Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:2.7.11.1 (protein kinase)
 81,284 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

An inwardly rectifying, ATP-regulated K+ channel with a distinctive molecular architecture, ROMK1, was recently cloned from rat kidney. Using patch clamp techniques, we have investigated the regulation of ROMK1 with particular emphasis on phosphorylation/dephosphorylation processes. Spontaneous channel rundown occurred after excision of membrane patches into ATP-free bath solutions in the presence of Mg2+. Channel rundown was almost completely abolished after excision of patches into either Mg(2+)-free bathing solutions or after preincubation with the broad-spectrum phosphatase inhibitor, orthovanadate, in the presence of Mg2+. MgATP preincubation also inhibited channel rundown in a dose-dependent manner. In addition, the effect of the specific phosphatase inhibitors okadaic acid (1 microM) and calyculin A (1 microM) was also investigated. The presence of either okadaic acid or calyculin A failed to inhibit channel rundown. Taken together, these data suggest that rundown of ROMK1 involves a Mg(2+)-dependent dephosphorylation process. Channel activity was also partially restored after the addition of MgATP to the bath solution. Addition of exogenous cAMP-dependent protein kinase A (PKA) catalytic subunit led to a further increase in channel open probability. Addition of Na2ATP, in the absence of Mg2+, was ineffective, suggesting that restoration of channel activity is a Mg(2+)-dependent process. Addition of the specific PKA inhibitor, PKI, to the bath solution led to a partial, reversible inhibition in channel activity. Thus, PKA-dependent phosphorylation processes are involved in the modulation of channel activity. This observation is consistent with the presence of potential PKA phosphorylation sites on ROMK1.
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PMID:Regulation of ROMK1 K+ channel activity involves phosphorylation processes. 805 60

Activity of the recently cloned ATP-sensitive epithelial K+ channel, ROMK (Ho, K., Nichols, C. G., Lederer, W. J., Lytton, J., Vassilev, P. M., Kanazirska, M. V., and Hebert, S. C. (1993) Nature 362, 31-38), is regulated by phosphorylation-dephosphorylation processes with cAMP-dependent protein kinase (PKA)-dependent phosphorylation events being required for maintenance of channel activity in excised membrane patches (McNicholas, C. M., Wang, W., Ho, K., Hebert, S. C., and Giebisch, G. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 8077-8081; Kubokawa, M., McNicholas, C. M., Higgins, M. A., Wang, W., and Giebisch, G. (1995) Am. J. Physiol. 269, F355-F362). To determine whether this channel is a substrate for PKA, ROMK tagged with the hemagglutinin epitope was transiently transfected into HEK293 cells. In vitro labeling of immunoprecipitated proteins from transfected cells showed that ROMK could be phosphorylated by PKA. Metabolic labeling of ROMK resulted in a significantly increased phosphorylation upon pretreatment of the cells with forskolin, consistent with an action of cAMP-dependent protein kinase. Phosphoamino acid analyses of the ROMK phosphoproteins revealed that phosphate was attached exclusively to serine residues. Three putative PKA phosphorylation sites containing serine residues in the predicted ROMK proteins are shown directly to be substrates for PKA. Site-directed mutagenesis of each of these sites or double mutation of any two sites showed that ROMK proteins retained the ability to be phosphorylated by PKA both in vivo and in vitro to a variable extent, while triple mutation of all three PKA sites abolished the phosphorylation induced by cAMP agonists in transfected cells. Two-electrode voltage clamp experiments showed that PKA-dependent phosphorylation was required for ROMK channel activity and that at least two of the three sites were required for channel function when expressed in X. laevis oocytes. Taken together, these results provide strong evidence that direct phosphorylation of the channel polypeptide by PKA is involved in channel regulation and PKA-dependent phosphorylation is essential for ROMK channel activity.
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PMID:Phosphorylation of the ATP-sensitive, inwardly rectifying K+ channel, ROMK, by cyclic AMP-dependent protein kinase. 862 94

