cAMP signaling in neurons: patterns of neuronal expression and intracellular localization for a novel protein, AKAP 150, that anchors the regulatory subunit of cAMP-dependent protein kinase II beta

Novel mechanism of regulation of Rac activity and lamellipodia formation by RET tyrosine kinase

Rac activation in neuronal cells plays an important role in lamellipodia formation that is a critical event for neuritogenesis. It is well known that the Rac activity is regulated via activation of phosphatidylinositol 3-kinase (PI3K) by a variety of receptor tyrosine kinases. Here we show that increased serine phosphorylation on RET receptor tyrosine kinase following cAMP elevation promotes lamellipodia formation of neuronal cells induced by glial cell line-derived neurotrophic factor (GDNF). We identified serine 696 in RET as a putative phosphorylation site by protein kinase A and found that mutation of this serine almost completely inhibited lamellipodia formation by GDNF without affecting activation of the PI3K/AKT signaling pathway. Mutation of tyrosine 1062 in RET, whose phosphorylation is crucial for activation of PI3K, also inhibited lamellipodia formation by GDNF. Inhibition of lamellipodia formation by mutation of either serine 696 or tyrosine 1062 was associated with decrease of the Rac1-guanine nucleotide exchange factor (GEF) activity, suggesting that this activity is regulated by two different signaling pathways via serine 696 and tyrosine 1062 in RET. Moreover, in the presence of serine 696 mutation, lamellipodia formation was rescued by replacing tyrosine 687 with phenylalanine. These findings propose a novel mechanism that receptor tyrosine kinase modulates actin dynamics in neuronal cells via its cAMP-dependent phosphorylation.

Modulation of Drosophila slowpoke calcium-dependent potassium channel activity by bound protein kinase a catalytic subunit

Drosophila Slowpoke (dSlo) calcium-dependent potassium channels bind directly to the catalytic subunit of cAMP-dependent protein kinase (PKAc). We demonstrate here that coexpression of PKAc with dSlo in mammalian cells results in a dramatic decrease of dSlo channel activity. This modulation requires catalytically active PKAc but is not mediated by phosphorylation of S942, the only PKA consensus site in the dSlo C-terminal domain. dSlo binds to free PKAc but not to the PKA holoenzyme that includes regulatory subunits and is inactive. Activators of endogenous PKA that stimulate dSlo phosphorylation, but do not produce detectable PKAc binding to dSlo, do not modulate channel function. Furthermore, the catalytically inactive PKAc mutant does bind to dSlo but does not modulate channel activity. These results are consistent with the hypothesis that both binding of active PKAc to dSlo and phosphorylation of dSlo or some other protein are necessary for channel modulation.

The role of PKA, CaMKII, and PKC in avoidance conditioning: permissive or instructive?

This article explores the causal and correlative relationships between kinases and learning and memory. Specifically, the contributions of three kinases-protein kinase A (PKA), calcium calmodulin-dependent kinase II (CaMKII), and protein kinase C (PKC)-are assessed during the consolidation phase of avoidance conditioning. The following sources of evidence are considered: inhibitor data, activity monitoring, and transgenic studies. An exhaustive effort is made to address several issues regarding the participation of these kinases in (a) posttraining timing and magnitude, (b) location across many brain regions, and (c) the use of multiple pharmacological agents and assays. In addition, this article attempts to integrate the behavioral data with the purported role of kinases in long-term potentiation (LTP).

A-kinase anchor proteins in endocrine systems and reproduction

Over the past few years, significant progress has been made in characterizing the expression and localization of proteins that act as scaffolds for cAMP-dependent protein kinase (PK-A). These A-kinase anchor proteins (AKAPs) tether PK-A to intracellular organelles and structures, sequestering the kinase near its physiological substrates. The compartmentalization of distinct pockets of PK-A activity serves to provide spatial regulation of this signaling pathway. In addition, other signaling proteins bind to AKAPs, as do some newly described proteins of unknown function, suggesting that proteins of various pathways are anchored through AKAPs.

The biological functions of A-kinase anchor proteins

cAMP-dependent protein kinase is targeted to discrete subcellular locations by a family of specific anchor proteins (A-kinase anchor proteins, AKAPs). Localization recruits protein kinase A (PKA) holoenzyme close to its substrate/effector proteins, directing and amplifying the biological effects of cAMP signaling.AKAPs include two conserved structural modules: (i) a targeting domain that serves as a scaffold and membrane anchor; and (ii) a tethering domain that interacts with PKA regulatory subunits. Alternative splicing can shuffle targeting and tethering domains to generate a variety of AKAPs with different targeting specificity. Although AKAPs have been identified on the basis of their interaction with PKA, they also bind other signaling molecules, mainly phosphatases and kinases, that regulate AKAP targeting and activate other signal transduction pathways. We suggest that AKAP forms a "transduceosome" by acting as an autonomous multivalent scaffold that assembles and integrates signals derived from multiple pathways. The transduceosome amplifies cAMP and other signals locally and, by stabilizing and reducing the basal activity of PKA, it also exerts long-distance effects. The AKAP transduceosome thus optimizes the amplitude and the signal/noise ratio of cAMP-PKA stimuli travelling from the membrane to the nucleus and other subcellular compartments.

Axonal outgrowth from adult mouse nodose ganglia in vitro is stimulated by neurotrophin-4 in a Trk receptor and mitogen-activated protein kinase-dependent way

The actions of neurotrophic factors on sensory neurons of the adult nodose ganglion were studied in vitro. The ganglia were explanted in an extracellular matrix-based gel that permitted observation of the growing axons. Neurotrophin-4 (NT-4) was a very efficient stimulator of outgrowth of axons from the nodose ganglion and had almost doubled the outgrowth length when this was analyzed after 2 days in culture. Brain-derived neurotrophic factor also stimulated outgrowth, but to a lesser degree, whereas NT-3 gave only weak stimulatory tendencies. Nerve growth factor and glial cell line-derived neurotrophic factor both lacked stimulatory effects. NT-4 is known to act via TrkB receptors, and the presence of these on growing nodose neurons was demonstrated immunohistochemically. In line with a Trk-mediated growth effect, the NT-4 stimulation was abolished by K252a, a selective inhibitor of neurotrophin receptor-associated tyrosine kinase activity. K252a had no effect on the unstimulated preparation. NT-4 treatment led to activation of the mitogen-activated protein kinase and inhibition of the latter pathway by PD98059 significantly reduced the NT-4 stimulated outgrowth, whereas the drug had no effect on the unstimulated growth. In conclusion, the data suggest that NT-4 can serve as a powerful growth factor for neurons of adult nodose ganglia and that the growth stimulation involves TrkB- and mitogen-activated protein kinase.

