Purification and characterization of calmodulin-dependent multifunctional protein kinase from smooth muscle: isolation of caldesmon kinase

Expression, purification and biochemical characterization of recombinant Ca-dependent protein kinase 2 of the malaria parasite Plasmodium falciparum

Calcium-dependent protein kinases (CDPKs) are serine/threonine kinases that react in response to calcium which functions as a trigger for several mechanisms in plants and invertebrates, but not in mammals. Recent structural studies have defined the role of calcium in the activation of CDPKs and have elucidated the important structural changes caused by calcium in order to allow the kinase domain of CDPK to bind and phosphorylate the substrate. However, the role of autophosphorylation in CDPKs is still not fully understood. In Plasmodium falciparum, seven CDPKs have been identified by sequence comparison, and four of them have been characterized and assigned to play a role in parasite motility, gametogenesis and egress from red blood cells. Although PfCDPK2 was already discovered in 1997, little is known about this enzyme and its metabolic role. In this work, we have expressed and purified PfCDPK2 at high purity in its unphosphorylated form and characterized its biochemical properties. Moreover, propositions about putative substrates in P. falciparum are made based on the analysis of the phosphorylation sites on the artificial substrate myelin basic protein (MBP).

A role for calcium/calmodulin-dependent protein kinase II in cardiac disease and arrhythmia

More than 20 years have passed since the discovery that a collection of specific calcium/calmodulin-dependent phosphorylation events is the result of a single multifunctional kinase. Since that time, we have learned a great deal about this multifunctional and ubiquitous kinase, known today as calcium/calmodulin-dependent protein kinase II (CaMKII). CaMKII is interesting not only for its widespread distribution and broad specificity but also for its biophysical properties, most notably its activation by the critical second messenger complex calcium/calmodulin and its autophosphorylating capability. A central role for CaMKII has been identified in regulating a diverse array of fundamental cellular activities. Furthermore, altered CaMKII activity profoundly impacts function in the brain and heart. Recent findings that CaMKII expression in the heart changes during hypertrophy, heart failure, myocardial ischemia, and infarction suggest that CaMKII may be a viable therapeutic target for patients suffering from common forms of heart disease.

Inhibition of CaM kinase II activation and force maintenance by KN-93 in arterial smooth muscle

Ca(+)/calmodulin-dependent protein kinase II (CaM kinase II) has been implicated in the regulation of smooth muscle contractility. The goals of this study were to determine: 1) to what extent CaM kinase II is activated by contractile stimuli in intact arterial smooth muscle, and 2) the effect of a CaM kinase II inhibitor (KN-93) on CaM kinase II activation, phosphorylation of myosin regulatory light chains (MLC(20)), and force. Both histamine (1 microM) and KCl depolarization activated CaM kinase II with a time course preceding maximal force development, and suprabasal CaM kinase II activation was sustained during tonic contractions. CaM kinase II activation was inhibited by KN-93 pretreatment (IC(50) approximately 1 microM). KN-93 inhibited histamine-induced tonic force maintenance, whereas early force development and MLC(20) phosphorylation responses during the entire time course were unaffected. Both force development and maintenance in response to KCl were inhibited by KN-93. Rapid increases in KCl-induced MLC(20) phosphorylation were also inhibited by KN-93, whereas steady-state MLC(20) phosphorylation responses were unaffected. In contrast, phorbol 12,13-dibutyrate (PDBu) did not activate CaM kinase II and PDBu-stimulated force development was unaffected by KN-93. Thus KN-93 appears to target a step(s) essential for force maintenance in response to physiological stimuli, suggesting a role for CaM kinase II in regulating tonic contractile responses in arterial smooth muscle. Pharmacological activation of protein kinase C bypasses the KN-93 sensitive step.