1. The effect of protein kinase activators on cloned inward rectifier channels expressed in Xenopus oocytes was examined using a two-electrode voltage clamp. PKA activators caused no change in KIR1.1, KIR2.1, or KIR2.3 current. The PKC activators phorbol 12-myristate 14-acetate (PMA) and phorbol 12, 13-dibutyrate (PDBu) inhibited KIR2.3 currents, but not KIR2.1 or KIR1.1 current. This inhibition was blocked by staurosporine. An inactive phorbol ester, 4 alpha-phorbol 12, 13-didecanoate (4 alpha-PDD), had no effect on KIR2.3. 2. Upon changing solution from 2 to 98 microM K+, KIR2.3 but not KIR1.1 or KIR2.1 currents typically 'ran down' over 5 min to 60-80% of maximum amplitude. Rundown occurred even if PMA was applied before changing to high [K+] solution, indicating that rundown was independent of PKC activity. Rundown was evoked by substituting NMG+ for Na+, showing that it results from low [Na+] and not from high [K+]. 3. These results suggest that KIR2.3, but not KIR1.1 or KIR2.1, is subject to regulation, both by PKC activation and as a consequence of low [Na+]o. The difference in secondary regulation may account for specific responses to PKC stimulation of tissues expressing otherwise nearly identical KIR channels.
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PMID:Protein kinase C inhibition of cloned inward rectifier (HRK1/KIR2.3) K+ channels expressed in Xenopus oocytes. 888 75

The renal ATP-sensitive low-conductance K+ channel (KATP) plays an important role in K+ recycling in the thick ascending limb and in K+ secretion in the collecting duct. The low-conductance KATP is stimulated by cAMP-dependent protein kinase A and inhibited by protein kinase C, arachidonic acid, acidic pH and sulfonylurea agents. We reviewed the progress concerning the properties of the recently cloned inward-rectifying K+ channel (ROMK or KirI) and compared their regulatory mechanisms with the native low-conductance KATP. The results are important to gain insight into molecular mechanisms by which ROMK channels are regulated.
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PMID:Structure and function of the low conductance KATP channel, ROMK. 926 88

The atomic force microscope (AFM) was used to continuously follow height changes of individual protein molecules exposed to physiological stimuli. A AFM tip was coated with ROMK1 (a cloned renal epithelial potassium channel known to be highly pH sensitive) and lowered onto atomically flat mica surface until the protein was sandwiched between AFM tip and mica. Because the AFM tip was an integral part of a highly flexible cantilever, any structural alterations of the sandwiched molecule were transmitted to the cantilever. This resulted in a distortion of the cantilever that was monitored by means of a laser beam. With this system it was possible to resolve vertical height changes in the ROMK1 protein of >/=0.2 nm (approximately 5% of the molecule's height) with a time resolution of >/=1 msec. When bathed in electrolyte solution that contained the catalytic subunit of protein kinase A and 0.1 mM ATP (conditions that activate the native ion channel), we found stochastically occurring height fluctuations in the ROMK1 molecule. These changes in height were pH-dependent, being greatest at pH 7.6, and lowering the pH (either by titration or by the application of CO2) reduced their magnitude. The data show that overall changes in shape of proteins occur stochastically and increase in size and frequency when the proteins are active. This AFM "molecular-sandwich" technique, called MOST, measures structural activity of proteins in real time and could prove useful for studies on the relationship between structure and function of proteins at the molecular level.
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PMID:Structural activity of a cloned potassium channel (ROMK1) monitored with the atomic force microscope: the "molecular-sandwich" technique. 939 Nov 67