Regulation of Na(+)-K(+)-ATPase by cAMP-dependent protein kinase anchored on membrane via its anchoring protein

Na(+)-K(+)- ATPase alpha-subunits in basolateral membrane vesicles (BLMVs) purified from rat parotid glands were (32)P-labeled within 5 s by incubation with [gamma-(32)P]ATP at 37 degrees C in the presence of cAMP, but no labeling occurred without cAMP. Phosphorylation of Na(+)-K(+)-ATPase was associated with a decrease in its activity. This alpha-subunit phosphorylation disappeared when BLMVs were briefly incubated with cAMP and subsequent washing before the incubation with [gamma-(32)P]ATP, indicating that catalytic subunit of protein kinase A (PKA) associated to BLMVs via binding with its RII regulatory subunit anchored on the membrane. In the absence of cAMP, a PKA catalytic subunit readily reassociated with the membrane-bound RII subunit. HT-31 peptide inhibited the Na(+)-K(+)-ATPase phosphorylation by membrane-bound endogenous PKA, indicating an involvement of A-kinase anchoring protein (AKAP). AKAP-150 protein in BLMVs was shown by immunoblotting and an RII overlay assay and was coimmunoprecipitated by anti-RII antibody. These results show that Na(+)-K(+)-ATPase of rat parotid gland acinar cells is regulated in vivo by membrane-anchored PKA via AKAP rather than by free cytosolic PKA.

Signal-processing machines at the postsynaptic density

Dendrites of individual neurons in the vertebrate central nervous system are contacted by thousands of synaptic terminals relaying information about the environment. The postsynaptic membrane at each synaptic terminal is the first place where information is processed as it converges on the dendrite. At the postsynaptic membrane of excitatory synapses, neurotransmitter receptors are attached to large protein "signaling machines" that delicately regulate the strength of synaptic transmission. These machines are visible in the electron microscope and are called the postsynaptic density. By changing synaptic strength in response to neural activity, the postsynaptic density contributes to information processing and the formation of memories.

Laminin acts via beta 1 integrin signalling to alter cholinergic regulation of L-type Ca(2+) current in cat atrial myocytes

A perforated patch recording method was used to determine how plating cells on laminin (20 microg ml(-1); >2 h) alters cholinergic regulation of L-type Ca(2+) current (I(Ca,L)) in atrial myocytes. Acetylcholine (ACh; 1 microm)-induced inhibition of basal I(Ca,L) was not different between cells on glass and laminin. However, stimulation of I(Ca,L) elicited by ACh withdrawal was significantly smaller in cells on laminin (10 +/- 2 %) than on glass (48 +/- 5 %) (P < 0.001). Stimulation of I(Ca,L) induced by either spermine-NO (200 microm), milrinone (10 microm), IBMX (100 microm) or forskolin (1 microm) was significantly smaller in cells plated on laminin than on glass. However, stimulation of I(Ca,L) by 100 microm 8-CPT-cAMP or intracellular dialysis with 50 microM cAMP was not different between cells plated on laminin or glass. Basal, forskolin- and IBMX-stimulated cAMP content was significantly smaller in cells plated on laminin than on glass. Stimulation of I(Ca,L) by ACh withdrawal was significantly smaller in cells plated on an alpha beta 1-integrin antibody (10 +/- 4 %) than on glass (3 +/- 6 %; P < 0.001). In cells on laminin, prior exposure to 100 microg ml-1 YIGSR, a laminin receptor-binding peptide, restored ACh-induced stimulation of I(Ca,L) (58 +/- 14 %)laminin alone (7 +/- 2 %; P < 0. 05). Addition of 20 microm cytochalasin D or 1 microM latrunculin A, agents that prevent actin polymerization, to cells on laminin restored ACh-induced stimulation of I(Ca,L). We conclude that laminin binding to beta 1 integrins acts in association with the actin-based cytoskeleton to attenuate adenylate cyclase activity. As a result, laminin inhibits NO-mediated stimulation of I(Ca,L) elicited by ACh withdrawal. Laminin-integrin signalling may be relevant to changes in autonomic regulation that occur during cardiac development and/or disease.

Localization of the A kinase anchoring protein AKAP79 in the human hippocampus

The phosphorylation state of the proteins, regulated by phosphatases and kinases, plays an important role in signal transduction and long-term changes in neuronal excitability. In neurons, cAMP-dependent protein kinase (PKA), protein kinase C (PKC) and calcineurin (CN) are attached to a scaffold protein, A kinase anchoring protein (AKAP), thought to anchor these three enzymes to specific sites of action. However, the localization of AKAP, and the predicted sites of linked phosphatase and kinase activities, are still unknown at the fine structural level. In the present study, we investigated the distribution of AKAP79 in the hippocampus from postmortem human brains and lobectomy samples from patients with intractable epilepsy, using preembedding immunoperoxidase and immunogold histochemical methods. AKAP79 was found in the CA1, presubicular and subicular regions, mostly in pyramidal cell dendrites, whereas pyramidal cells in the CA3, CA2 regions and dentate granule cells were negative both in postmortem and in surgical samples. In some epileptic cases, the dentate molecular layer and hilar interneurons also became immunoreactive. At the subcellular level, AKAP79 immunoreactivity was present in postsynaptic profiles near, but not attached to, the postsynaptic density of asymmetrical (presumed excitatory) synapses. We conclude that the spatial selectivity for the action of certain kinases and phosphatases regulating various ligand- and voltage-gated channels may be ensured by the selective presence of their anchoring protein, AKAP79, at the majority of glutamatergic synapses in the CA1, but not in the CA2/CA3 regions, suggesting profound differences in signal transduction and long-term synaptic plasticity between these regions of the human hippocampus.

Yotiao protein, a ligand for the NMDA receptor, binds and targets cAMP-dependent protein kinase II(1)

A yeast two-hybrid screen revealed that regulatory subunits (RII) of PKAII bind the Yotiao protein. Yotiao interacts with the NR1 subunit of the NMDA receptor. A purified C-terminal fragment of Yotiao binds PKAII, via an RII binding site constituted by amino acid residues 1452-1469, with a dissociation constant (K(d)) between 50 and 90 nM in vitro. A stable complex composed of Yotiao, RII and NR1 was immunoprecipitated from whole rat brain extracts. Immunostaining analysis disclosed that Yotiao, RIIbeta and NR1 colocalize in striatal and cerebellar neurons. Co-assembly of Yotiao/PKAII complexes with NR1 subunits may promote cAMP-dependent modulation of NMDA receptor activity at synapses, thereby influencing brain development and synaptic plasticity.