Characterization of Ca2+/calmodulin-dependent protein kinase II from smooth muscle

We have characterized chicken gizzard smooth muscle Ca2+/calmodulin-dependent protein kinase II (CaM-PKII) with particular focus on its autophosphorylation. The autophosphorylation of smooth muscle CaMPKII produced a partially constitutively active enzyme, as occurs with the alpha- and beta-isoforms of this enzyme, but the autophosphorylation kinetics were significantly slower. Phosphorylation during the initial rapid phase coincided with the production of constitutively active enzyme. The phosphorylation was on both serine and threonine residues, which is distinct from the brain enzyme where threonine phosphorylation is much faster and more prevalent than serine phosphorylation. The major autophosphorylation sites identified were different from the known autophosphorylation sites of the alpha- and beta-isoforms. During the initial autophosphorylation phase Ser-319, Ser-352 and a Thr residue within residues 345-368 were found to be phosphorylated. During the subsequent gradual phase two serine residues in the variable region and Ser-280 were phosphorylated, but Thr-286 and Thr-305, which are the known major autophosphorylation sites for the alpha- and beta-isoforms, were not detected as the major autophosphorylation sites of smooth muscle CaMPKII. By comparing the phosphopeptide sequence with the known sequences of various isoforms, we concluded that isoform gamma-b, which contains a unique insertion and two deletions at the C-terminal side of the calmodulin binding domain, is the dominant CaMPKII isoform in smooth muscle. The molecular mass of smooth muscle CaMPKII was estimated to be 240 kDa which would comprise four subunits, fewer than in the alpha- and beta-isoforms. The results show that smooth muscle CaMPKII is functionally distinct from the alpha- and beta-isoforms of this enzyme, which might be crucial for its physiological relevance.

Phosphorylation of caldesmon by smooth-muscle casein kinase II

A caldesmon kinase activity was partially purified from an extract of chicken gizzard smooth muscle by sequential chromatography on columns of DEAE-Sephacel, MonoQ and Superose 12. This kinase was identified as casein kinase II by Western blotting using peptide-directed antibodies raised against the alpha, alpha' and beta subunits of human casein kinase II; the smooth muscle enzyme consisted of similar subunits of M(r) 43,000 (alpha), 39,000 (alpha'), and 27,000 (beta). Phosphorylation of caldesmon and casein by smooth muscle casein kinase II was optimal at approximately 0.1 M NaCl, did not require second messengers, and was inhibited by heparin. The kinase utilized either GTP or ATP as a substrate. Caldesmon was phosphorylated to approximately 1 mol Pi mol-1 caldesmon by smooth muscle casein kinase II with a Km for caldesmon of 4.9 microM. Two-dimensional thin-layer electrophoresis indicated phosphate incorporation into both serine and threonine. All the incorporated phosphate was recovered in the N-terminal peptide (residues 1-152) generated by cleavage at cysteine 153 with 2-nitro-5-thiocyanobenzoic acid. Purification of tryptic phosphopeptides and N-terminal sequencing revealed two principal sites of phosphorylation: serine 73 and threonine 83. The following four synthetic peptides corresponding to this domain of caldesmon were examined as substrates of casein kinase II: A = RRREVNAQNSVAEEE; B = AQNSVAEEE; C = RSTDDEAA; D = SVAEEETKRSTDDE. Interestingly, only peptides C and D were phosphorylated and both only at threonine. Phosphorylation of intact caldesmon did not affect the pattern of chymotryptic digestion suggesting that it does not induce a significant conformational change in the protein substrate. Phosphorylation also had no effect on the binding of caldesmon to actin or on the caldesmon-mediated inhibition of actomyosin MgATPase activity. However, phosphorylation completely abolished the interaction of caldesmon with immobilized smooth muscle myosin. These results are consistent with the localization of the myosin-binding domain near the N-terminus of caldesmon and of the actin-binding domain near the opposite end of the elongated molecule. Casein kinase II may therefore play a role in regulating caldesmon-myosin interaction and the ability of caldesmon to cross-link actin and myosin filaments in smooth muscle.

New isoforms of Ca2+/calmodulin-dependent protein kinase II in smooth muscle

Four novel isoforms of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) were found in rat aorta smooth muscle. Two of them were related to gamma-isoform of brain CaM kinase II (gamma-a). Differences in the primary structure of these isoforms were located in the variable region. One of them (gamma-b) contained 23 unique amino acid residues, whereas the other (gamma-c) did not contain this sequence. Both isoforms lacked the two segments (Val-316 to Gln-337 and Lys-353 to Leu-362) present in gamma-a. The DNA sequence of these gamma-isoforms except the variable region was exactly the same, suggesting that they are produced by alternative splicing. Another two isoforms were related to the delta-isoform of brain CaM kinase II (delta-a). delta-b contained a unique 11-residue sequence in the variable region whereas delta-c did not. As found for gamma-isoforms, the sequence analysis suggested that the three delta-isoforms are also produced by alternative splicing. Analysis of RNA by reverse transcription PCR confirmed the existence of specific messages for gamma-b, delta-a and delta-b. The variety of isoforms of CaM kinase II suggest that each isoform may play a specialized role in cell regulation.

