosphorylated MCOLN1 C-terminal tail. Activation of PKA with forskolin promoted MCOLN1 phosphorylation, both in vitro and in vivo. In contrast, addition of the PKA inhibitor H89 abolished MCOLN1 phosphorylation. We also found that PKA-mediated phosphorylation regulates MCOLN1 channel activity. Forskolin treatment decreased MCOLN1 channel activity, whereas treatment with H89 increased MCOLN1 channel activity. The stimulatory effect of H89 on MCOLN1 function was not observed when Ser(557) and Ser(559) were mutated to alanine residues, indicating that these two residues are essential for PKA-mediated negative regulation of MCOLN1. This paper presents the first example of regulation of a member of the mucolipin family by phosphorylation.     

Regulation of a rat lung protein tyrosine kinase activity by reversible phosphorylation/dephosphorylation

Incubation of a partially purified protein tyrosine kinase from rat lung with Mg2+ and ATP resulted in about 10-15-fold activation of the enzyme activity as judged by the phosphorylation of poly(Glu:Tyr,4:1), an exogenous substrate. The activation was time dependent and was associated with the phosphorylation of a single protein band of 50 kDa. Phosphoamino acid analysis of the phosphorylated protein indicated that tyrosine was the amino acid being phosphorylated. Upon gel filtration on a Sephacryl S-200 column, the phosphorylated protein co-eluted with protein tyrosine kinase and ATP-binding activities, suggesting that all three activities are part of the same protein. In addition, pretreatment of the partially purified protein tyrosine kinase with alkaline phosphatase inhibited its enzyme activity which could be restored by reincubation with Mg2+ and ATP. These data suggest that a temporal relationship exists between the phosphorylation and the activation states of rat lung protein tyrosine kinase, and that the phospho- and dephospho- forms represent the active and inactive (or less active) forms, respectively, of the enzyme.     

Modulation of the phosphorylation and activity of calcium/calmodulin-dependent protein kinase II by zinc

Calcium/calmodulin-dependent protein kinase II (CaMPK-II) is a key regulatory enzyme in living cells. Modulation of its activity, therefore, could have a major impact on many cellular processes. We found that Zn(2+) has multiple functional effects on CaMPK-II. Zn(2+) generated a Ca(2+)/CaM-independent activity that correlated with the autophosphorylation of Thr(286), inhibited Ca(2+)/CaM binding that correlated with the autophosphorylation of Thr(306), and inhibited CaMPK-II activity at high concentrations that correlated with the autophosphorylation of Ser(279). The relative level of autophosphorylation of these three sites was dependent on the concentration of zinc used. The autophosphorylation of at least these three sites, together with Zn(2+) binding, generated an increased mobility form of CaMPK-II on sodium dodecyl sulfate gels. Overall, autophosphorylation induced by Zn(2+) converts CaMPK-II into a different form than the binding of Ca(2+)/CaM. In certain nerve terminals, where Zn(2+) has been shown to play a neuromodulatory role and is present in high concentrations, Zn(2+) may turn CaMPK-II into a form that would be unable to respond to calcium signals.     

Protein kinase C activity, protein phosphorylation and germinal vesicle breakdown in Spisula oocytes

To test the possible role of protein kinase C (C-kinase) in regulating germinal vesicle breakdown (GVBD) in Spisula oocytes, we studied the effects of phorbol esters and antagonists of C-kinase on GVBD and protein phosphorylation. Responses to these agents were compared to those elicited by fertilization or increased extracellular K+. The tumor-promoting phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), a potent agonist of C-kinase, elicited GVBD with half-maximal stimulation at 20 nM. By contrast, 4 alpha-phorbol-12,13-didecanoate, a phorbol ester which does not stimulate C-kinase, did not trigger GVBD. TPA accelerated GVBD when induced by excess K+, but it did not affect the time course of the process when initiated by fertilization. Three structurally different antagonists of C-kinase (W-7, H-7, and retinol) all blocked GVBD when induced by fertilization or TPA. When oocytes were preincubated with [32P]orthophosphate and then stimulated to undergo GVBD by fertilization, TPA, or 45 mM K+, protein phosphorylation was greatly increased, especially for a polypeptide(s) of about 45 kDa. Phosphorylation increased prior to GVBD. Retinol inhibited phosphorylation in activated eggs. C-kinase activity was demonstrated in oocyte extracts. These results strongly suggest that protein phosphorylation by C-kinase is involved in the pathway that regulates GVBD in Spisula oocytes.     

