Role of G protein beta gamma subunits in the regulation of the plasma membrane Ca2+ pump

Analysis of cholesterol-recognition motifs of the plasma membrane Ca2+-ATPase

The plasma membrane Ca2+-ATPase (PMCA) is crucial for the fine tuning of intracellular calcium levels in eukaryotic cells. In this study, we show the presence of CARC sequences in all human and rat PMCA isoforms and we performed further analysis by molecular dynamics simulations. This analysis focuses on PMCA1, containing three CARC motifs, and PMCA4, with four CARC domains. In PMCA1, two CARC motifs reside within transmembrane domains, while the third is situated at the intracellular interface. The simulations depict more stable RMSD values and lower RMSF fluctuations in the presence of cholesterol, emphasizing its potential stabilizing effect. In PMCA4, a distinct dynamic was found. Notably, the total energy differences between simulations with cholesterol and phospholipids are pronounced in PMCA4 compared to PMCA1. RMSD values for PMCA4 indicate a more energetically favorable conformation in the presence of cholesterol, suggesting a robust interaction between CARCs and this lipid in the membranes. Furthermore, RMSF analysis for CARCs in both PMCA isoforms exhibit lower values in the presence of cholesterol compared to POPC alone. The analysis of H-bond occupancy and total energy values strongly suggests the potential interaction of CARCs with cholesterol. Given the crucial role of PMCAs in physiological calcium regulation and their involvement in diverse pathological processes, this study underscores the significance of CARC motifs and their interaction with cholesterol in elucidating PMCA function. These insights into the energetic preferences associated with CARC-cholesterol interactions offer valuable implications for understanding PMCA function in maintaining calcium homeostasis and addressing potential associated pathologies.

Immunohistochemical localization of G protein betagamma subunits in the lateral wall of the rat cochlea

The role of G protein-mediated signal transduction in the production of endolymph, an extracellular fluid of unusual ionic composition, is beginning to be understood. The identity of Galpha subunits in the stria vascularis and the spiral ligament of the lateral wall of the cochlear duct is well established. However, little is known about the presence of betagamma subunits. This study used immunohistochemistry to investigate the distribution of G protein betagamma subunits in the lateral wall of the cochlea. Temporal bones of 6- to 8-week-old rats were fixed in 4% paraformaldehyde and 0.1% glutaraldehyde and processed for embedding in paraffin wax. The dewaxed, midmodiolar sections of the cochlea were incubated with subunit-specific polyclonal antibodies. The results show that the pattern of immunoreactivity varies for the G protein beta1-4 and gamma1-3, 5 and 7 subunits in the stria vascularis and spiral ligament. In the stria vascularis, immunoreactivity was detected for beta2, beta3, beta4, gamma1, gamma2 and gamma7 subunits. All five types of fibrocytes in the spiral ligament exhibited positive staining for gamma2 and gamma7. However, immunoreactivity for beta1-4 subunits was variable. Immunoreactivity for gamma3 and gamma5 subunits was not detected in the lateral cochlear wall. The expression pattern of G protein betagamma subunits in lateral wall provides a basis for interpreting the functions of G protein-coupled receptors in cochlear fluid homeostasis.

Is there a specific role for the plasma membrane Ca2+ -ATPase in the hepatocyte?

The plasma membrane Ca2+ -ATPase (PMCA) is responsible for the fine, long-term regulation of the cytoplasmic calcium concentration by extrusion of this cation from the cell. Although the general kinetic mechanisms for the action of both, well coordinated hydrolytic activity and calcium transport are reasonably understood in the majority of cell types, due to the complex physiologic and biochemical characteristics shown by the hepatocyte, the study of this enzyme in this cell type has become a real challenge. Here, we review the various molecular aspects known to date to be associated with liver PMCA activity, and outline the strategies to follow for establishing the role of this enzyme in the overall physiology of the hepatocyte. In this way, we first concentrate on the basic biochemical aspects of liver cell PMCA, and place an important emphasis on expression of its molecular forms to finally focus on the critical hormonal regulation of the enzyme. Although these complex aspects have been studied mainly under normal conditions, the significance of PMCA in the calcium homeostasis of an abnormal liver cell is also reviewed.

