Human erythrocyte calmodulin. Further chemical characterization and the site of its interaction with the membrane

Piezo1 regulates shear-dependent nitric oxide production in human erythrocytes

Mature circulating red blood cells (RBC) are classically viewed as passive participants in circulatory function, given erythroblasts eject their organelles during maturation. Endogenous production of nitric oxide (NO) and its effects are of particular significance; however, the integration between RBC sensation of the local environment and subsequent activation of mechano-sensitive signaling networks that generate NO remain poorly understood. The present study investigated endogenous NO-production via the RBC-specific nitric oxide synthase-isoform (RBC-NOS), connecting membrane strain with intracellular enzymatic processes. Isolated RBC were obtained from apparently healthy humans. Intracellular NO was compared at rest and following shear (cellular deformation) using semi-quantitative fluorescent imaging. Concurrently, RBC-NOS phosphorylation at its Serine¹¹⁷⁷ (ser¹¹⁷⁷) residue was measured. The contribution of cellular deformation to shear-induced NO-production in RBC was determined by rigidifying RBC with the thiol-oxidizing agent diamide; rigid RBC exhibited significantly impaired (up to 80%) capacity to generate NO via RBC-NOS during shear. Standardizing membrane strain of rigid RBC by applying increased shear did not normalize NO-production, or RBC-NOS activation. Calcium-imaging with Fluo-4 revealed that diamide-treated RBC exhibited a 42%-impairment in Piezo1-mediated calcium-movement when compared with untreated RBC. Pharmacological inhibition of Piezo1 with GsMTx4 during shear inhibited RBC-NOS activation in untreated RBC, while Piezo1-activation with Yoda1 in the absence of shear stimulated RBC-NOS activation. Collectively, a novel, mechanically-activated signaling pathway in mature RBC is described. Opening of Piezo1 and subsequent influx of calcium appears to be required for endogenous production of NO in response to mechanical shear, which is accompanied by phosphorylation of RBC-NOS at ser¹¹⁷⁷.

Calcium Channels and Calcium-Regulated Channels in Human Red Blood Cells

Free Calcium (Ca²⁺) is an important and universal signalling entity in all cells, red blood cells included. Although mature mammalian red blood cells are believed to not contain organelles as Ca²⁺ stores such as the endoplasmic reticulum or mitochondria, a 20,000-fold gradient based on a intracellular Ca²⁺ concentration of approximately 60 nM vs. an extracellular concentration of 1.2 mM makes Ca²⁺-permeable channels a major signalling tool of red blood cells. However, the internal Ca²⁺ concentration is tightly controlled, regulated and maintained primarily by the Ca²⁺ pumps PMCA1 and PMCA4. Within the last two decades it became evident that an increased intracellular Ca²⁺ is associated with red blood cell clearance in the spleen and promotes red blood cell aggregability and clot formation. In contrast to this rather uncontrolled deadly Ca²⁺ signals only recently it became evident, that a temporal increase in intracellular Ca²⁺ can also have positive effects such as the modulation of the red blood cells O₂ binding properties or even be vital for brief transient cellular volume adaptation when passing constrictions like small capillaries or slits in the spleen. Here we give an overview of Ca²⁺ channels and Ca²⁺-regulated channels in red blood cells, namely the Gárdos channel, the non-selective voltage dependent cation channel, Piezo1, the NMDA receptor, VDAC, TRPC channels, Ca_V2.1, a Ca²⁺-inhibited channel novel to red blood cells and i.a. relate these channels to the molecular unknown sickle cell disease conductance P_sickle. Particular attention is given to correlation of functional measurements with molecular entities as well as the physiological and pathophysiological function of these channels. This view is in constant progress and in particular the understanding of the interaction of several ion channels in a physiological context just started. This includes on the one hand channelopathies, where a mutation of the ion channel is the direct cause of the disease, like Hereditary Xerocytosis and the Gárdos Channelopathy. On the other hand it applies to red blood cell related diseases where an altered channel activity is a secondary effect like in sickle cell disease or thalassemia. Also these secondary effects should receive medical and pharmacologic attention because they can be crucial when it comes to the life-threatening symptoms of the disease.