Protein kinase A-dependent phosphorylation of aquaporin-1

The molecular mechanisms for regulating water balance in many tissues are unknown. Like the kidney, the eye contains multiple water channel proteins (aquaporins) that transport water through membranes, including two (AQP1 and AQP4) in the ciliary body, the site of aqueous humor production. Previous results from our laboratory demonstrated that water channel activity of AQP1 was significantly increased by protein kinase A (PKA) activators such as cyclic-AMP (cAMP) and forskolin. The purpose of this study is to determine whether PKA-dependent protein phosphorylation is involved in the regulation of water channel activity of AQP1. Results presented here suggest that catalytic subunit of protein kinase A significantly increased the amount of phosphorylated AQP1 protein. In addition, these results indicated that cAMP-responsive redistribution of AQP1 may be regulated by phosphorylation of AQP1. Moreover, they provide new insights on the molecular mechanisms for regulating water balance in several tissues involving rapid water transport such as ciliary epithelium. In addition, they suggest important potential roles for AQP1 in several clinical disorders involving rapid water transport such as glaucoma.     

Regulation of the clathrin-coated vesicle cycle by reversible phosphorylation

Reversible phosphorylation has long been an attractive mechanism to control cycles of coat assembly and disassembly during clathrin-mediated endocytosis. Many of the coat proteins are phosphorylated in vivo and in vitro. Our work has focused on the role of phosphorylation of the mu2 subunit of AP-2 (adaptor protein 2), which appears to be necessary for efficient cargo recruitment. Studies to probe the regulation of mu2 phosphorylation demonstrated that clathrin is a specific activator of the mu2 kinase, and, in permeabilized cells, cargo sequestration, driven by exogenously added clathrin, results in elevated levels of m2 phosphorylation. Furthermore, phosphorylated mu2 is mainly associated with assembled clathrin in vivo and its steady-state level is strongly reduced in cells depleted of clathrin heavy chain. Our results imply a central role for clathrin in the regulation of cargo selection via modulation of phospho-mu2 levels. This is therefore a novel regulatory role for clathrin that is independent of its structural role and that provides elegant spatial control of AP-2 and cargo interactions, ensuring that AP-2 is only activated at the correct cellular location and in the correct functional context. Ongoing studies are exploring further the roles of reversible phosphorylation in the coated vesicle cycle.     

Suppression of Kir2.3 activity by protein kinase C phosphorylation of the channel protein at threonine 53

Kir2.3 plays an important part in the maintenance of membrane potential in neurons and myocardium. Identification of intracellular signaling molecules controlling this channel thus may lead to an understanding of the regulation of membrane excitability. To determine whether Kir2.3 is modulated by direct phosphorylation of its channel protein and identify the phosphorylation site of protein kinase C (PKC), we performed experiments using several recombinant and mutant Kir2.3 channels. Whole-cell Kir2.3 currents were inhibited by phorbol 12-myristate 13-acetate (PMA) in Xenopus oocytes. When the N-terminal region of Kir2.3 was replaced with that of Kir2.1, another member in the Kir2 family that is insensitive to PMA, the chimerical channel lost its PMA sensitivity. However, substitution of the C terminus was ineffective. Four potential PKC phosphorylation sites in the N terminus were studied by comparing mutations of serine or threonine with their counterpart residues in Kir2.1. Whereas substitutions of serine residues at positions 5, 36, and 39 had no effect on the channel sensitivity to PMA, mutation of threonine 53 completely eliminated the channel response to PMA. Interestingly, creation of this threonine residue at the corresponding position (I79T) in Kir2.1 lent the mutant channel a PMA sensitivity almost identical to the wild-type Kir2.3. These results therefore indicate that Kir2.3 is directly modulated by PKC phosphorylation of its channel protein and threonine 53 is the PKC phosphorylation site in Kir2.3.     