MDM2 promotes p21waf1/cip1 proteasomal turnover independently of ubiquitylation

The CDK inhibitor p21waf1/cip1 is degraded by a ubiquitin-independent proteolytic pathway. Here, we show that MDM2 mediates this degradation process. Overexpression of wild-type or ring finger-deleted, but not nuclear localization signal (NLS)-deleted, MDM2 decreased p21waf1/cip1 levels without ubiquitylating this protein and affecting its mRNA level in p53(-/-) cells. This decrease was reversed by the proteasome inhibitors MG132 and lactacystin, by p19(arf), and by small interfering RNA (siRNA) against MDM2. p21waf1/cip1 bound to MDM2 in vitro and in cells. The p21waf1/cip1-binding-defective mutant of MDM2 was unable to degrade p21waf1/cip1. MDM2 shortened the half-life of both exogenous and endogenous p21waf1/cip1 by 50% and led to the degradation of its lysine-free mutant. Consequently, MDM2 suppressed p21waf1/cip1-induced cell growth arrest of human p53(-/-) and p53(-/-)/Rb(-/-)cells. These results demonstrate that MDM2 directly inhibits p21waf1/cip1 function by reducing p21waf1/cip1 stability in a ubiquitin-independent fashion.

The Stoichiometry of Gbeta gamma binding to G-protein-regulated inwardly rectifying K+ channels (GIRKs)

G-protein-coupled inwardly rectifying K(+) (GIRK; Kir3.x) channels are the primary effectors of numerous G-protein-coupled receptors. GIRK channels decrease cellular excitability by hyperpolarizing the membrane potential in cardiac cells, neurons, and secretory cells. Although direct regulation of GIRKs by the heterotrimeric G-protein subunit Gbetagamma has been extensively studied, little is known about the number of Gbetagamma binding sites per channel. Here we demonstrate that purified GIRK (Kir 3.x) tetramers can be chemically cross-linked to exogenously purified Gbetagamma subunits. The observed laddering pattern of Gbetagamma attachment to GIRK4 homotetramers was consistent with the binding of one, two, three, or four Gbetagamma molecules per channel tetramer. The fraction of channels chemically cross-linked to four Gbetagamma molecules increased with increasing Gbetagamma concentrations and approached saturation. These results suggest that GIRK tetrameric channels have four Gbetagamma binding sites. Thus, GIRK (Kir 3.x) channels, like the distantly related cyclic nucleotide-gated channels, are tetramers and exhibit a 1:1 subunit/ligand binding stoichiometry.

Role of alternative splicing in generating isoform diversity among plasma membrane calcium pumps

Calcium pumps of the plasma membrane (also known as plasma membrane Ca(2+)-ATPases or PMCAs) are responsible for the expulsion of Ca(2+) from the cytosol of all eukaryotic cells. Together with Na(+)/Ca(2+) exchangers, they are the major plasma membrane transport system responsible for the long-term regulation of the resting intracellular Ca(2+) concentration. Like the Ca(2+) pumps of the sarco/endoplasmic reticulum (SERCAs), which pump Ca(2+) from the cytosol into the endoplasmic reticulum, the PMCAs belong to the family of P-type primary ion transport ATPases characterized by the formation of an aspartyl phosphate intermediate during the reaction cycle. Mammalian PMCAs are encoded by four separate genes, and additional isoform variants are generated via alternative RNA splicing of the primary gene transcripts. The expression of different PMCA isoforms and splice variants is regulated in a developmental, tissue- and cell type-specific manner, suggesting that these pumps are functionally adapted to the physiological needs of particular cells and tissues. PMCAs 1 and 4 are found in virtually all tissues in the adult, whereas PMCAs 2 and 3 are primarily expressed in excitable cells of the nervous system and muscles. During mouse embryonic development, PMCA1 is ubiquitously detected from the earliest time points, and all isoforms show spatially overlapping but distinct expression patterns with dynamic temporal changes occurring during late fetal development. Alternative splicing affects two major locations in the plasma membrane Ca(2+) pump protein: the first intracellular loop and the COOH-terminal tail. These two regions correspond to major regulatory domains of the pumps. In the first cytosolic loop, the affected region is embedded between a putative G protein binding sequence and the site of phospholipid sensitivity, and in the COOH-terminal tail, splicing affects pump regulation by calmodulin, phosphorylation, and differential interaction with PDZ domain-containing anchoring and signaling proteins. Recent evidence demonstrating differential distribution, dynamic regulation of expression, and major functional differences between alternative splice variants suggests that these transporters play a more dynamic role than hitherto assumed in the spatial and temporal control of Ca(2+) signaling. The identification of mice carrying PMCA mutations that lead to diseases such as hearing loss and ataxia, as well as the corresponding phenotypes of genetically engineered PMCA "knockout" mice further support the concept of specific, nonredundant roles for each Ca(2+) pump isoform in cellular Ca(2+) regulation.