Calcium in red blood cells-a perilous balance

Ca2+ is a universal signalling molecule involved in regulating cell cycle and fate, metabolism and structural integrity, motility and volume. Like other cells, red blood cells (RBCs) rely on Ca2+ dependent signalling during differentiation from precursor cells. Intracellular Ca2+ levels in the circulating human RBCs take part not only in controlling biophysical properties such as membrane composition, volume and rheological properties, but also physiological parameters such as metabolic activity, redox state and cell clearance. Extremely low basal permeability of the human RBC membrane to Ca2+ and a powerful Ca2+ pump maintains intracellular free Ca2+ levels between 30 and 60 nM, whereas blood plasma Ca2+ is approximately 1.8 mM. Thus, activation of Ca2+ uptake has an impressive impact on multiple processes in the cells rendering Ca2+ a master regulator in RBCs. Malfunction of Ca2+ transporters in human RBCs leads to excessive accumulation of Ca2+ within the cells. This is associated with a number of pathological states including sickle cell disease, thalassemia, phosphofructokinase deficiency and other forms of hereditary anaemia. Continuous progress in unravelling the molecular nature of Ca2+ transport pathways allows harnessing Ca2+ uptake, avoiding premature RBC clearance and thrombotic complications. This review summarizes our current knowledge of Ca2+ signalling in RBCs emphasizing the importance of this inorganic cation in RBC function and survival.

On the functional use of the membrane compartmentalized pool of ATP by the Na+ and Ca++ pumps in human red blood cell ghosts

Previous evidence established that a sequestered form of adenosine triphosphate (ATP pools) resides in the membrane/cytoskeletal complex of red cell porous ghosts. Here, we further characterize the roles these ATP pools can perform in the operation of the membrane's Na(+) and Ca(2+) pumps. The formation of the Na(+)- and Ca(2+)-dependent phosphointermediates of both types of pumps (E(Na)-P and E(Ca)-P) that conventionally can be labeled with trace amounts of [gamma-(3)P]ATP cannot occur when the pools contain unlabeled ATP, presumably because of dilution of the [gamma-(3)P]ATP in the pool. Running the pumps forward with either Na(+) or Ca(2+) removes pool ATP and allows the normal formation of labeled E(Na)-P or E(Ca)-P, indicating that both types of pumps can share the same pools of ATP. We also show that the halftime for loading the pools with bulk ATP is 10-15 minutes. We observed that when unlabeled "caged ATP" is entrapped in the membrane pools, it is inactive until nascent ATP is photoreleased, thereby blocking the labeled formation of E(Na)-P. We also demonstrate that ATP generated by the membrane-bound pyruvate kinase fills the membrane pools. Other results show that pool ATP alone, like bulk ATP, can promote the binding of ouabain to the membrane. In addition, we found that pool ATP alone functions together with bulk Na(+) (without Mg(2+)) to release prebound ouabain. Curiously, ouabain was found to block bulk ATP from entering the pools. Finally, we show, with red cell inside-outside vesicles, that pool ATP alone supports the uptake of (45)Ca by the Ca(2+) pump, analogous to the Na(+) pump uptake of (22)Na in this circumstance. Although the membrane locus of the ATP pools within the membrane/cytoskeletal complex is unknown, it appears that pool ATP functions as the proximate energy source for the Na(+) and Ca(2+) pumps.