Identification of cAMP-dependent protein kinase holoenzymes in preantral- and preovulatory-follicle-enriched ovaries, and their association with A-kinase-anchoring proteins

Undifferentiated cells from preantral (PA) follicles respond to high levels of cAMP in a different manner than do differentiated cells from preovulatory (PO) follicles. We hypothesized that this differential response of PA and PO cells to cAMP could be due, in part, to either a difference in the profile of isoforms that comprise the cAMP-dependent protein kinase (PKA) holoenzymes and/or a difference in the interaction of PKA with A-kinase-anchoring proteins (AKAPs). To test these hypotheses, PKA activity, PKA holoenzymes, PKA subunits and AKAPs from PA and PO ovaries were compared. Soluble PKA holoenzymes and regulatory (R) subunits were separated by DEAE-cellulose chromatography and sucrose-density-gradient centrifugation. PKA R subunits were distinguished by photoaffinity labelling, autophosphorylation, size, isoelectric point and immunoreactivity. AKAPs were identified by RII subunit overlay assays and immunoreactivity. The results showed that extracts from PA and PO ovaries exhibited equivalent PKA holoenzyme profiles and activities, characterized by low levels of PKA type I (PKAI) holoenzyme and two distinct PKAII holoenzyme peaks, one containing only RIIbeta subunits (PKAIIbeta) and one containing both PKAIIbeta and PKAIIalpha holoenzymes. Both PA and PO ovarian extracts also contained PKA catalytic (C)-subunit-free RIalpha, while only PO ovaries exhibited C-subunit-free RIIbeta. Consistent with the elevated levels of C-subunit-free RIIbeta in PO cells, PKA activation in PO cells required higher concentrations of forskolin than that in PA cells. While extracts of PA and PO ovaries exhibited a number of similar AKAPs, including four prominent ones reactive with anti-AKAP-KL antisera (where AKAP-KL is an AKAP especially abundant in kidney and liver), cAMP-agarose affinity chromatography revealed two major differences in AKAP binding to purified R subunits. PO ovaries contained increased levels of AKAP80 (AKAP of 80 kDa) bound selectively to R subunits in DEAE-cellulose peak 2 (comprising PKAIIbeta and RIalpha), but not to R subunits in DEAE-cellulose peak 3 (comprising PKAIIalpha, PKAIIbeta and RIIbeta). PO ovaries also showed increased binding of R subunits to AKAPs reactive with anti-AKAP-KL antisera at 210, 175, 150 and 115 kDa. Thus in PO ovaries, unlike in PA ovaries, the majority of AKAPs are bound to R subunits. These results suggest that altered PKA-AKAP interactions may contribute to the distinct responses of PA and PO follicles to high levels of cAMP, and that higher cAMP levels are required to activate PKA in PO ovaries.

Membrane-bound cAMP-dependent protein kinase controls cAMP-induced differentiation in PC12 cells

The A126 cell line, a derivative of PC12, is defective in cAMP-induced transcription and does not differentiate in the presence of cAMP. In these cells overexpression of a cAMP-dependent protein kinase (PKA) anchor protein, AKAP75, and of the PKA catalytic subunit substantially increased the fraction of PKAII bound to the membrane, stimulated the transcription of cAMP-induced genes, and induced terminal differentiation. Conversely, wild type PC12 cells expressing a derivative of the AKAP75 protein, AKAP45, which binds the PKA regulatory subunits RII, but fails to locate them to the membranes, induced translocation of PKAII to the cytosol. These cells did not efficiently accumulate PKA catalytic subunit in the nuclei when stimulated with cAMP, did not transcribe cAMP-induced genes, and failed to differentiate when exposed to cAMP. These data indicate that membrane-bound PKA positively controls the transcription of cAMP-induced genes and differentiation in PC12 cells.

Mechanism of A-kinase-anchoring protein 79 (AKAP79) and protein kinase C interaction

The A-kinase-anchoring protein AKAP79 co-ordinates the location of cAMP-dependent protein kinase, phosphatase 2B (PP2B/calcineurin) and protein kinase C (PKC) at postsynaptic sites in neurons. In this report we focus on the mechanism of interaction between AKAP79 and PKC. We show that neither lipid activators nor kinase activation are required for association with AKAP79. The anchoring protein binds and inhibits the conserved catalytic core of PKCbetaII. AKAP79 also associates with conventional, novel and atypical isoforms of PKC in vitro and in vivo, and immunofluorescence staining of rat hippocampal neurons demonstrates that the murine anchoring-protein homologue AKAP150 is co-distributed with PKCalpha/beta, PKCepsilon or PKCiota. Binding of the AKAP79(31-52) peptide, which inhibits kinase activity, exposes the pseudosubstrate domain of PKCbetaII, allowing endoproteinase Arg-C proteolysis in the absence of kinase activators. Reciprocal experiments have identified two arginine residues at positions 39 and 40 that are essential for AKAP79(31-52) peptide inhibition of PKCbetaII. Likewise, the same mutations in the full-length anchoring protein reduced inhibition of PKCbetaII. Thus AKAP79 associates with multiple PKC isoforms through a mechanism involving protein-protein interactions at the catalytic core where binding of the anchoring protein inhibits kinase activity through displacement of the pseudosubstrate.

The A-kinase anchor protein MAP2B and cAMP-dependent protein kinase are associated with class C L-type calcium channels in neurons

Phosphorylation by cAMP-dependent protein kinase (PKA) increases the activity of class C L-type Ca(2+) channels which are clustered at postsynaptic sites and are important regulators of neuronal functions. We investigated a possible mechanism that could ensure rapid and efficient phosphorylation of these channels by PKA upon stimulation of cAMP-mediated signaling pathways. A kinase anchor proteins (AKAPs) bind to the regulatory R subunits of PKA and target the holoenzyme to defined subcellular compartments and substrates. Class C channels isolated from rat brain extracts by immunoprecipitation contain an endogenous kinase that phosphorylates kemptide, a classic PKA substrate peptide, and also the main phosphorylation site for PKA in the pore-forming alpha(1) subunit of the class C channel complex, serine 1928. The kinase activity is inhibited by the PKA inhibitory peptide PKI(5-24) and stimulated by cAMP. Physical association of the catalytic C subunit of PKA with the immunoisolated class C channel complex was confirmed by immunoblotting. A direct protein overlay binding assay performed with (32)P-labeled RIIbeta revealed a prominent AKAP with an M(r) of 280,000 in class C channel complexes. The protein was identified by immunoblotting as the microtubule-associated protein MAP2B, a well established AKAP. Class C channels did not contain tubulin and MAP2B association was not disrupted by dilution or addition of nocodazole, two treatments that cause dissociation of microtubules. In vitro experiments show that MAP2B can directly bind to the alpha(1) subunit of the class C channel. Our findings indicate that PKA is an integral part of neuronal class C L-type Ca(2+) channels and suggest that the AKAP MAP2B may mediate this interaction. Neither PKA nor MAP2B were detected in immunoprecipitates of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-type glutamate receptors or class B N-type Ca(2+) channels. Accordingly, MAP2B docked at class C Ca(2+) channels may be important for recruiting PKA to postsynaptic sites.

Cyclic nucleotide-dependent protein kinases: intracellular receptors for cAMP and cGMP action

Intracellular cAMP and cGMP levels are increased in response to a variety of hormonal and chemical stimuli; these nucleotides play key roles as second messenger signals in modulating myriad physiological processes. The cAMP-dependent protein kinase and cGMP-dependent protein kinase are major intracellular receptors for these nucleotides, and the actions of these enzymes account for much of the cellular responses to increased levels of cAMP or cGMP. This review summarizes many studies that have contributed significantly to an improved understanding of the catalytic, regulatory, and structural properties of these protein kinases. These accumulated findings provide insights into the mechanisms by which these enzymes produce their specific physiological effects and are helpful in considering the actions of other protein kinases as well.