Dissection of the cAMP induced cytosolic calcium response in Dictyostelium discoideum: the role of cAMP receptor subtypes and G protein subunits

The cAMP signaling cascade leading to changes in [Ca2+]i in Dictyostelium discoideum was analyzed using cell lines overexpressing single cAMP receptor subtypes (cAR1-cAR3) or lacking the G(alpha2) or G(beta) subunit of the G protein. Imaging of fura2-dextran-loaded amoebae revealed cAMP-induced [Ca2+]i changes characteristic for each receptor subtype activated. Cells expressing distinct subtypes sort to defined zones during multicellular development suggesting involvement of the specific [Ca2+]i transients in patterning processes. Whereas generation of the [Ca2+]i increase was G(alpha2)-independent, only few cells devoid of G(beta) displayed a [Ca2+]i change after stimulation indicating its participation in the regulation of the calcium homeostasis.

G protein beta gamma subunits

Guanine nucleotide binding (G) proteins relay extracellular signals encoded in light, small molecules, peptides, and proteins to activate or inhibit intracellular enzymes and ion channels. The larger G proteins, made up of G alpha beta gamma heterotrimers, dissociate into G alpha and G beta gamma subunits that separately activate intracellular effector molecules. Only recently has the G beta gamma subunit been recognized as a signal transduction molecule in its own right; G beta gamma is now known to directly regulate as many different protein targets as the G alpha subunit. Recent X-ray crystallography of G alpha, G beta gamma, and G alpha beta gamma subunits will guide the investigation of structure-function relationships.

Heterotrimeric G proteins in heart disease

Heterotrimeric G proteins couple many types of cell surface receptors to intracellular effectors such as enzymes or ion channels. In the mammalian heart, G protein-mediated signalling pathways are involved in the regulation of contractile force, heart rate, conduction velocity, and relaxation. In the first part of this review we summarize some important structural and functional features of receptors, G proteins, and effectors with special focus on the heart. In the second part, we review the current knowledge about alterations of G protein-mediated signalling in heart disease such as myocardial hypertrophy and heart failure.

Stimulatory effect of hormonal signaling factors on (Ca(2+)-Mg2+)-ATPase activity in rat liver plasma membranes: cross talk with regucalcin

The effect of hormonal signaling factors on (Ca(2+)-Mg2+)-ATPase activity in rat liver plasma membranes was investigated. The presence of inositol-glycan (10(-7)-10(-5) M), dibutyryl cAMP (10(-4) and 10(-3) M) or inositol 1,4,5-trisphosphate (IP3; 10(-6) and 10(-5) M) in the enzyme reaction mixture produced a significant increase in (Ca(2+)-Mg2+)-ATPase activity. These effects were completely inhibited by the presence of vanadate (10(-4) M), an inhibitor of the enzyme phosphorylation, and N-ethylmaleimide (5 x 10(-3) M), a SH group modifying reagent. Meanwhile, regucalcin, a Ca(2+)-binding protein isolated from rat liver cytosol, increased the enzyme activity by binding to the SH groups of (Ca(2+)-Mg2+)-ATPase in liver plasma membranes. The presence of regucalcin (0.25 microM) with an effective concentration completely inhibited the effect of inositol-glycan (10(-5) M) to increase (Ca(2+)-Mg2+)-ATPase activity, while the effect of dibutyryl cAMP (10(-3) M) or IP3 (10(-5) M) was not altered. The inositol-glycan effect was not modulated by the presence of dibutyryl cAMP or IP3. Now, the preincubation of the plasma membranes with regucalcin did not modify the effect of inositol-glycan on the enzyme activity, suggesting that regucalcin competes with inositol-glycan for the binding to the plasma membranes. The present results suggest that there may be a cross talk with regucalcin and hormonal signaling factors in the regulation of (Ca(2+)-Mg2+)-ATPase activity in liver plasma membranes.