Free and bound intracellular calmodulin measurements in cardiac myocytes

Calmodulin (CaM) is a ubiquitous Ca2+ binding protein and Ca2+-CaM activates many cellular targets and functions. While much of CaM is thought to be protein bound, quantitative data in cardiac myocytes is lacking regarding CaM location, [CaM]free and CaM redistribution during changes in [Ca2+]i. Here, we demonstrated that in adult rabbit cardiac myocytes, CaM is highly concentrated at Z-lines (confirmed by Di-8-ANEPPS staining of transverse tubules) using three different approaches: immunocytochemistry (endogenous CaM), Alexa Fluor 488 conjugate CaM (F-CaM) in both permeabilized cells (exogenous CaM) and in patch clamped intact cells (via pipette dialysis). Using 100 nM [CaM]free we washed F-CaM into permeabilized myocytes and saw a two-phase (fast and slow) CaM binding curve with a plateau after 40 min of F-CaM wash-in. We also measured myocyte [CaM]free using two modified null-point titration methods, finding [CaM]free to be 50-75 nM (which is only 1% of total [CaM]). Higher [Ca2+]i increased CaM binding especially in the nucleus and at Z-lines and significantly slowed F-CaM dissociation rate when F-CaM was washed out of permeabilized myocytes. Additionally, in both permeabilized and intact myocytes, CaM moved into the nucleus when [Ca2+]i was elevated, and this was reversible. We conclude that [CaM]free is very low in myocytes even at resting [Ca2+]i, indicating intense competition of CaM targets for free CaM. Bound CaM is relatively concentrated at Z-lines at rest but translocates significantly to the nucleus upon elevation of [Ca2+]i, which may influence activation of different targets and cellular functions.

Oligonucleotide trapping method for transcription factor purification systematic optimization using electrophoretic mobility shift assay

Oligonucleotide trapping, where a transcription factor-DNA response element complex is formed in solution and then recovered (trapped) on a column, was optimized for the purification of CAAT/enhancer binding protein (C/EBP) from rat liver nuclear extract. Electrophoretic mobility shift assays (EMSAs) with ACEP24(GT)5 oligonucleotide, containing the CAAT element, was used to estimate thebinding affinity and concentration of C/EBP in the nuclear extract and then low concentrations of protein and oligonucleotide, which favor specific binding, were used for all further experiments. Also using EMSA, the highest concentrations of competitors, which inhibit non-specific binding but do not inhibit oligonucleotide binding by C/EBP, were determined to be 932 nM T18 (single-stranded DNA), 50 ng/ml heparin (non-DNA competitor), and 50 microg/ml poly(dI:dC) (duplex DNA). Inclusion of 0.1% Tween-20 improved DNA binding. For complex formation, 110 microg nuclear extract was diluted to 0.2 nM C/EBP (apparent Kd of C/EBP) and 1.34 nM ACEP24(GT)5 was added, along with Tween-20 and the competitors. After incubation, the complex was trapped by annealing the (GT)5 tail of the C/EBP-[ACEP24(GT)5] complex to an (AC)5-Sepharose column under flow at 4 degrees C. The column was washed with 0.4 M NaCl and the protein eluted with 1.2 M NaCl. The purification typically resulted in two proteins of apparent molecular mass 32000 and 38000. The smaller one, the major product, was identified to be C/EBP-alpha. The yield was 2.1 microg (66 pmol) of purified C/EBP-alpha p32. This systematic approach to oligonucleotide trapping is generally applicable for the purification of other transcription factors.

Calcium, calmodulin, and calcium-calmodulin kinase II: heartbeat to heartbeat and beyond

Calcium (Ca) is the key regulator of cardiac contraction during excitation-contraction (E-C) coupling. However, differences exist between the amount of Ca being transported into the myocytes upon electrical stimulation as compared to Ca released from the sarcoplasmic reticulum (SR). Moreover, alterations in E-C coupling occur in cardiac hypertrophy and heart failure. In addition to the direct effects of Ca on the myofilaments, Ca plays a pivotal role in activation of a number of Ca-dependent proteins or second messengers, which can modulate E-C coupling. Of these proteins, calmodulin (CaM) and Ca-CaM-dependent kinase II (CaMKII) are of special interest in the heart because of their role of modulating Ca influx, SR Ca release, and SR Ca uptake during E-C coupling. Indeed, CaM and CaMKII may be associated with some ion channels and Ca transporters and both can modulate acute cellular Ca handling. In addition to the changes in Ca, CaM and CaMKII signals from beat-to-beat, changes may occur on a longer time scale. These may occur over seconds to minutes involving phosphorylation/dephosphorylation reactions, and even a longer time frame in altering gene transcription (excitation-transcription (E-T) coupling) in hypertrophic signaling and heart failure. Here we review the classical role of Ca in E-C coupling and extend this view to the role of the Ca-dependent proteins CaM and CaMKII in modulating E-C coupling and their contribution to E-T coupling.