Modulation of terminal deoxynucleotidyltransferase activity by the DNA-dependent protein kinase.

Rare Ig and TCR coding joints can be isolated from mice that have a targeted deletion in the gene encoding the 86-kDa subunit of the Ku heterodimer, the regulatory subunit of the DNA-dependent protein kinase (DNA-PK). However in the coding joints isolated from Ku86-/- animals, there is an extreme paucity of N regions (the random nucleotides added during V(D)J recombination by the enzyme TdT). This finding is consistent with a decreased frequency of coding joints containing N regions isolated from C.B-17 SCID mice that express a truncated form of the catalytic subunit of the DNA-PK (DNA-PKCS). This finding suggests an unexpected role for DNA-PK in addition of N nucleotides to coding ends during V(D)J recombination. In this report, we establish that TdT forms a stable complex with DNA-PK. Furthermore, we show that DNA-PK modulates TdT activity in vitro by limiting both the length and composition of nucleotide additions.     

New evidences for a regulation of deoxycytidine kinase activity by reversible phosphorylation

Recent studies indicate that deoxycytidine kinase (dCK), which activates various nucleoside analogues used in antileukemic therapy, can be regulated by post-translational modification, most probably through reversible phosphorylation. To further unravel its regulation, dCK was overexpressed in HEK-293 cells as a His-tag fusion protein. Western blot analysis showed that purified overexpressed dCK appears as doublet protein bands. The slower band disappeared after treatment with protein phosphatase lambda (PP lambda) in parallel with a decrease of dCK activity, providing additional arguments in favor of both phosphorylated and unphosphorylated forms of dCK.     

Modulation of Kv3.1b potassium channel phosphorylation in auditory neurons by conventional and novel protein kinase C isozymes

In fast-spiking neurons such as those in the medial nucleus of the trapezoid body (MNTB) in the auditory brainstem, Kv3.1 potassium channels are required for high frequency firing. The Kv3.1b splice variant of this channel predominates in the mature nervous system and is a substrate for phosphorylation by protein kinase C (PKC) at Ser-503. In resting neurons, basal phosphorylation at this site decreases Kv3.1 current, reducing neuronal ability to follow high frequency stimulation. We used a phospho-specific antibody to determine which PKC isozymes control serine 503 phosphorylation in Kv3.1b-tranfected cells and in auditory neurons in brainstem slices. By using isozyme-specific inhibitors, we found that the novel PKC-delta isozyme, together with the novel PKC-epsilon and conventional PKCs, contributed to the basal phosphorylation of Kv3.1b in MNTB neurons. In contrast, only PKC-epsilon and conventional PKCs mediate increases in phosphorylation produced by pharmacological activation of PKC in MNTB neurons or by metabotropic glutamate receptor activation in Kv3.1/mGluR1-cotransfected cells. We also measured the time course of dephosphorylation and recovery of basal phosphorylation of Kv3.1b following brief high frequency electrical stimulation of the trapezoid body, and we determined that the recovery process is mediated by both novel PKC-delta and PKC-epsilon isozymes and by conventional PKCs. The association between Kv3.1b and PKC isozymes was confirmed by reciprocal coimmunoprecipitation of Kv3.1b with multiple PKC isozymes. Our results suggest that the Kv3.1b channel is regulated by both conventional and novel PKC isozymes and that novel PKC-delta contributes specifically to the maintenance of basal phosphorylation in auditory neurons.     