Prolonged exposure of hamsters to cold changes the levels of G proteins in brown adipose tissue plasma membranes

The levels of G proteins in plasma membranes prepared from brown adipose tissue of control and cold-exposed hamsters were determined by quantitative immunoblotting and competitive ELISA. Prolonged (four weeks) exposure of hamsters to cold decreased significantly the total content of the alpha subunits of the stimulatory (Gs alpha) as well as inhibitory (Gi alpha (1,2)) G proteins. Interestingly, the reduction in the Gs alpha content was solely due to a large reduction in the content of the short (45 kDa) isoform of Gs alpha, while the level of the long (52 kDa) isoform of Gs alpha remained unchanged. The level of the beta subunit of G protein was decreased comparably to the reduction in the total content of the alpha subunits. Cold-induced alterations in the G protein network associated with plasma membranes of brown adipose tissue were accompanied by changed characteristics of AlF(4-)-sensitive adenylyl cyclase activity.

Seven helix cAMP receptors stimulate Ca2+ entry in the absence of functional G proteins in Dictyostelium

Surface cAMP receptors (cARs) in Dictyostelium transmit a variety of signals across the plasma membrane. The best characterized cAR, cAR1, couples to the heterotrimeric guanine nucleotide-binding protein (G protein) alpha-subunit G alpha 2 to mediate activation of adenylyl and guanylyl cyclases and cell aggregation. cAR1 also elicits other cAMP-dependent responses including receptor phosphorylation, loss of ligand binding (LLB), and Ca2+ influx through a G alpha 2-independent pathway that may not involve G proteins. Here, we have expressed cAR1 and a related receptor, cAR3, in a g beta- strain (Lilly, P., Wu. L., Welker, D. L., and Devreotes, P. N. (1993) Genes & Dev. 7,986-995), which lacks G protein activity. Both cell lines failed to aggregate, a process requiring the G alpha 2 and G beta- subunits. In contrast, cAR1 phosphorylation in cAR1/g beta- cells showed a time course and cAMP dose dependence indistinguishable from those of cAR1/G beta+ controls. cAMP-induced LLB was also normal in the cAR1/g beta- cells. Finally, cAR1/g beta- cells and cAR3/g beta- cells showed a Ca2+ response with kinetics, agonist dependence, ion specificity, and sensitivity to depolarization agents that were like those of G beta+ controls, although they accumulated fewer Ca2+ ions per cAMP receptor than the control strains. Together, these results suggest that the G beta-subunit is not required for the activation or attenuation of cAR1 phosphorylation, LLB, or Ca2+ influx. It may, however, serve to amplify the Ca2+ response, possibly by modulating other intracellular Ca2+ signal transduction pathways.

Lactogen enhances Nb2 cell GTPase activity after 4 hours incubation

The lactogen receptor has been suggested to associate with one or more G proteins despite the absence of a 7-transmembrane spanning sequence. These studies were designed to determine whether lactogens acutely increase GTP binding to or GTPase activity in Nb2 cell membrane. Incubation of Nb2 cell membrane with either ovine PRL (10 ng/ml) or diluent for 0-1 h resulted in a decrease in total(35)S-GTP binding to both with no difference in GTP binding between PRL- and diluent-treated membranes. There was also no change in(35)S-GTP binding to Nb2 cell membrane incubated with increasing oPRL concentrations (0.001-100 ng/ml) for 60 min. α-(32)P-GTP photoaffinity labelling was used to evaluate changes in GTP binding to specific G proteins. Photoaffinity labelling of α-(32)P-GTP to no G protein was changed after preincubation with oPRL (10 ng/ml) for 0-60 min or with oPRL (0.01-10 ng/ml) for 60 min. Finally, it was determined whether oPRL had any acute effect on GTPase activity, as determined by release of(32)Pi from γ-(32)P-GTP. When Nb2 cell membrane was preincubated for 0-60 min with oPRL (10 ng/ml) or a range of oPRL concentrations (0-10 ng/ml), no change in GTPase activity was observed. However, when Nb2 cells were incubated with lactogen for 0-7 h, GTPase activity in equal quantities of Nb2 cell membrane prepared from those cells increased over time. Increased GTPase activity (64.9-74.4%;P<0.03 compared to 0 h) was observed after 4-7 h incubation with lactogen.In summary, addition of lactogen to Nb2 cell membrane did not acutely increase either GTP binding or GTPase activity. Yet when Nb2 cells were incubated with lactogen for 4 h prior to preparation of membrane, GTPase activity was significantly increased. This evidence, in addition to our previous results showing that 4 h incubation with lactogen increased G protein β subunit concentration and pertussis toxin-stimulated ADP-ribosylation of Gi, support a role for delayed lactogen modulation of one or more G proteins in the Nb2 cell, requiring at least 4 h for maximal effect.