Oxidatively modified calmodulin binds to the plasma membrane Ca-ATPase in a nonproductive and conformationally disordered complex

Oxidation of either Met(145) or Met(146) in wheat germ calmodulin (CaM) to methionine sulfoxide prevents the CaM-dependent activation of the plasma membrane (PM) Ca-ATPase (D. Yin, K. Kuczera, and T. C. Squier, 2000, Chem. Res. Toxicol. 13:103-110). To investigate the structural basis for the inhibition of the PM-Ca-ATPase by oxidized CaM (CaM(ox)), we have used circular dichroism (CD) and fluorescence spectroscopy to resolve conformational differences within the complex between CaM and the PM-Ca-ATPase. The similar excited-state lifetime and solvent accessibility of the fluorophore N-1-pyrenyl-maleimide covalently bound to Cys(26) in unoxidized CaM and CaM(ox) indicates that the globular domains within CaM(ox) assume a native-like structure following association with the PM-Ca-ATPase. However, in comparison with oxidized CaM there are increases in the 1) molar ellipticity in the CD spectrum and 2) conformational heterogeneity between the opposing globular domains for CaM(ox) bound to the CaM-binding sequence of the PM-Ca-ATPase. Furthermore, CaM(ox) binds to the PM-Ca-ATPase with high affinity at a distinct, but overlapping, site to that normally occupied by unoxidized CaM. These results suggest that alterations in binding interactions between CaM(ox) and the PM-Ca-ATPase block important structural transitions within the CaM-binding sequence of the PM-Ca-ATPase that are normally associated with enzyme activation.

The rate of activation by calmodulin of isoform 4 of the plasma membrane Ca(2+) pump is slow and is changed by alternative splicing

A reconstitution system allowed us to measure the ATPase activity of specific isoforms of the plasma membrane Ca(2+) pump continuously, and to measure the effects of adding or removing calmodulin. The rate of activation by calmodulin of isoform 4b was found to be very slow, with a half-time (at 235 nM calmodulin and 0.5 microM free Ca(2+)) of about 1 min. The rate of inactivation of isoform 4b when calmodulin was removed was even slower, with a half-time of about 20 min. Isoform 4a has a lower apparent affinity for calmodulin than 4b, but its activation rate was surprisingly faster (half time about 20 s). This was coupled with a much faster inactivation rate, consistent with its low affinity. A truncated mutant of isoform 4b also had a more rapid activation rate, indicating that the downstream inhibitory region of full-length 4b contributed to its slow activation. The results indicate that the slow activation is due to occlusion of the calmodulin-binding domain of 4b, caused by its strong interaction with the catalytic core. Since the activation of 4b occurs on a time scale comparable to that of many Ca(2+) spikes, this phenomenon is important to the function of the pump in living cells. The slow response of 4b indicates that this isoform may be the appropriate one for cells which respond slowly to Ca(2+) signals.

Dynamic structure of the calmodulin-binding domain of the plasma membrane Ca-ATPase in native erythrocyte ghost membranes