In vivo phosphorylation of regulatory light chain of myosin II during mitosis of cultured cells

Phosphorylation of the regulatory light chain of myosin II (MLC) controls the contractility of actomyosin in nonmuscle and muscle cells. It has been reported that cdc2 phosphorylates MLC in vitro at Ser-1 or Ser-2 and Thr-9 which protein kinase C phosphorylates (Satterwhite, L. L., M. J. Lohka, K. L. Wilson, T. Y. Scherson, L. K. Cisek, J. L. Corden, and T. D. Pollard. 1992 J. Cell Biol. 118:595-605). We have examined in vivo phosphorylation of MLC during mitosis and after the release of mitotic arrest. Phosphate incorporation of MLC in mitotic cells is found to be 6-12 times greater than that in nonmitotic cells. Phosphopeptide maps have revealed that the MLC from mitotic cells is phosphorylated at Ser-1 and/or Ser-2 (Ser-1/2), but not at Thr-9. MLC is also phosphorylated to a much lesser extent at Ser-19 which myosin light chain kinase phosphorylates. On the other hand, MLC of nonmitotic cells is phosphorylated at Ser-19 but not at Ser-1/2. The extent of phosphate incorporation is doubled at 30 min after the release of mitotic arrest when some cells start cytokinesis. Phosphopeptide analyses have revealed that the phosphorylation at Ser-19 is increased 20 times, while the phosphorylation at Ser-1/2 is decreased by half. This high extent of MLC phosphorylation at Ser-19 is maintained for another 30 min and gradually decreased to near the level of interphase cells as cells complete spreading at 180 min. On the other hand, phosphorylation at Ser-1/2 is decreased to 18% at 60 min, and is practically undetectable at 180 min after the release of mitotic arrest. The stoichiometry of MLC phosphorylation has been determined by quantitation of phosphorylated and unphosphorylated forms of MLC separated on 2D gels. The molar ratio of phosphorylated MLC to total MLC is found to be 0.16 +/- 0.06 and 0.31 +/- 0.05 in interphase and mitotic cells, respectively. The ratio is increased to 0.49 +/- 0.05 at 30 min after the release of mitotic arrest. These results suggest that the change in the phosphorylation site from Ser-1/2 to Ser-19 plays an important role in signaling cytokinesis.

Smooth-muscle caldesmon phosphatase is SMP-I, a type 2A protein phosphatase

Caldesmon phosphatase was identified in chicken gizzard smooth muscle by using as substrates caldesmon phosphorylated at different sites by protein kinase C, Ca2+/calmodulin-dependent protein kinase II and cdc2 kinase. Most (approximately 90%) of the phosphatase activity was recovered in the cytosolic fraction. Gel filtration after (NH4)2SO4 fractionation of the cytosolic fraction revealed a single major peak of phosphatase activity which coeluted with calponin phosphatase [Winder, Pato and Walsh (1992) Biochem. J. 286, 197-203] and myosin LC20 phosphatase. Further purification of caldesmon phosphatase was achieved by sequential chromatography on columns of DEAE-Sephacel, omega-amino-octyl-agarose, aminopropyl-agarose and thiophosphorylated myosin LC20-Sepharose. A single peak of caldesmon phosphatase activity was detected at each step of the purification. The purified phosphatase was identified as SMP-I [Pato and Adelstein (1980) J. Biol. Chem. 255, 6535-6538] by subunit composition (three subunits, of 60, 55 and 38 kDa) and Western blotting using antibodies against the holoenzyme which recognize all three subunits and antibodies specific for the 38 kDa catalytic subunit. SMP-I is a type 2A protein phosphatase [Pato, Adelstein, Crouch, Safer, Ingebritsen and Cohen (1983) Eur. J. Biochem. 132, 283-287; Winder et al. (1992), cited above]. Consistent with the conclusion that SMP-I is the major caldesmon phosphatase of smooth muscle, purified SMP-I from turkey gizzard dephosphorylated all three phosphorylated forms of caldesmon, whereas SMP-II, -III and -IV were relatively ineffective. Kinetic analysis of dephosphorylation by chicken gizzard SMP-I of the three phosphorylated caldesmon species and calponin phosphorylated by protein kinase C indicates that calponin is a significantly better substrate of SMP-I than are any of the three phosphorylated forms of caldesmon. We therefore suggest that caldesmon phosphorylation in vivo can be maintained after kinase inactivation due to slow dephosphorylation by SMP-I, whereas calponin and myosin are rapidly dephosphorylated by SMP-I and SMP-III/SMP-IV respectively. This may have important functional consequences in terms of the contractile properties of smooth muscle.

