Phosphorylation of low molecular weight proteins in purified preparations of rat heart sarcolemma and sarcoplasmic reticulum

Phosphorylation of purified bovine cardiac sarcolemma and potassium-stimulated calcium uptake

Sarcolemmal vesicles were prepared from bovine cardiac muscle by differential and discontinuous sucrose density gradient centrifugation. Na+/K+-ATPase was purified 33-fold to a specific activity of 53 +/- 0.5 (12) mumol Pi X mg-1 X h-1, binding sites for strophantin 20-fold to a density of 56.3 +/- 5.3 (14) pmol/mg and that for the calcium antagonist nitrendipine 5.5-fold to a density of 0.72 +/- 0.07 (6) pmol/mg. The specific activity of the Na+/Ca2+ exchanger was 61.1 +/- 3.7 (6) nmol/mg. The vesicles had an intravesicular volume of 20 +/- 4 (4) microliter/mg and 56.9 +/- 6 (4)% of the vesicles were right-side-out oriented. Several peptides of the purified membranes were phosphorylated in the presence of Mg . ATP and EGTA. Most of the radioactive phosphate was incorporated into a peptide with an apparent molecular mass of 22 kDa. Denaturation of the membranes at 100 degrees C changed the mobility of this peptide to 15 kDa and 11 kDa. This peptide could not be distinguished from a sarcoplasmic reticulum peptide of similar molecular mass. The phosphorylation of the sarcolemmal peptide was stimulated by Ca2+/calmodulin, cAMP and the catalytic subunit of cAMP-dependent protein kinase. A comparison of the phosphorylation of sarcolemmal membranes with that of sarcoplasmic reticulum showed that Ca2+/calmodulin stimulated in each membrane, the phosphorylation of the 22-kDa peptide and a 44-kDa peptide, and in the sarcoplasmic reticulum the phosphorylation of an additional peptide of 55-kDa. Ca2+/calmodulin-dependent phosphorylation of a 55-kDa peptide could not be demonstrated in sarcolemma, regardless if sarcolemmal membranes were incubated together with sarcoplasmic reticulum or if the phosphorylation was carried out in the presence of purified cardiac myosin light chain kinase or phosphorylase kinase. 'Depolarization' induced Ca2+ uptake which was measured according to Bartschat, D.K., Cyr, D.L. and Lindenmayer, G.E. [(1980) J. Biol. Chem. 255, 10044-10047] was 5 nmol/mg protein. This uptake was not enhanced after preincubation of the vesicles with Mg . ATP or Mg . ATP and cAMP-dependent protein kinase. The value of 5 nmol/mg protein is in agreement with the theoretical amount of Ca2+ which can be accumulated by the bovine cardiac sarcolemma in the absence of a driving force other than the Ca2+ gradient. The potassium-stimulated Ca2+ uptake was not blocked by the organic Ca2+ channel blockers. Prolonged incubation of Mg . ATP with sarcolemmal vesicles in the presence of various ATPase inhibitors led to the hydrolysis of ATP. The liberated phosphate precipitated with Ca2+ in the presence of LaCl3. These precipitates amounted to an apparent Ca2+ uptake ranging from 50 to over 1000 nmol/mg. The results suggest that potassium-stimulated Ca2+ uptake of bovine cardiac sarcolemmal vesicles is not enhanced in the presence of ATP or by phosphorylation of a 22-kDa peptide.

Phospholamban of cardiac sarcoplasmic reticulum consists of two functionally distinct proteolipids

Phosphorylation of phospholamban by either a cAMP-dependent or a calmodulin-dependent kinase stimulates the Ca2+ transporting activity of cardiac sarcoplasmic reticulum membranes. It has now been found that phospholamban consists of 2 distinct proteins; one is the specific substrate for the cAMP-dependent phosphorylation, and the other for the calmodulin-dependent kinase. In spite of functional diversity, the 2 polypeptides share a number of properties. Among them, the proteolipid character, Mr, resistance to trypsinization, and subunit composition.

Injection of catalytic subunit of cAMP-dependent protein kinase into isolated cardiac myocytes

Cyclic adenosine 3',5'-monophosphate (cAMP) or the free catalytic subunit (C) of the cAMP-dependent protein kinase were pressure injected into single guinea pig ventricular cells. The following results were obtained: Injection of cAMP prolonged the action potential and shifted the action potential plateau to a more positive level. Under voltage clamp, cAMP injection increased the amplitude of the slow inward calcium current (Isi). Injection of C permanently prolonged the action potential and enhanced the amplitude of Isi by a factor of 2-4, depending on the amount of injected C. In the current-voltage relations the potential of maximum Isi and the apparent current reversal did not change. After maximum prolongation of the action potential due to repeated injections of C, even high concentrations of adrenaline did not further change the configuration of the action potential. In many experiments transient depolarizations appeared after the injection. Correspondingly, under voltage clamp transient inward currents occurred. C injection increased both the time-dependent and time-independent potassium outward current. In response to injection of the catalytic subunit, the isotonic contraction was larger in amplitude and relaxation was faster. It is concluded that the cAMP-dependent protein kinase increases the slow inward calcium current in the heart, presumably by phosphorylation of some membrane proteins.