Expression of a kinase anchoring protein 79 in the human fetal amygdala

The expression of AKAP (a kinase anchoring protein) 79 enriched in postsynaptic densities has been investigated in the human amygdaloid nuclei of the 8th gestational month. Nuclear specific diffuse and cellular immunostaining is observed. An outstanding feature of cellular immunostaining is the labelling of somata and dendritic trees in a manner that allows neuronal classification. Bipolar, multipolar, and pyramidal AKAP79-positive neurons are found throughout the amygdala; the highest packing density of immunostained neurons is seen within the central and lateral nucleus. Dense diffuse immunolabelling is observed in the lateral and accessory basal nucleus. The results indicate that AKAP79 is expressed in various neuronal types (projection as well as local circuit neurons). Diffuse staining does not always match with cellular labelling within a nucleus, thus, AKAP79 may be particularly enriched in dendrites in some nuclei. The widespread distribution of AKAP79 indicates its possible role in various amygdaloid circuitries; thus, AKAP79 does not seem to be restricted to definite functional systems in the 8th month.

Differential localization of protein kinase A type II isozymes in the Golgi-centrosomal area

Selectivity in the action of cAMP may be mediated by compartmentalized pools of cyclic AMP-dependent protein kinase (PKA). PKA type II is directed to different subcellular loci by interaction of the type II regulatory subunits (RIIalpha, RIIbeta) with A-kinase anchoring proteins. In order to separately investigate the subcellular localization of PKA type II isozymes, monospecific antibodies to human RIIalpha and RIIbeta subunits of PKA were developed. We demonstrate that centrosomes bind both RIIalpha and RIIbeta. Centrosomes were the preferred intracellular anchoring site for RIIbeta. However, centrosomal localization of RIIbeta was observed only in some highly differentiated cells such as keratinocytes, granulosa cells, and macrophages and in all neoplastic cell lines examined. Centrosomal localization of RIIbeta was not observed in normal undifferentiated cells such as fibroblasts, myoblasts, and T and B cells. In contrast, RIIalpha was abundant in the Golgi area and in the trans-Golgi network (TGN). Furthermore, although RIIalpha appeared to colocalize with microtubules in the Golgi/TGN, extractions with nonionic detergent demonstrated that RIIalpha was mainly membrane-associated. In addition, alterations of microtubule dynamics with Nocodazole or Taxol affected the distribution of the detergent-extractable pool of RIIalpha, indicating that RIIalpha may localize with microtubule-associated vesicles. Thus, RIIalpha and RIIbeta clearly localize differently in the Golgi-centrosomal region. This indicates specific roles for PKA isozymes containing either RIIalpha or RIIbeta.

The type and the localization of cAMP-dependent protein kinase regulate transmission of cAMP signals to the nucleus in cortical and cerebellar granule cells

cAMP signals are received and transmitted by multiple isoforms of cAMP-dependent protein kinases, typically determined by their specific regulatory subunits. In the brain the major regulatory isoform RIIbeta and the RII-anchor protein, AKAP150 (rat) or 75 (bovine), are differentially expressed. Cortical neurons express RIIbeta and AKAP75; conversely, granule cerebellar cells express predominantly RIalpha and RIIalpha. Cortical neurons accumulate PKA catalytic subunit and phosphorylated cAMP responsive element binding protein very efficiently into nuclei upon cAMP induction, whereas granule cerebellar cells fail to do so. Down-regulation of RIIbeta synthesis by antisense oligonucleotides inhibited cAMP-induced nuclear signaling in cortical neurons. Expression in cerebellar granule cells of RIIbeta and AKAP75 genes by microinjection of specific expression vectors, markedly stimulated cAMP-induced transcription of the lacZ gene driven by a cAMP-responsive element promoter. These data indicate that the composition of PKA in cortical and granule cells underlies the differential ability of these cells to transmit cAMP signals to the nucleus.

Identification of tethering domains for protein kinase A type Ialpha regulatory subunits on sperm fibrous sheath protein FSC1

The fibrous sheath is a unique cytoskeletal structure in the sperm flagellum believed to modulate sperm motility. FSC1 is the major structural protein of the fibrous sheath. The yeast two-hybrid system was used to identify other proteins that contribute to the structure of the fibrous sheath or participate in sperm motility. When FSC1 was used as the bait to screen a mouse testis cDNA library, two clones were isolated encoding the type Ialpha regulatory subunit (RIalpha) of cAMP-dependent protein kinase. Deletion analysis using the yeast two-hybrid system and in vitro binding assays with glutathione S-transferase-FSC1 fusion proteins identified two RIalpha tethering domains on FSC1. A domain located at residues 219-232 (termed domain A) corresponds to the reported tethering domain for a type II regulatory subunit (RII) of cAMP-dependent protein kinase, indicating that this binding domain has dual specificity to RI and RII. Another RIalpha tethering site (termed domain B) at residues 335-344 shows specific binding of RIalpha and had no significant sequence homology with known RII tethering domains. However, helical wheel projection analysis indicates that domain B is likely to form an amphipathic helix, the secondary structure of RII tethering domains of protein kinase A anchoring proteins. This was supported by the finding that site-directed mutagenesis to disrupt the amphipathic helix eliminated RIalpha binding. This is apparently the first report of an RIalpha-specific protein kinase A anchoring protein tethering domain.

Cyclic AMP signaling and gene regulation

Cyclic adenosine monophosphate (cAMP) is a ubiquitous second messenger produced in cells in response to hormones and nutrients. The production of cAMP is dependent upon the actions of many different proteins that affect its synthesis and degradation. An important function of cAMP is to activate the phosphorylating enzyme, protein kinase A. The key roles of cAMP and protein kinase A in the phosphorylation and regulation of enzyme substrates involved in intermediary metabolism are well known. A newly discovered role for protein kinase A is in the phosphorylation and activation of transcription factors that are critical for the control of the transcription of genes in response to elevated levels of cAMP.

Subcellular localization of rolipram-sensitive, cAMP-specific phosphodiesterases. Differential targeting and activation of the splicing variants derived from the PDE4D gene

Biochemical and immunofluorescence analyses revealed that phosphodiesterase variants encoded by the PDE4D gene are targeted to discrete subcellular structures. In quiescent FRTL-5 thyroid cells, the rolipram-sensitive phosphodiesterase (PDE) activity (cAMP-PDE) was recovered both in the soluble and particulate fractions of the homogenate. Although an immunoreactive 93-kDa PDE (PDE4D3) variant was recovered in both compartments, a 105-kDa variant with the properties of PDE4D4 was recovered mostly in the particulate fraction. The PDE4D3 form was readily solubilized with nonionic detergents. Conversely, the PDE4D4 form required buffers containing ionic detergents for extraction, suggesting that different mechanisms target these variants to insoluble structures. A 15-min stimulation with thyroid-stimulating hormone (TSH) led to an activation of the cAMP-PDE in both compartments and was correlated with a shift in electrophoretic mobility of the PDE4D3 polypeptide. Long term incubation with TSH caused an increase of the PDE activity in the soluble fraction and the appearance of a 68-kDa immunoreactive polypeptide with the properties of PDE4D2. Immunofluorescence analysis showed, in addition to diffuse staining, a signal localized on regions adjacent to the plasma membrane on cytoskeletal structures and in a perinuclear region of quiescent cells. Long term incubation with TSH caused an increase in the immunofluorescence signal in the soluble compartment. These data demonstrate that three PDE4D splicing variants are targeted to discrete subcellular compartments and that hormones cause the activation of these isoforms in a temporally and spatially dependent manner.