Regucalcin modulates hormonal effect on (Ca(2+)-Mg2+)-ATPase activity in rat liver plasma membranes

The interaction of various hormones and regucalcin on (Ca(2+)-Mg2+)-ATPase activity in rat liver plasma membranes was investigated. The presence of epinephrine (10(-6)-10(-4) M), phenylephrine (10(-6)-10(-4) M), and insulin (10(-8)-10(-7) M) in the reaction mixture produced a significant increase in (Ca(2+)-Mg7+)-ATPase activity, while the enzyme activity was decreased significantly by calcitonin (3 x 10(-8)-3 x 10(-6) M). These hormonal effects, except for calcitonin, were clearly inhibited by the presence of vanadate (10(-4) M) which can inhibit the Ca(2+)-dependent phosphorylation of enzyme. Meanwhile, regucalcin (0.25 and 0.50 microM), isolated from rat liver cytosol, elevated significantly (Ca(2+)-Mg2+)-ATPase activity in the plasma membranes, although this elevation was not inhibited by vanadate (10(-4) M). The epinephrine (10(-5) M) or phenylephrine (10(-4) M)-induced increase in (Ca(2+)-Mg2+)-ATPase activity was disappeared in the presence of regucalcin; in this case the effect of regucalcin was also weakened. However, the inhibitory effect of calcitonin (3 x 10(-6) M) was not weakened by the presence of regucalcin (0.5 microM). Moreover, GTP (10(-5) and 10(-4) M)-induced increase in (Ca(2+)-Mg2+)-ATPase activity was not seen in the presence of regucalcin (0.25 microM). The present finding suggests that the activating mechanism of regucalcin on (Ca(2+)-Mg2+)-ATPase is not involved on GTP-binding protein which modulates the receptor-mediated hormonal effect in rat liver plasma membranes.

Stable changes in expression or activation of G protein alpha i or alpha q subunits affect the expression of both beta 1 and beta 2 subunits

G proteins consist of three subunits: alpha, beta and gamma. Four beta subunits have been cloned: beta 1 and beta 4 (36 kDa), and beta 2 and beta 3 (35 kDa). We studied endogenous beta subunits in mouse NIH 3T3 fibroblasts stably expressing high levels of G protein alpha subunits after transfection with cDNAs encoding alpha i1, alpha i2, alpha i3 and alpha q. Immunoblots showed that NIH 3T3 cells express beta 36 and beta 35 subunits; in these cells, beta 35 subunits are four times more abundant than beta 36 subunits. We could detect beta 1 and beta 2 mRNA, but neither beta 3 nor beta 4 mRNA. We found that a stable increase in expression of wild-type alpha i1, alpha i2, alpha i3 or alpha q subunits is always accompanied by an increase in beta 1 and beta 2 mRNA and protein levels. There was no evidence of selectivity for an increase in beta 1 rather than beta 2 subunits depending on the type of alpha subunit overexpressed. However, constitutive activation or inactivation of alpha subunits induced specific changes in beta subunits. Expression of constitutively inactivated alpha i2 subunits was accompanied by an increase in mRNA and protein levels of both beta subunits. In contrast, cells expressing constitutively activated alpha i2 subunits did not show any change in the amount of beta proteins expressed in membranes, despite a significant increase in beta 1 and beta 2 mRNA. We conclude that stable changes in the levels of expression or degree of activation of G alpha subunits affect the level of expression, and possibly the turn-over, of beta subunits, without selectivity among beta 1 and beta 2 subunits.