Early presence of phospholamban in developing a chick heart

Phosphorylation of phospholamban and development of reticular Ca2+ transport were studied in crude membrane preparations of embryonic, newborn and adult chick heart. Maximal phosphorylation of phospholamban by added catalytic subunit of cyclic AMP-dependent protein kinase increases from embryonic day 4-15. It decreases with further development. In the same membrane preparations active Ca2+-uptake into vesicles of sarcoplasmic reticulum rises from day 4-7 and decreases then slightly until day 20. A several-fold increase in Ca2+-transport activity occurs at the time of hatching. The data indicate separate genetic control for synthesis of phospholamban and sarcoplasmic reticulum Ca2+-ATPase.

Properties of 5'-nucleotidase in rat heart sarcolemma

The activity of 5'-nucleotidase (5'-ribonucleotide phosphohydrolase, EC 3.1.3.5) was examined in membrane fractions isolated by hypotonic shock-LiBr treatment (fraction HL) and sucrose gradient separation (fraction S) of rat ventricle homogenate. The enzyme activity in these two fractions differed significantly in several respects. In fraction HL, 5'-nucleotidase had a high affinity for AMP (Km 35 microM), and ATP was a potent competitive inhibitor. In contrast, the 5'-nucleotidase displayed by fraction S showed a low substrate affinity (Km 130 microM) and less sensitivity to ATP. Treatment of membranes with trypsin and neuraminidase markedly stimulated 5'-nucleotidase in fraction HL, whereas only a modest effect was observed in fraction S. Exposure of the membranes to Triton X-100 resulted in a 60% and 10% increase in the enzyme activity in fractions HL and S, respectively. The characteristic activity ratios of 5'-nucleotidase at 200 microM relative to 50 microM AMP in fractions HL and S were modified by alamethicin in an opposite way and became identical. Although concanavalin A almost completely inhibited the 5'-nucleotidase activity in both membrane preparations at a concentration of 2 microM, Hill plots of the data on concanavalin A inhibition revealed a coefficient of 2.2 for fraction S and 1.1 for fraction HL. The differences in 5'-nucleotidase activity of the two membrane fractions are considered to be due to differences in the orientation of the vesicles of the sarcolemmal preparations. These results suggest that two distinct catalytic sites for 5'-nucleotidase are present at the intra- and extracellular surface of the rat heart sarcolemma.

Isolation of cardiac membrane proteolipids by high pressure liquid chromatography. A comparison of reticular and sarcolemmal proteolipids, phospholamban and calciductin

Membrane-bound phosphorylatable proteolipids were reported to play a role in the regulation of transmembrane Ca2+ fluxes by catecholamines. A generally applicable purification procedure is described by which such proteolipids as the cardiac sarcoplasmic reticulum phospholamban is purified by solvent extraction followed by high pressure liquid chromatography on microparticulate silica. Phospholamban is thereby purified with a yield of 3.37 mg from 100 mg of sarcoplasmic reticulum proteins, significantly higher than that obtained by any of the previously reported procedures. It appeared homogeneous upon dodecyl sulfate-polyacrylamide gel electrophoresis where it is stained by Coomassie blue and detected by autoradiography. The same procedure is applicable to cardiac sarcolemmal calciductin. Both proteolipids exhibit the same Mr 11 000 and pI 3.7 upon two-dimensional gel electrophoresis. Their amino acid compositions are very similar if not identical. This raises the intriguing possibility that phospholamban and calciductin are identical though they obviously belong to different membranes.

Comparison of cyclic AMP-dependent phosphorylation of sarcolemma and sarcoplasmic reticulum from rat cardiac ventricle muscle

1. The phosphorylation by cAMP and protein kinase I of rat cardiac sarcolemma (SL) and sarcoplasmic reticulum (SR) isolated from the same homogenate, was compared. 2. In both fractions, the phosphate incorporation is strongly dependent on the ATP and the membrane protein concentration. 3. SDS-gel electrophoresis reveals that in the SL preparation a protein of Mr = 24,500 and a glycoprotein of Mr = 17,500 are mainly phosphorylated, while in the SR fraction the main phosphate incorporation is found in a protein having a Mr = 37,000. 4. Isoprenaline stimulates the phosphorylation of SL but not of SR. Propranolol abolished that stimulatory action of isoprenaline completely, suggesting that the beta-adrenoceptor is involved.