Defective motor behavior and neural gene expression in RIIbeta-protein kinase A mutant mice

Motor behavior is modulated by dopamine-responsive neurons in the striatum, where dopaminergic signaling uses G-protein-coupled pathways, including those that result in the activation of cAMP-dependent protein kinase (PKA). The RIIbeta isoform of PKA is highly enriched in the striatum, and targeted disruption of the RIIbeta gene in mice leads to a dramatic reduction in total PKA activity in this region. Although the mutant mice show typical locomotor responses after acute administration of dopaminergic drugs, they display abnormalities in two experience-dependent locomotor behaviors: training on the rotarod task and locomotor sensitization to amphetamine. In addition, amphetamine induction of fos is absent, and the basal expression of dynorphin mRNA is reduced in the striatum. These results demonstrate that motor learning and the regulation of neuronal gene expression require RIIbeta PKA, whereas the acute locomotor effects of dopaminergic drugs are relatively unaffected by this PKA deficiency.

Defective motor behavior and neural gene expression in RIIbeta-protein kinase A mutant mice

Motor behavior is modulated by dopamine-responsive neurons in the striatum, where dopaminergic signaling uses G-protein-coupled pathways, including those that result in the activation of cAMP-dependent protein kinase (PKA). The RIIbeta isoform of PKA is highly enriched in the striatum, and targeted disruption of the RIIbeta gene in mice leads to a dramatic reduction in total PKA activity in this region. Although the mutant mice show typical locomotor responses after acute administration of dopaminergic drugs, they display abnormalities in two experience-dependent locomotor behaviors: training on the rotarod task and locomotor sensitization to amphetamine. In addition, amphetamine induction of fos is absent, and the basal expression of dynorphin mRNA is reduced in the striatum. These results demonstrate that motor learning and the regulation of neuronal gene expression require RIIbeta PKA, whereas the acute locomotor effects of dopaminergic drugs are relatively unaffected by this PKA deficiency.

Association of protein kinase A type I with detergent-resistant structures of mammalian sperm cells

The finding that flagellar movement in detergent-permeabilized sperm cells is restored when Mg ATP and cAMP are added implicated detergent-resistant protein kinase A (PKA) in the regulation of sperm motility. It is widely believed that only the PKA regulatory subunit RII can associate with the cytoskeleton and/or organelles. In this paper we used monoclonal antibodies against the PKA catalytic subunit and RI subunit and demonstrated that PKA type I is also associated with the sperm cytoskeleton. To our knowledge, this is the first report showing anchored PKA type I. This association was found in sperm of nonrodent mammalian species and, to a lesser extent, also in mouse sperm. The PKA catalytic subunit is bound to the cytoskeleton secondarily via its complex with the regulatory subunit. The detergent-resistant complexes of RI and catalytic subunits localize predominantly to the flagellum. Ultrastructural immunogold labeling revealed the association of detergent-resistant PKA type I with outer dense fibers (ODF) and the fibrous sheath (FS) but not with microtubules. This location is consistent with a proposed role of PKA in regulation of FS sliding on underlying ODF.

Membrane-targeting sequences on AKAP79 bind phosphatidylinositol-4, 5-bisphosphate

Protein kinases and phosphatases are targeted through association with anchoring proteins that tether the enzymes to subcellular structures and organelles. Through in situ fluorescent techniques using a Green Fluorescent Protein tag, we have mapped membrane-targeting domains on AKAP79, a multivalent anchoring protein that binds the cAMP-dependent protein kinase (PKA), protein kinase C (PKC) and protein phosphatase 2B, calcineurin (CaN). Three linear sequences termed region A (residues 31-52), region B (residues 76-101) and region C (residues 116-145) mediate targeting of AKAP79 in HEK-293 cells and cortical neurons. Analysis of these targeting sequences suggests that they contain putative phosphorylation sites for PKA and PKC and are rich in basic and hydrophobic amino acids similar to a class of membrane-targeting domains which bind acidic phospholipids and calmodulin. Accordingly, the AKAP79 basic regions mediate binding to membrane vesicles containing acidic phospholipids including phosphatidylinositol-4, 5-bisphosphate [PtdIns(4,5)P2] and this binding is regulated by phosphorylation and calcium-calmodulin. Finally, AKAP79 was shown to be phosphorylated in HEK-293 cells following stimulation of PKA and PKC, and activation of PKC or calmodulin was shown to release AKAP79 from membrane particulate fractions. These findings suggest that AKAP79 might function in cells not only as an anchoring protein but also as a substrate and effector for the anchored kinases and phosphatases.

A novel lipid-anchored A-kinase Anchoring Protein facilitates cAMP-responsive membrane events

Compartmentalization of protein kinases with substrates is a mechanism that may promote specificity of intracellular phosphorylation events. We have cloned a low-molecular weight A-kinase Anchoring Protein, called AKAP18, which targets the cAMP-dependent protein kinase (PKA) to the plasma membrane, and permits functional coupling to the L-type calcium channel. Membrane anchoring is mediated by the first 10 amino acids of AKAP18, and involves residues Gly1, Cys4 and Cys5 which are lipid-modified through myristoylation and dual palmitoylation, respectively. Transient transfection of AKAP18 into HEK-293 cells expressing the cardiac L-type Ca2+ channel promoted a 34 9% increase in cAMP-responsive Ca2+ currents. In contrast, a targeting-deficient mutant of AKAP18 had no effect on Ca2+ currents in response to the application of a cAMP analog. Further studies demonstrate that AKAP18 facilitates GLP-1-mediated insulin secretion in a pancreatic beta cell line (RINm5F), suggesting that membrane anchoring of the kinase participates in physiologically relevant cAMP-responsive events that may involve ion channel activation.

