Phosphorylation of the inhibitory subunit of troponin in perfused hearts of mice deficient in phosphorylase kinase. Evidence for the phosphorylation of troponin by adenosine 3':5'-phosphate-dependent protein kinase in vivo

Inhibition of phosphorylation of troponin I in rat heart by adenosine and 5'-chloro-5'-deoxyadenosine

We have investigated the effects of adenosine on protein phosphorylation in extracts of rat heart. Incubation of a myofibrillar fraction with [gamma-32P]ATP resulted in the phosphorylation of several proteins by endogenous protein kinases. The adenosine analog 5'-chloro-5'-deoxyadenosine inhibited the phosphorylation of a 29 kD protein in this preparation. The protein was identified as cardiac troponin I (cTnI) by two-dimensional gel electrophoresis, using purified cTnI as standard. Addition of the catalytic subunit of cAMP-dependent protein kinase to the myofibrillar fraction increased phosphorylation of cTnI; this increase was inhibited by 5'-chloro-5'-deoxyadenosine and adenosine. Phosphorylation of purified cTnI by the catalytic subunit was also inhibited by 5'-chloro-5'-deoxyadenosine. Under these conditions used, 50% inhibition of phosphorylation by either endogenous or exogenous kinase was observed at approximately 50 microM 5'-chloro-5'-deoxyadenosine or adenosine. The inhibition described here occurred independently of catecholamines. The effects of ADP, AMP, and adenine on cTnI phosphorylation are also described.

Effects of phosphorylated and unphosphorylated C-protein on cardiac actomyosin ATPase

C-protein, a component of the thick filaments of striated muscles, is reversibly phosphorylated and dephosphorylated in heart. It has been hypothesized that C-protein may be involved in regulating contraction, because the extent of C-protein phosphorylation correlates with the rate of cardiac relaxation. To test this hypothesis, the effects of phosphorylated and unphosphorylated C-protein on the actin-activated ATPase activity of myosin filaments prepared from DEAE-Sephadex-purified myosin were examined. Unphosphorylated C-protein (0.1 microM to 1.5 microM) stimulated actin-activated myosin ATPase activity in a dose-dependent manner. With a myosin: C-protein molar ratio of approximately 1, actin-activated myosin ATPase activity was elevated up to 3.2 times that of the control. Phosphorylated C-protein (2.5 mol PO4/mol C-protein) stimulated the activity somewhat less (2.5 times that of control). The stimulation of ATPase activity by C-protein was due to an increase in the Vmax value (from 0.25/second to 0.62/second) and a decrease in the Km value (from 11.9 microM to 6.7 microM). The addition of C-protein to actomyosin solutions produced an increase in the light-scattering of the actomyosin solution and a distinct precipitation of the actomyosin with time. Phosphorylated C-protein had a smaller effect on light-scattering than dephosphorylated C-protein. C-protein had a negligible effect on Ca-ATPase, EDTA-K-ATPase, or Mg-ATPase activities in the absence of actin. C-protein had only small effects on the actin-activated ATPase of heavy meromyosin. These results suggest that C-protein stimulates actin-activated myosin ATPase activity by enhancing the formation of stable aggregates between actin and myosin filaments.

The phosphorylase kinase activity of hearts from phosphorylase kinase-deficient mice

In an assay measuring radioactive incorporation from gamma--P32P]ATP into phosphorylase b, cardiac muscle extracts from mice with the phosphorylase kinase deficiency mutation showed significant, calcium-dependent phosphorylase kinase activity that was 10 to 15% of that of Swiss mice, the control strain. Isoproterenol stimulated significant phosphorylase a accumulation in both isolated atria and right ventricular strips of phosphorylase kinase-deficient mice, and the drug-stimulated increases in phosphorylase a activity the the contractile responses of right ventricular strips were similar in Swiss and phosphorylase kinase/deficient mice.

Cardiac function and phosphorylation of contractile proteins

The primary regulation of cardiac contractility is probably through changes in the level of cytoplasmic free Ca 2+ . In the stimulation of contraction by catecholamines, secondary controls may be present at the level of the contractile proteins. Troponin-I, a subunit of the troponin complex of the thin filament, and C-protein, a thick filament component, are both phosphorylated in perfused hearts in response to catecholamines over time courses similar to that for the increase in contraction. Both proteins are also phosphorylated rapidly in vitro by cyclic-AMP-dependent protein kinase. Phosphorylation of troponin-I causes a decrease in the sensitivity of both cardiac myofibrillar ATPase and tension development of skinned fibre preparations to Ca 2+ , and also an increase in the rate of dissociation of Ca 2+ from isolated troponin. These results support the hypothesis that the role of phosphorylation of cardiac troponin-I is to contribute to the increased rate of relaxation of the heart that is observed with catecholamines. C-protein is phosphorylated to a maximum of 4-5 mol phosphate per mole protein both in vivo and in vitro . At present, however, the functions of both C-protein itself and its phosphorylation are unknown. Dephosphorylation of these contractile proteins after catecholamine stimulation is slow in perfused heart, although the rate can be increased by cholinergic agents. Phosphorylase, in contrast, is rapidly dephosphorylated under these circumstances. Phosphoprotein phosphatases relatively specific for phosphorylase have been identified in rat heart, whereas troponin-I appears to be dephosphorylated by general phosphatases. These observations account for the different rates of dephosphorylation of phosphorylase and the contractile proteins, but do not explain the slow dephosphorylation of the latter. In control perfused hearts myosin P-light chain was 50 % phosphorylated, and this was not changed by perfusion with positive inotropic agents or by short-term ischaemia. It was also unchanged during long-term hormonal modifications. Perfusions with 32 P 1 in rat heart give a half-time for the turnover of phosphate bound to the P-light chain of 2-4 min, showing that the myosin light chain kinase and phosphatase are active in the heart. It is hypothesized that under control conditions the kinase is already fully active, and that an increase in cytoplasmic Ca 2+ cannot therefore cause further activation of the enzyme.