In the present study, we have used the two-electrode voltage-clamp and patch-clamp techniques to study the effects of forskolin and cAMP on the ROMK1 channels, which are believed to be the native K+ secretory channels in the kidney. Addition of 1 microM forskolin or 100 microM 8-bromo-cAMP, within 10 min, has no significant effect on the current of ROMK1 channels expressed in Xenopus oocytes. In contrast, application of 1 microM forskolin, within 3 min, significantly increased whole-cell K+ current by 35%, when ROMK1 channels were coexpressed with the A kinase anchoring protein AKAP79, which was cloned from neuronal tissue. Two lines of evidence indicate that the effect of forskolin is mediated by a cAMP-dependent pathway: (i) Addition of 100 microM 8-bromo-cAMP mimics the effect of forskolin and (ii) the effect of forskolin and cAMP is not additive. That AKAP is required for the effect of cAMP is further supported by experiments in which addition of ATP (100 microM) and cAMP (100 microM) restored the activity of run-down ROMK1 channels in inside-out patches in oocytes that coexpressed ROMK1 and AKAP79 but not in those that expressed ROMK1 alone. Moreover, when we used RII, the regulatory subunit of type II protein kinase A, in an overlay assay, we identified a RII-binding protein in membranes obtained from the kidney cortex but not in membranes from oocytes. This suggests that the insensitivity of ROMK1 channels to forskolin and cAMP is due to the absence of AKAPs. We conclude that AKAP may be a critical component that mediates the effect of protein kinase A on the ROMK channels in the kidney.
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PMID:The A kinase anchoring protein is required for mediating the effect of protein kinase A on ROMK1 channels. 970 37

ROMK inward-rectifier K+ channels control renal K+ secretion. The activity of ROMK is regulated by protein kinase A (PKA), but the molecular mechanism for regulation is unknown. Having found that direct interaction with membrane phosphatidylinositol 4, 5-bisphosphate (PIP2) is essential for channel activation, we investigate here the role of PIP2 in regulation of ROMK1 by PKA. By using adenosine-5'-[gamma-thio]triphosphate) (ATP[gammaS]) as the substrate, we found that PKA does not directly activate ROMK1 channels in membranes that are devoid of PIP2. Rather, phosphorylation by PKA + ATP[gammaS] lowers the concentration of PIP2 necessary for activation of the channels. In solution-binding assays, anti-PIP2 antibodies bind PIP2 and prevent PIP2-channel interaction. In inside-out membrane patches, antibodies inhibit the activity of the channels. PKA treatment then decreases the sensitivity of ROMK1 for inhibition by the antibodies, indicating an enhanced interaction between PIP2 and the phosphorylated channels. Conversely, mutation of the PKA phosphorylation sites in ROMK1 decreases PIP2 interaction with the channels. Thus, PKA activates ROMK1 channels by enhancing PIP2-channel interaction.
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PMID:Regulation of ROMK1 channel by protein kinase A via a phosphatidylinositol 4,5-bisphosphate-dependent mechanism. 1031 68

The ROMK channel plays an important role in K recycling in the thick ascending limb (TAL) and K secretion in the cortical collecting duct (CCD). A large body of evidence indicates that the ROMK channel is a key component of the native K secretory channel identified in the apical membrane of the TAL and the CCD. Although the ROMK channel shares several key regulatory mechanisms with the native K secretory channel in a variety of respects, differences in the channel modulatory mechanism are clearly present between the ROMK channel and the native K secretory channel. Therefore, it is possible that additional associate proteins are required to interact with the ROMK channel to assemble the native K secretory channel. This notion is supported by recent reports showing that cystic fibrosis transmembrane conductance regulator (CFTR) and A kinase anchoring proteins (AKAP) interact with the ROMK channels to restore the response to ATP sensitivity and protein kinase A stimulation. This review is an attempt to summarize the up-to-date progress regarding the interaction between the ROMK channel and the associate proteins in forming the native K secretory channel.
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PMID:Regulation of the ROMK channel: interaction of the ROMK with associate proteins. 1060 Sep 28