Molecular characterization of a cDNA that encodes six isoforms of a novel murine A kinase anchor protein

We have cloned cDNA that encodes six novel A kinase anchor proteins (collectively named AKAP-KL). AKAP-KL diversity is generated by alternative mRNA splicing and utilization of two translation initiation codons. AKAP-KL polypeptides are evident in lung, kidney, and cerebellum, but are absent from many tissues. Different isoforms predominate in different tissues. Thus, AKAP-KL expression is differentially regulated in vivo. All AKAP-KL isoforms contain a 20-residue domain that avidly binds (Kd approximately 10 nM) regulatory subunits (RII) of protein kinase AII and is highly homologous with the RII tethering site in neuronal AKAP75. The distribution of AKAP-KL is strikingly asymmetric (polarized) in situ. Anchor protein accumulates near the inner, apical surface of highly polarized epithelium in tubules of nephrons. Both RII and AKAP-KL are enriched at an intracellular site that lies just below the plasma membrane of alveolar epithelial cells in lung. AKAP-KL interacts with and modulates the structure of the actin cytoskeleton in transfected cells. We also demonstrate that the tethering domain of AKAP-KL avidly ligates RII subunits in intact cells. AKAP-KL may be involved in (a) establishing polarity in signaling systems and (b) physically and functionally integrating PKAII isoforms with downstream effectors to capture, amplify, and precisely focus diffuse, trans-cellular signals carried by cAMP.

A-kinase anchor protein 75 increases the rate and magnitude of cAMP signaling to the nucleus

A-kinase anchor protein 75 (AKAP75) binds regulatory subunits (RIIalpha and RIIbeta) of type II protein kinase A (PKAII) isoforms and targets the resulting complexes to sites in the cytoskeleton that abut the plasma membrane [1-7]. Co-localization of AKAP75-PKAII with adenylate cyclase and PKA substrate/effector proteins in cytoskeleton and plasma membrane effects a physical and functional integration of up-stream and downstream signaling proteins, thereby ensuring efficient propagation of signals carried by locally generated cyclic AMP (cAMP) [4-9]. An important, but previously untested, prediction of the AKAP model is that efficient, cyclic nucleotide-dependent liberation of diffusible PKA catalytic subunits from cytoskeleton-bound AKAP75-PKAII complexes will also enhance signaling to distal organelles, such as the nucleus. We tested this idea by suing HEK-A75 cells, in which PKAII isoforms are immobilized in cortical cytoskeleton by AKAP75. Abilities of HEK-A75 and control cells (with cytoplasmically dispersed PKAII isoforms) to respond to increases in cAMP content were compared. Cells with anchored PKAII exhibited a threefold higher level of nuclear catalytic subunit content and 4-10-fold greater increments in phosphorylation of a regulatory serine residue in cAMP response element binding protein (CREB) and in phosphoCREB-stimulated transcription of the c-fos gene. Each effect occurred more rapidly in cells containing targeted AKAP75-PKAII complexes. Thus, anchoring of PKAII in actin cortical cytoskeleton increases the rate, magnitude and sensitivity of cAMP signaling to the nucleus.

Loss of haloperidol induced gene expression and catalepsy in protein kinase A-deficient mice

The antipsychotic drug, haloperidol, elicits the expression of neurotensin and c-fos mRNA in the dorsal lateral region of the striatum and produces an acute cataleptic response in rodents that correlates with the motor side effects of haloperidol in humans. Mice harboring a targeted disruption of the RIIbeta subunit of protein kinase A have a profound deficit in cAMP-stimulated kinase activity in the striatum. When treated with haloperidol, RIIbeta mutant mice fail to induce either c-fos or neurotensin mRNA and the acute cataleptic response is blocked. However, both wild-type and mutant mice become cataleptic when neurotensin peptide is directly injected into the lateral ventricle, demonstrating that the kinase deficiency does not interfere with the action of neurotensin but rather its synthesis and release. These results establish a direct role for protein kinase A as a mediator of haloperidol induced gene induction and cataleptic behavior.

Molecular characterization of a novel A kinase anchor protein from Drosophila melanogaster

Activation of protein kinase A (PKA) at discrete intracellular sites facilitates oogenesis and development in Drosophila. Thus, PKA-anchor protein complexes may be involved in controlling these crucial biological processes. Evaluation of this proposition requires knowledge of PKA binding/targeting proteins in the fly. We now report the discovery and characterization of cDNAs encoding a novel, Drosophila A kinase anchor protein, DAKAP550. DAKAP550 is a large (>2300 amino acids) acidic protein that is maximally expressed in anterior tissues. It binds regulatory subunits (RII) of both mammalian and Drosophila PKAII isoforms. The tethering region of DAKAP550 includes two proximal, but non-contiguous RII-binding sites (B1 and B2). The B1 domain (residues 1406-1425) binds RII approximately 20-fold more avidly than B2 (amino acids 1350-1369). Affinity-purified anti-DAKAP550 IgGs were exploited to demonstrate that the anchor protein is expressed in many cells in nearly all tissues throughout the lifespan of the fly. However, DAKAP550 is highly enriched and asymmetrically positioned in subpopulations of neurons and in apical portions of cells in gut and trachea. The combination of RII (PKAII) binding activity with differential expression and polarized localization is consistent with a role for DAKAP550 in creating target loci for the reception of signals carried by cAMP. The DAKAP550 gene was mapped to the 4F1.2 region of the X chromosome; flies that carry a deletion for this portion of the X chromosome lack DAKAP550 protein.

Organelle-specific targeting of protein kinase AII (PKAII). Molecular and in situ characterization of murine A kinase anchor proteins that recruit regulatory subunits of PKAII to the cytoplasmic surface of mitochondria

Experiments were designed to test the idea that A kinase anchor proteins (AKAPs) tether regulatory subunits (RII) of protein kinase AII (PKAII) isoforms to surfaces of organelles that are bounded by phospholipid bilayers. S-AKAP84, one of three RII-binding proteins encoded by a single-copy murine gene, was studied as a prototypic organelle-associated AKAP. When S-AKAP84 was expressed in HEK293 cells, the anchor protein was targeted to mitochondria and excluded from other cell compartments. The RII tethering site is located in the cytoplasm adjacent to the mitochondrial surface. Endogenous RII subunits are not associated with mitochondria isolated from control cells. Expression of S-AKAP84 in transfected HEK293 cells triggered a redistribution of 15% of total RII to mitochondria. Thus, the tethering region of the organelle-inserted anchor protein is properly oriented and avidly binds RII (PKAII) isoforms in intact cells. Two critical domains in S-AKAP84 were mapped. Residues 1 to 30 govern insertion of the polypeptide into the outer mitochondrial membrane; amino acids 306-325 constitute the RII-binding site. Properties established for S-AKAP84 in vitro and in situ strongly suggest that a physiological function of this protein is to concentrate and immobilize RII (PKAII) isoforms at the cytoplasmic face of a phospholipid bilayer.