Primary structure required for the inhibition of smooth muscle myosin light chain kinase

Myosin light chain kinase (MLCK) contains the autoinhibitor sequence right next to the N-terminus side of the calmodulin binding region. In this paper, the structural requirement of the inhibition of MLCK activity was studied using synthetic peptide analogs. Peptides Ala-783-Lys-799 and Ala-783-Arg-798 inhibited calmodulin independent MLCK at the same potency as the peptide Ala-783-Gly-804. Deletion of Arg-797-Lys-799 or substitution of these residues to Ala markedly increased the Ki while the substitution of Lys-792 and Lys-793 to Ala and the deletion of Lys-784-Lys-785 did not affect the inhibitory activity of the peptides. The results suggest that Arg-797-Arg-798 are especially important for the inhibitory activity among other basic residues in the autoinhibitory region.

Calcium-dependent enhancement of calcium current in smooth muscle by calmodulin-dependent protein kinase II

Calcium entry through voltage-activated Ca2+ channels is important in regulating many cellular functions. Activation of these channels in many cell types results in feedback regulation of channel activity. Mechanisms linking Ca2+ channel activity with its downregulation have been described, but little is known of the events responsible for the enhancement of Ca2+ current that in many cells follows Ca2+ channel activation and an increase in cytoplasmic Ca2+ concentration. Here we investigate how this positive feedback is achieved in single smooth muscle cells. We find that in these cells voltage-activated calcium current is persistently but reversibly enhanced after periods of activation. This persistent enhancement of the Ca2+ current is mediated by activation of calmodulin-dependent protein kinase II because it is blocked when either the rise in cytoplasmic Ca2+ is inhibited or activation of calmodulin-dependent protein kinase II is prevented by specific peptide inhibitors of calcium-calmodulin or calmodulin-dependent protein kinase II itself. This mechanism may be important in different forms of Ca2+ current potentiation, such as those that depend on prior Ca2+ channel activation or are a result of agonist-induced release of Ca2+ from internal stores.

Actin mediated regulation of muscle contraction

Striated and smooth muscles have different mechanisms of regulation of contraction which can be the basis for selective pharmacological alteration of the contractility of these muscle types. The progression in our understanding of the tropomyosin-troponin regulatory system of striated muscle from the early 1970s through the early 1990s is described along with key concepts required for understanding this complex system. This review also examines the recent history of the putative contractile regulatory proteins of smooth muscle, caldesmon and calponin. A contrast is made between the actin linked regulatory systems of striated and smooth muscle.

The molecular anatomy of caldesmon

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Determination of the phosphorylation sites of smooth muscle caldesmon by protein kinase C

Smooth muscle caldesmon was phosphorylated by protein kinase C up to 1.90 mol P/mol caldesmon. Phosphorylated caldesmon was completely digested by trypsin and the produced phosphopeptides were purified by C-8 and C-18 reverse phase chromatography. Four phosphopeptides were determined and two phosphoserines were identified. Both were localized in the C-terminal domain at serine-587 and serine-726. By following the time course of phosphorylation, serine-587 was found to be the preferred site. Effects of the phosphorylation of caldesmon by protein C on the inhibition of acto-H-meromyosin ATPase activity was also examined. While unphosphorylated caldesmon inhibited the ATPase activity by 60%, phosphorylated caldesmon hardly inhibited the ATPase activity. Therefore, it was concluded that the phosphorylation at serine-726 and serine-587 reverses the inhibitory activity of caldesmon.