Modulation of red cell band 4.1 function by cAMP-dependent kinase and protein kinase C phosphorylation

Human erythrocyte protein 4.1 is phosphorylated in vivo by several protein kinases including protein kinase C and cAMP-dependent kinase. We have used cAMP-dependent kinase purified from red cells and protein kinase C purified from brain to test the effects of phosphorylation on band 4.1 function. In solution, each kinase catalyzed the incorporation of 1-4 mol of PO4/mol of band 4.1. Phosphorylation of band 4.1 by each kinase resulted in a significant (50-80%) reduction in the ability of band 4.1 to promote spectrin binding to F-actin. Direct measurement of spectrin-band 4.1 binding showed that phosphorylation by each kinase also caused dramatic reduction in this association. Phosphorylation of band 4.1 by each kinase for increasing time periods enabled us to demonstrate an approximately linear inverse relationship between PO4 incorporation into band 4.1 and spectrin binding. These results show that phosphorylation of band 4.1 by cAMP-dependent kinase and protein kinase C may be central to the regulation of red cell cytoskeletal organization and membrane mechanical properties.     

Ca2+/calmodulin-dependent protein kinase II regulates Tiam1 by reversible protein phosphorylation

A number of guanine nucleotide exchange factors have been identified that activate Rho family GTPases, by promoting the binding of GTP to these proteins. We have recently demonstrated that lysophosphatidic acid and several other agonists stimulate phosphorylation of the Rac1-specific exchange factor Tiam1 in Swiss 3T3 fibroblasts, and that protein kinase C is involved in Tiam1 phosphorylation (Fleming, I. N., Elliott, C. M., Collard, J. G., and Exton, J. H. (1997) J. Biol. Chem. 272, 33105-33110). We now show, through manipulation of intracellular [Ca2+] and the use of protein kinase inhibitors, that both protein kinase Calpha and Ca2+/calmodulin-dependent protein kinase II are involved in the phosphorylation of Tiam1 in vivo. Furthermore, we show that Ca2+/calmodulin-dependent protein kinase II phosphorylates Tiam1 in vitro, producing an electrophoretic retardation on SDS-polyacrylamide gel electrophoresis. Significantly, phosphorylation of Tiam1 by Ca2+/calmodulin-dependent protein kinase II, but not by protein kinase C, enhanced its nucleotide exchange activity toward Rac1, by approximately 2-fold. Furthermore, Tiam1 was preferentially dephosphorylated by protein phosphatase 1 in vitro, and treatment with this phosphatase abolished the Ca2+/calmodulin-dependent protein kinase II activation of Tiam1. These data demonstrate that protein kinase Calpha and Ca2+/calmodulin-dependent protein kinase II phosphorylate Tiam1 in vivo, and that the latter kinase plays a key role in regulating the activity of this exchange factor in vitro.     

Modulation of protein kinase C activity by palmitoyl esters of maltose.

Mixtures of maltose palmitates containing predominantly maltose tetrapalmitate (designated MTP) possess immune potentiating and antitumor properties. Immune potentiation derives from macrophage activation and B lymphocyte mitogenicity and antitumor action from anti-angiogenic activity. Their mode of action at the cellular level is not known. Since high performance liquid chromatography (HPLC) provided purified maltose palmitates, we tested whether they individually and as a mixture could modulate activity of protein Kinase C (PKC), an enzyme implicated in mitogenic and release reactions. MTP activated crude lymphocyte and purified brain PKC in the absence of phosphatidyl serine (PS). It also augmented labeled dibutyryl phorbol (PDBu) binding to the brain enzyme in the absence of phospholipid. HPLC purified maltose tetrapalmitates (two isomers) were insoluble in aqueous solvent, and activated PKC slightly after incorporation into PS liposomes. Purified maltose di- and tri-palmitates were inhibitory to the enzyme. The activation of PKC was, therefore, due to higher saturated maltose palmitates, well dispersed by less substituted maltose palmitates acting as emulsifiers.     

Modulation of testicular galactolipid sulphotransferase activity by phosphorylation. Stimulation of enzyme activity in vitro by an endogenous kinase.