Cloning and characterization of a testis-specific, developmentally regulated A-kinase-anchoring protein (TAKAP-80) present on the fibrous sheath of rat sperm

cAMP is important for the initiation of mammalian sperm motility. Previously we established that a type II cAMP-dependent protein kinase is tightly associated with the fibrous sheath of rat sperm. This unique cytoskeletal structure surrounds the 9+2 axonemal network in the principal piece of the flagellum. Association of the kinase to the fibrous sheath is mediated via its regulatory subunit, RII. An RII-binding overlay procedure was used to document that RII could specifically associate with fibrous sheath polypeptides of 120 and 80 kDa. In this study, we report the cloning of a rat testis-specific, developmentally regulated, RII-binding protein (TAKAP-80). A 1.2-kb cDNA clone, isolated by screening a rat testis expression library with 32P-labeled RII, hybridized to a 1.8-kb mRNA transcript present exclusively in testis. This transcript appeared at detectable levels at 30 days after birth. Over the next 10 days the mRNA levels increased greatly. This time interval corresponds to the initiation of spermiogenesis. The complete nucleotide sequence of TAKAP-80 cDNA was obtained by polymerase chain reaction and contained a continuous open reading frame of 502 amino acids. The deduced amino acid sequence showed a clear demarcation of charged and hydrophobic amino acid residues. Amino acids 1-147 of the protein contained 45% charged residues, with lysine and arginine predominating. Similarly, amino acids 268-502 also contained a high percentage of charged amino acids (35%). In contrast, amino acids 148-267 were mostly hydrophobic and contained clusters of a repeating PXXP motif where X was predominantly valine and alanine or sometimes proline. The 1.2-kb cDNA clone was inserted into the pRSET vector and expressed as a His6 tag fusion protein in Escherichia coli. The recombinant protein was soluble and bound RIIalpha, RIIbeta and type IIalpha holoenzyme by the RII-binding overlay procedure. Deletion analysis revealed that the high-affinity interaction site for RII was contained within amino acids 258-378 of TAKAP-80. Antibodies prepared against the fusion protein recognized an 80-kDa protein present in the urea-insoluble particulate fraction of rat testis and in purified fibrous sheath preparations isolated from rat epididymal sperm. Levels of the 80-kDa immunoreactive protein were significantly higher in mature (60 days old) compared with immature (30 days old) rat testis, correlating with the mRNA levels.

PKA isoforms, neural pathways, and behaviour: making the connection

In mammals, the cAMP-dependent protein kinase (PKA) family of enzymes is assembled from the products of four regulatory and two catalytic subunit genes, all of which are expressed in neurons. Specific isoforms of PKA display differences in biochemical properties and subcellular localization, but it has been difficult to ascribe specific physiological functions to any given isoform. The recent development of gene knockout and transgenic mouse models has allowed for a more integrated examination of the in vivo roles of specific PKA isoforms in gene expression, synaptic plasticity, and behaviour.

Characterization of a pretranscriptional pathway for induction by phenobarbital of cytochrome P450 3A23 in primary cultures of adult rat hepatocytes

Our laboratory has proposed that phenobarbital (PB), a typical lipophilic agent that induces some members of the supergene family of liver microsomal cytochromes P450 (e.g., CYP2B1/2 and CYP3A23), acts through a complex process inhibitable by the presence of growth hormone (GH), the absence of some components of the extracellular matrix, or a disrupted cytoskeleton. To verify that these manipulations of the culture environment block specific steps in the PB induction pathway rather than simply exerting nonspecific or toxic effects on CYP2B1/2 gene transcription, we have now examined PB induction of CYP3A23, a gene known to also be transcriptionally activated by dexamethasone (DEX) through a "nonclassical" pathway apparently involving the glucocorticoid receptor. We found that in primary cultures of adult rat hepatocytes treated with PB, induction of CYP3A23 mRNA, just as we reported for induction of CYP2B1/2 mRNA, required the use of Matrigel (a reconstituted basement membrane) and was blocked by the presence of cytoskeletal inhibitors (colchicine or cytochalasins) or of physiologic concentrations of GH in the culture medium. Moreover, PB induction of CYP3A23 and of CYP2B1/2 mRNAs was greatly diminished by inhibitors of cAMP-dependent protein kinase (PKA). In striking contrast, induction of CYP3A23 mRNA by DEX was unaffected by any of these alterations of the culture conditions that block its induction by PB. We conclude that the effects of extracellular matrix, GH, disruption of the cytoskeleton, and activation of cAMP-dependent protein kinase, pharmacologically define multiple, pretranscriptional steps in the pathway(s) for PB induction of liver cytochromes P450.

Identification of a novel protein kinase A anchoring protein that binds both type I and type II regulatory subunits

Compartmentalization of cAMP-dependent protein kinase is achieved in part by interaction with A-kinase anchoring proteins (AKAPs). All of the anchoring proteins identified previously target the kinase by tethering the type II regulatory subunit. Here we report the cloning and characterization of a novel anchoring protein, D-AKAP1, that interacts with the N terminus of both type I and type II regulatory subunits. A novel cDNA encoding a 125-amino acid fragment of D-AKAP1 was isolated from a two-hybrid screen and shown to interact specifically with the type I regulatory subunit. Although a single message of 3.8 kilobase pairs was detected for D-AKAP1 in all embryonic stages and in most adult tissues, cDNA cloning revealed the possibility of at least four splice variants. All four isoforms contain a core of 526 amino acids, which includes the R binding fragment, and may be expressed in a tissue-specific manner. This core sequence was homologous to S-AKAP84, including a mitochondrial signal sequence near the amino terminus (Lin, R. Y., Moss, S. B., and Rubin, C. S. (1995) J. Biol. Chem. 270, 27804-27811). D-AKAP1 and the type I regulatory subunit appeared to have overlapping expression patterns in muscle and olfactory epithelium by in situ hybridization. These results raise a novel possibility that the type I regulatory subunit may be anchored via anchoring proteins.

Membrane localization of cAMP-dependent protein kinase amplifies cAMP signaling to the nucleus in PC12 cells

The A126 cell line, in contrast to its PC12 parent, does not differentiate, accumulate nuclear cAMP-dependent protein kinase A (PKA) catalytic subunit, or transcribe cAMP-dependent promoters in response to cAMP. Total PKA is reduced by 50% and is partly resistant to cAMP-induced dissociation in vivo. Unlike PC12, where PKAII is membrane-associated, PKAII is exclusively cytosolic in A126. Cotransfection with the RII anchor protein (AKAP75) and the PKA catalytic subunit (C-PKA) restored cAMP-induced transcription to levels found in PC12. These data indicate that membrane-bound PKAII amplifies cAMP signaling to the nucleus and suggest that cAMP-mediated responses are specified by the type and cellular localization of the PKA isoform.

The v-Ki-Ras oncogene alters cAMP nuclear signaling by regulating the location and the expression of cAMP-dependent protein kinase IIbeta

The v-Ki-Ras oncoprotein dedifferentiates thyroid cells and inhibits nuclear accumulation of the catalytic subunit of cAMP-dependent protein kinase. After activation of v-Ras or protein kinase C, the regulatory subunit of type II protein kinase A, RIIbeta, translocates from the membranes to the cytosol. RIIbeta mRNA and protein were eventually depleted. These effects were mimicked by expressing AKAP45, a truncated version of the RII anchor protein, AKAP75. Because AKAP45 lacks membrane targeting domains, it induces the translocation of PKAII to the cytoplasm. Expression of AKAP45 markedly decreased thyroglobulin mRNA levels and inhibited accumulation of C-PKA in the nucleus. Our results suggest that: 1) The localization of PKAII influences cAMP signaling to the nucleus; 2) Ras alters the localization and the expression of PKAII; 3) Translocation of PKAII to the cytoplasm reduces nuclear C-PKA accumulation, resulting in decreased expression of cAMP-dependent genes, including RIIbeta, TSH receptor, and thyroglobulin. The loss of RIIbeta permanently down-regulates thyroid-specific gene expression.