Phosphorylation of rabbit cardiac-muscle troponin I by phosphorylase kinase. The effect of adrenaline

1. Incubation of rabbit cardiac-muscle troponin I with phosphorylase b kinase leads to the incorporation of .07-1.2 mol of Pi/mol. 2. The major site of phosphorylation is a serine residue at position 72. 3. Lesser amounts of phosphate are incorporated into threonine-138, threonine-162 and serine 20. 4. Serine-20 is the only site that contains a significant amount of phosphate before incubation with phosphorylase b kinase. 5. Unlike the situation with serine-20, the extent of phosphorylation of serine-72 and threonine-138 in the perfused rabbit heart does not change when the heart is exposed to adrenaline (4 microM).

The site of phosphorylation of troponin I in the perfused rabbit heart. The effect of adrenaline

1. On treatment of the perfused rabbit heart with adrenaline, the total covalently bound phosphate of troponin I increased from 1.14 mol of phosphate/mol to 1.86 mol of phosphate/mol. 2. Covalently bound phosphate could be identified only in the region of the molecule of cardiac troponin I consisting of residues 1--48. 3. When 32P-labelled orthophosphate was present in the perfusion medium the phosphate at serine-20 became radioactively labelled. This residue was the only significant site of phosphorylation that could be identified. 4. The addition of adrenaline caused a 4--5-fold increase in covalently bound [32P]phosphate. Virtually all of the 32P was located at serine-20. 5. It was concluded from these studies that the extent of phosphorylation of serine-20 of cardiac troponin I increased from 30--40% in the control perfused heart to about 100% in the presence of adrenaline.

Phosphorylation of a bovine cardiac actin complex

A bovine cardiac actin-tropomyosin-troponin complex was phosphorylated in the presence of [gamma-32P]ATP, Mg2+, adenosine 3',5'-monophosphate (cyclic AMP), and bovine cardiac cyclic-AMP-dependent protein kinase. Approximately 81% of the [32P]phosphate incorporated was identified as phosphoserine and phosphothreonine. Gel electrophoresis studies showed that 55% of the [32P]phosphate was associated with the inhibitory component of troponin (Tn-I) and 24% with a protein resembling the tropomyosin-binding component of troponin in the actin complex, respectively. The phosphorylation of Tn-I in the actin complex was inhibited 30% when Ca2+ was increased from 0.1 to 50 muM, but phosphorylation of other components was not affected by increasing Ca2+ concentration. Half-maximal calcium activation of the ATPase activity of reconstituted actomyosins made with the [32P]phosphorylated cardiac actin complex and cardiac myosin was shifted to Ca2+ values higher than those of actomyosins made with the nonphosphorylated actin complex.

Phosphorylase kinase isoenzymes in deficient ICR/IAn mice

ICR/IAn mice present a deficiency in phosphorylase kinase activity; the extent of this deficiency is less in some tissues [Lyon, S.B. Biochem. Genet. 4, 169--185 (1970)] than in skeletal muscle, where enzyme activity is 0.3% of normal [Cohen, P.T. W & Cohen, P. FEBS Lett. 29, 113--115 (1973)]. New-born mice of this strain were also reported (Lyon, 1970) to reveal a small amount of skeletal muscle enzyme activity. The properties of these residual phosphorylase kinases were compared to those of control C57 BL mice, with reference to control muscle and liver enzymes which were shown to be of different molecular species [Daegelen-Proux et al. Biochim. Biophys Acta, 452, 398--405 (1976)]. The properties investigated were the immunological reactivity against an antiserum raised against muscle phosphorylase kinase, the thermal stability and the Ca2+ dependency. The results suggest that the muscle enzyme from the new-born ICR/IAn mice and the heart enzyme from adult deficient mice are different to the muscle enzyme from adult normal mice, but they have properties in common with normal adult liver enzyme. These results lead to the conclusion that there exists in the muscle of I strain a "foetal form" of phosphorylase kinase, the activity of which decreases progressively after birth. Out work also confirmed the observations made by Cohen et al. [Eur. J. Biochem. 66, 347--356 (1976)] which showed that there is no evidence for the existence of a cross-reacting material in the muscle of adult deficient mice.