Evidence is presented for a testicular protein kinase activity capable of stimulating the activity in vitro of a partially purified preparation of the testicular galactolipid sulphotransferase. This enzyme is responsible for the synthesis of the major mammalian testicular glycolipid, sulphogalactosylglycerol, and is an early marker of differentiation during spermatogenesis. This stimulatory activity has been separated by affinity chromatography, using 3',5'-ADP-agarose, from both the detergent-solubilized microsomes (microsomal fractions) and the soluble fraction of the testicular homogenate. The stimulator was eluted from the affinity matrix by either a salt, or, more selectively, a cyclic AMP gradient. Thus this matrix can function as an analogue of 3',5'-cyclic AMP. The activity of the sulphotransferase stimulator was ATP-dependent and coincident with protein kinase activity. Sulphotransferase activity was also stimulated in the presence of commercial preparations of cyclic AMP-dependent protein kinase and stimulation was prevented in the presence of kinase inhibitors. Our results suggest that sulphogalactolipid biosynthesis is regulated by a phosphorylation process during spermatogenesis. In addition, our results suggest that affinity chromatography on 3',5'-ADP-agarose may provide a general method for the purification of cyclic AMP-dependent kinases.     

Shear stress stimulates phosphorylation of eNOS at Ser(635) by a protein kinase A-dependent mechanism

Shear stress stimulates nitric oxide (NO) production by phosphorylating endothelial NO synthase (eNOS) at Ser(1179) in a phosphoinositide-3-kinase (PI3K)- and protein kinase A (PKA)-dependent manner. The eNOS has additional potential phosphorylation sites, including Ser(116), Thr(497), and Ser(635). Here, we studied these potential phosphorylation sites in response to shear, vascular endothelial growth factor (VEGF), and 8-bromocAMP (8-BRcAMP) in bovine aortic endothelial cells (BAEC). All three stimuli induced phosphorylation of eNOS at Ser(635), which was consistently slower than that at Ser(1179). Thr(497) was rapidly dephosphorylated by 8-BRcAMP but not by shear and VEGF. None of the stimuli phosphorylated Ser(116). Whereas shear-stimulated Ser(635) phosphorylation was not affected by phosphoinositide-3-kinase inhibitors wortmannin and LY-294002, it was blocked by either treating the cells with a PKA inhibitor H89 or infecting them with a recombinant adenovirus-expressing PKA inhibitor. These results suggest that shear stress stimulates eNOS by two different mechanisms: 1) PKA- and PI3K-dependent and 2) PKA-dependent but PI3K-independent pathways. Phosphorylation of Ser(635) may play an important role in chronic regulation of eNOS in response to mechanical and humoral stimuli.     

Deletion of protein kinase A phosphorylation sites in the HERG potassium channel inhibits activation shift by protein kinase A.

We investigated the role of protein kinase A (PKA) in regulation of the human ether-a-go-go-related gene (HERG) potassium channel activation. HERG clones with single mutations destroying one of four consensus PKA phosphorylation sites (S283A, S890A, T895A, S1137A), as well as one clone carrying all mutations with no PKA phosphorylation sites (HERG 4M) were constructed. These clones were expressed heterologously in Xenopus oocytes, and HERG potassium currents were measured with the two microelectrode voltage clamp technique. Application of the cAMP-specific phosphodiesterase (PDE IV) inhibitor Ro-20-1724 (100 microM), which results in an increased cAMP level and PKA stimulation, induced a reduction of HERG wild type outward currents by 19.1% due to a shift in the activation curve of 12.4 mV. When 100 microM Ro-20-1724 was applied to the HERG 4M channel, missing all PKA sites, there was no significant shift in the activation curve, and the current amplitude was not reduced. Furthermore, the adenylate cyclase activator forskolin that leads to PKA activation (400 microM, 60 min), shifted HERG wild type channel activation by 14.1 mV and reduced currents by 39.9%, whereas HERG 4M channels showed only a small shift of 4.3 mV and a weaker current reduction of 22.3%. We conclude that PKA regulates HERG channel activation, and direct phosphorylation of the HERG channel protein has a functional role that may be important in regulation of cardiac repolarization.