A kinase anchor protein 75 targets regulatory (RII) subunits of cAMP-dependent protein kinase II to the cortical actin cytoskeleton in non-neuronal cells

Neuronal A kinase anchor protein (AKAP) homologs, such as AKAPs 75 and 150, tether cAMP-dependent protein kinase II (PKAII) isoforms to the postsynaptic cytoskeleton, thereby creating target sites for cAMP action. These AKAPs, which bind regulatory subunits (RIIs) of PKAII, are also expressed in certain non-neuronal cells. Non-neuronal cell lines that stably express wild type and mutant AKAP75 transgenes were generated to investigate the extraneuronal function of AKAPs. In non-neuronal cells, AKAP75 accumulates selectively in the actin-rich, cortical cytoskeleton in close proximity with the plasma membrane. AKAP75 efficiently sequesters cytoplasmic RIIalpha and RIIbeta (PKAII isoforms) and translocates these polypeptides to the cell cortex. Two structural modules in AKAP75, T1 (residues 27-48), and T2 (residues 77-100), are essential for targeting AKAP75.RII complexes to the cortical cytoskeleton. Deletions or amino acid substitutions in T1 and/or T2 result in the dispersion of both AKAP75 and RII subunits throughout the cytoplasm. AKAP75 is co-localized with F-actin and fodrin in the cortical cytoskeleton. Incubation of cells with 5 microM cytochalasin D disrupts actin filaments and dissociates actin from the cell cortex. In contrast, the bulk of AKAP75 and fodrin remain associated with the cortical region of cytochalasin D-treated cells. Thus, targeting of AKAP75 does not depend upon direct binding with F-actin. Rather, AKAP75 (like fodrin) may be associated with a multiprotein complex that interacts with integral plasma membrane proteins.

Functional co-localization of transfected Ca(2+)-stimulable adenylyl cyclases with capacitative Ca2+ entry sites

Three adenylyl cyclases (ACI, ACIII, and ACVIII) have been described, which are putatively Ca(2+)-stimulable, based on in vitro assays. However, it is not clear that these enzymes can be regulated by physiological rises in [Ca2+]i when expressed in intact cells. Furthermore, it is not known whether transfected adenylyl cyclases might display the strict requirement for capacitative Ca2+ entry that is shown by the Ca(2+)-inhibitable ACVI, which is indigenous to C6-2B glioma cells (Chiono, M., Mahey, R., Tate, G., and Cooper, D. M. F. (1995) J. Biol. Chem. 270, 1149-1155). In the present study, ACI, ACIII, and ACVIII were heterologously expressed in HEK 293 cells, and conditions were devised that distinguished capacitative Ca2+ entry from both internal release and nonspecific elevation in [Ca2+]i around the plasma membrane. Remarkably, not only were ACI and ACVIII largely insensitive to Ca2+ release from stores, but they were robustly stimulated only by capacitative Ca2+ entry and not al all by a substantial increase in [Ca2+]i at the plasma membrane elicited by ionophore. (ACIII, reflecting its feeble in vitro sensitivity to Ca2+, was unaffected by any [Ca2+]i rise.) These results suggest a quite unsuspected, essential association of Ca(2+)-sensitive adenylyl cyclases with capacitative Ca2+ entry sites, even when expressed heterologously.

Activity, expression and function of a second Drosophila protein kinase A catalytic subunit gene

The DC2 gene was isolated previously on the basis of sequence similarity to DC0, the major Drosophila protein kinase A (PKA) catalytic subunit gene. We show here that the 67-kD Drosophila DC2 protein behaves as a PKA catalytic subunit in vitro. DC2 is transcribed in mesodermal anlagen of early embryos. This expression depends on dorsal but on neither twist nor snail activity. DC2 transcriptional fusions mimic this embryonic expression and are also expressed in subsets of cells in the optic lamina, wing disc and leg discs of third instar larvae. A saturation screen of a small deficiency interval containing DC2 for recessive lethal mutations yielded no DC2 alleles. We therefore isolated new deficiencies to generate deficiency trans-heterozygotes that lacked DC2 activity. These animals were viable and fertile. The absence of DC2 did not affect the viability or phenotype of imaginal disc cells lacking DC0 activity or embryonic hatching of animals with reduced DC0 activity. Furthermore, transgenes expressing DC2 from a DC0 promoter did not efficiently rescue a variety of DC0 mutant phenotypes. These observations indicate that DC2 is not an essential gene and is unlikely to be functionally redundant with DC0, which has multiple unique functions during development.

Localization of protein kinases by anchoring proteins: a theme in signal transduction

A fundamental question in signal transduction is how stimulation of a specific protein kinase leads to phosphorylation of particular protein substrates throughout the cell. Recent studies indicate that specific anchoring proteins located at various sites in the cell compartmentalize the kinases to their sites of action. Inhibitors of the interactions between kinases and their anchoring proteins inhibit the functions mediated by the kinases. These data indicate that the location of these anchoring proteins provides some of the specificity of the responses mediated by each kinase and suggest that inhibitors of the interaction between the kinases and their anchoring proteins may be useful as therapeutic agents.

Adenylyl cyclases and the interaction between calcium and cAMP signalling

Adenylyl cyclase is the prototypical second messenger generator. Nearly all of the eight cloned adenylyl cyclases are regulated by one or other arm of the phospholipase C pathway. Functional and ultrastructural investigations have shown that adenylyl cyclases are intimately associated with sites of calcium ion entry into the cell. Oscillations in cellular cyclic AMP levels are predicted to arise because of feedback inhibition of adenylyl cyclase by Ca2+. Such findings inextricably intertwine cellular signalling by cAMP and internal Ca2+ and extend the known regulatory modes available to cAMP.

Localization and functions of protein kinase A during Drosophila oogenesis

We have characterized the requirements for the Protein Kinase A (PKA) catalytic subunit, DC0, in Drosophila oogenesis. Intercellular bridges in egg chambers from PKA deficient females are unstable, leading to the formation of multinucleate nurse cells by fusions of adjacent cells. Germline clones of cells homozygous for null mutations of DC0 indicate that PKA acts autonomously in the germline. Highest levels of PKA catalytic subunit protein are associated with germ cell membranes, suggesting that targets of PKA are associated with the membrane or membrane skeleton and contribute to the stabilization of intercellular bridges. The migration of a subset of follicle cells, the border cells, is also disrupted by germline PKA mutations, implying that nurse cell junctions provide an essential path for border cell migrations.