Close similarity between the rat native low-conductance K(+) channel in the apical membrane of renal cortical collecting duct principal cells and the cloned rat ROMK channel strongly suggest that the two are identical. Prominent features of ROMK regulation are a steep pH dependence and activation by protein kinase A (PKA)-dependent phosphorylation. In this study, we investigated the pH dependence of cloned renal K(+) channel (ROMK2), wild-type (R2-WT), and PKA site mutant channels (R2-S25A, R2-S200A, and R2-S294A). Ba(2+)-sensitive outward whole cell currents (holding voltage -50 mV) were measured in two-electrode voltage-clamp experiments in Xenopus laevis oocytes expressing either R2-WT or mutant channels. Intracellular pH (pH(i)) was measured with pH-sensitive microelectrodes in a different group of oocytes from the same batch on the same day. Resting pH(i) of R2-WT and PKA site mutants was the same: 7.32 +/- 0.02 (n = 22). The oocytes were acidified by adding 3 mM Na butyrate with external pH (pH(o)) adjusted to 7.4, 6.9, 6.4, or 5.4. At pH(o) 7.4, butyrate led to a rapid (tau: 163 +/- 14 s, where tau means time constant, n = 4) and stable acidification of the oocytes (DeltapH(i) 0.13 +/- 0. 02 pH units, where Delta means change, n = 12). Intracellular acidification reversibly inhibited ROMK2-dependent whole cell current. The effective acidic dissociation constant (pK(a)) value of R2-WT was 6.92 +/- 0.03 (n = 8). Similarly, the effective pK(a) value of the N-terminal PKA site mutant R2-S25A was 6.99 +/- 0.02 (n = 6). The effective pK(a) values of the two COOH-terminal PKA site mutant channels, however, were significantly shifted to alkaline values; i.e., 7.15 +/- 0.06 (n = 5) for R2-S200A and 7.16 +/- 0.03 (n = 8) for R2-S294A. The apparent DeltapH shift between the R2-WT and the R2-S294A mutant was 0.24 pH units. In excised inside-out patches, alkaline pH 8.5 activated R2-S294A channel current by 32 +/- 6.7%, whereas in R2-WT channel patches alkalinzation only marginally increased current by 6.5 +/- 1% (n = 5). These results suggest that channel phosphorylation may substantially influence the pH sensitivity of ROMK2 channel. Our data are consistent with the hypothesis that in the native channel PKA activation involves a shift of the pK(a) value of ROMK channels to more acidic values, thus relieving a H(+)-mediated inhibition of ROMK channels.
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PMID:PKA site mutations of ROMK2 channels shift the pH dependence to more alkaline values. 1105 53

We have used the patch-clamp technique to explore the role of A Kinase Anchor Proteins (AKAP) in mediating the effect of cAMP on ROMK1 channels expressed in the Xenopus oocytes. Addition of membrane permeant cAMP analogs increased channel activity only in oocytes injected with ROMK1 and AKAP79 cRNA but had no effect on channel activity in oocytes injected with ROMK1 alone. Using the two-electrode voltage clamp technique, we determined that application of H89, a potent inhibitor of protein kinase A (PKA), abolished the stimulatory effect of cAMP/forskolin. To investigate the role of AKAP specificity in conferring cAMP responses to ROMK1 channels, we examined channel activity in oocytes expressing ROMK1 and either AKAP18, AKAP-KL or AKAP75. Addition of cAMP failed to increase channel current in oocytes expressing ROMK1 and either AKAP18 or AKAP-KL. In contrast, cAMP increased ROMK1 channel activity by 33% in oocytes coexpressing AKAP75, the bovine homologue of AKAP79. The effect of cAMP on ROMK1 in oocytes coexpressing AKAP75 is inhibited by H89. Since all three AKAPs bind PKAII, the results suggest that a unique structural domain in AKAP75/79 collaborates with the PKAII binding site and enables a productive association of PKA with ROMK1 channels. Deletion of either the membrane targeting region of AKAP75 (AKAP45) or PKAII binding domain of AKAP75 (AKAP75DeltaC) abolished the effects of forskolin on ROMK1 channels. This suggests that the membrane targeting and the PKA binding domains of AKAP75 are essential for the effect of cAMP. However, the nature of the AKAP, that interacts with ROMK1 in the native tissue, remains to be determined because AKAP75/79 are not expressed in the kidney. We conclude that the regulation of ROMK1 channels by PKA requires the involvement of the cell membrane-directed AKAPs that are able to specifically link PKA to the target channel protein.
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PMID:PKA-induced stimulation of ROMK1 channel activity is governed by both tethering and non-tethering domains of an A kinase anchor protein. 1141 Jul 9


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