Chick heart plasma membranes. Isolation and analysis of autophosphorylation

Plasma membranes have been isolated from hearts of 10-day embryonic and newborn chicks. The membranes obtained were highly enriched in muscarinic acetylcholine receptors, K+ -stimulated, ouabain-sensitive p-nitrophenylphosphatase and 5'-nucleotidase. There was little contamination of the membrane fractions by the mitochondrial membranes or by contractile proteins. The autophosphorylation of the isolated membrane fractions was analyzed by measuring 32P incorporation from [gamma-32P]ATP into total membrane protein and into individual membrane components. Membranes obtained from embryonic hearts contained significantly more cAMP-dependent and -independent protein kinase activities than membranes from newborn chick hearts. Treatment of the membranes with Triton X-100 or the peptide ionophore alamethicin increased phosphorylation in membranes from either newborn or embryonic hearts. Membranes from embryonic hearts contained substrates for membrane-bound cAMP-dependent and -independent protein kinases either not observed or present in low amount in membranes from newborn hearts, and vice-versa. Notably, a 38 kDa protein was markedly phosphorylated by endogenous cAMP dependent protein kinase in plasma membrane enriched fractions from embryonic hearts. This phosphoprotein was not easily detected in any fraction obtained from newborn hearts. One cAMP-dependent phosphoprotein had an Mr of 27000 or 11000, depending on the conditions used to solubilize it. This protein was present in sarcolemma-enriched membranes as well as membrane fractions containing sarcoplasmic reticulum. There was more of this phosphoprotein in newborn heart membranes than in embryonic hearts. The phosphorylation of this protein was markedly enhanced by the peptide ionophore alamethicin. A second cAMP-dependent phosphoprotein with an Mr of 27000 was also detected in the sarcolemma-enriched membranes.

Phosphorylation of low-molecular-weight proteins in preparations of rat heart sarcolemma and sarcoplasmic reticulum

Two substrate proteins for cAMP-dependent protein kinase detected in a rat heart sarcolemma preparation displayed molecular weights of 24,000 and 9000 in sodium dodecyl sulfate gels and were shown to be interconvertible. The 9000-dalton protein could readily be separated from other low molecular weight phosphoproteins (mol. wt. 14,000 and 7000) by the use of 15% polyacrylamide gels. In addition to an endogenous cAMP-dependent protein kinase the membrane preparation also contained a protein-phosphorylation system that required Ca2+ and calmodulin. It appeared that both 24,000- and 55,000-dalton proteins were substrates for the endogenous Ca2+- and calmodulin-dependent protein kinase. Contaminating sarcoplasmic reticulum vesicles, first loaded with calcium oxalate, could be separated from the enriched sarcolemma preparation by sucrose gradient centrifugation. The separation was confirmed by comparative analysis of 5'-nucleotidase, Na+ -Ca2+ antiporter, and (Ca2+ + Mg2+)-dependent ATPase activities and by determination of gel electrophoretic (phospho)protein composition, sialic acid, cholesterol, and phospholipid contents. The 24,000-dalton phosphoprotein complex was equally distributed between sarcolemmal and sarcoplasmic reticulum fractions, whereas the 55,000- and 7000-dalton proteins were predominantly found in the sarcolemmal fraction. The 24,000-dalton protein was most likely phospholamban, because no other phosphoprotein was found in the 20,000 molecular weight range.

An electrogenic Na+/Ca2+ antiporter in addition to the Ca2+ pump in cardiac sarcolemma

Vesicles isolated from rat heart, particularly enriched in sarcolemma markers, were examined for their sidedness by investigation of side-specific interactions of modulators with the asymmetric (Na+ + K+)-ATPase and adenylate cyclase complex. The membrane preparation with the properties expected for inside-out vesicles showed the highest rate of ATP-driven Ca2+ transport. The Ca2+ pump was stimulated 1.7- and 2.1-fold by external Na+ and K+, respectively, the half-maximal activation occurring at 35 mM monovalent cation concentration. In vesicles loaded with Ca2+ by pump action in a medium containing 160 mM KCl, a slow spontaneous release of Ca2+ started after 2 min. The rate of this release could be dramatically increased by the addition of 40 mM NaCl to the external medium. In contrast, 40 mM KCl exerted no appreciable effect on vesicles loaded with Ca2+ in a medium containing 160 mM NaCl. Ca2+ movements were also studied in the absence of ATP and Mg2+. Vesicles containing an outwardly directed Na+ gradient showed the highest Ca2+ uptake activity. These findings suggested the operation of a Ca2+/Na+ antiporter in addition to the active Ca2+ pump in these sarcolemmal vesicles. A valinomycin-induced inward K+-diffusion potential stimulated the Na+-Ca2+ exchange, suggesting its electrogenic nature. If in the absence of ATP and Mg2+ the transmembrane Nai+/Nao+ gradient exceeded 160/15 mM concentrations, Ca2+ uptake could be stimulated by the addition of 5 mM oxalate, indicating Na+ gradient-induced Ca2+ uptake to be a translocation of Ca2+ to the lumen of the vesicle. A sarcoplasmic reticulum contamination, removed by further sucrose gradient fractionation, contained rather low Na+-Ca2+ exchange activity. This result suggests that the activity can be entirely accounted for by the sarcolemmal content of the cardiac membrane preparation.