We have used frequency-domain fluorescence resonance energy transfer (FRET) and anisotropy measurements to identify the structural properties of wheat germ calmodulin (CaM) bound to either the plasma membrane Ca-ATPase (PM-Ca-ATPase) in native erythrocyte ghost membranes or a peptide (C25W) that has an identical sequence to the CaM-binding domain on the PM-Ca-ATPase. Cross-linking experiments using benzophenone labeled CaM in conjunction with immunoblots using antibodies specific for either CaM or the PM-Ca-ATPase indicate that one molecule of CaM selectively binds one PM-Ca-ATPase polypeptide chain in native erythrocyte ghost membranes. There are no other proteins in the erythrocyte membrane that bind CaM with high affinity, permitting the measurement of the structural properties of CaM bound to the PM-Ca-ATPase in native erythrocyte ghost membranes. FRET measurements between the fluorophore pyrene maleimide (PMal) located at Cys27 in calcium binding loop I and nitrotyrosine139 in calcium binding loop IV on wheat germ CaM indicate that the average spatial separation and conformational heterogeneity associated with the two opposing globular domains of CaM are virtually identical upon CaM binding to either the PM-Ca-ATPase or C25W. Measurements of the solvent accessibility and segmental rotational dynamics of PMal-CaM bound to either the PM-Ca-ATPase or C25W further indicate that the local environment around the pyrene label located at Cys27 is very similar. However, the overall rotational dynamics of CaM bound to the PM-Ca-ATPase is much slower (phi 2 = 83 +/- 14 ns) than observed when CaM binds C25W (phi 2 = 10.3 +/- 0.5 ns). This implies that CaM is tightly associated with the CaM-binding domain of the PM-Ca-ATPase and that the observed rotational motion of pyrenylmaleimide labeled CaM is characteristic of the global motion of the CaM-binding domain on the PM-Ca-ATPase. The similar conformational heterogeneity and local environment of CaM bound to either the PM-Ca-ATPase or C25W indicates that CaM binds to a contiguous sequence of amino acids on the Ca-ATPase that are analogous to C25W and that there are no significant interactions with other structural elements within the PM-Ca-ATPase. The rate of rotational motion associated with CaM bound to the PM-Ca-ATPase is consistent with hydrodynamic calculation in which the calmodulin-binding domain located at the carboxyl-terminus of the PM-Ca-ATPase has a stable and defined tertiary structure that is independent of the other cytoplasmic domains of the PM-Ca-ATPase.

Activation of epidermal growth factor receptor by epidermal growth factor

The binding of epidermal growth factor (EGF) to epidermal growth factor receptor (EGF receptor) induces dimerization of the receptor and activation of its protein tyrosine kinase. Each of these three steps was followed as a function of the concentrations of EGF and of EGF receptor. Binding of EGF was followed by sedimentation of the complex between [3H]EGF and EGF receptor, dimerization was measured by quantitative cross-linking with glutaraldehyde, and the activation of the protein tyrosine kinase was monitored under the same conditions by following the initial velocity of the phosphorylation of peptides containing tyrosine. The binding of epidermal growth factor to its receptor was measured as a function of the concentration of epidermal growth factor, and the relationship was sigmoid with an average value of 1.7 for the Hill coefficient. Both dimerization and the activation of the tyrosine kinase displayed saturation as a function of the concentration of EGF. The ranges of the concentrations of EGF where dimerization and activation of the tyrosine kinase activity were half-maximal were 15-30 and 50-200 nM, respectively, but the value for the concentration of EGF at the half-maximum for the activation of the tyrosine kinase was a complex function of the concentration of EGF receptor. The observed behavior of the binding of EGF, the dimerization of EGF receptor, and the activation of the tyrosine kinase were used as criteria against which to test mechanisms for the process of activation. Equations were derived for various reversible and irreversible mechanisms and used to calculate the theoretical behaviors of the three properties. In direct comparisons of the experimental and the theoretical data, several of the previously proposed reversible and irreversible mechanisms for the activation of EGF receptor were found to be inadequate, but a reasonable mechanism was formulated that was compatible with the experimental data. In this mechanism, dimeric EGF receptor must be occupied by two molecules of EGF for enzymatic activity to be expressed.

A continuous spectrophotometric assay for the activation of plant NAD kinase by calmodulin, calcium(II), and europium(III) ions

A continuous spectrophotometric assay has been developed to quantify the calmodulin, calcium(II) ion, and europium(III) ion dependence of the activation of NAD kinase from pea seedlings. Experimental enzyme activation data are compared with the theoretical curves for the binding of calcium(II) ions to the individual calcium binding sites of calmodulin. These results indicate that the binding of three calcium(II) ions is necessary for activation of plant NAD kinase. Further studies demonstrate that europium(III) ions can replace calcium(II) ions in calmodulin with retention of its ability to activate NAD kinase.

Multiple sites of methyl esterification of calmodulin in intact human erythrocytes

Aspartyl and asparaginyl residues are susceptible to spontaneous chemical degradation reactions that result in the formation of isomerized and racemized aspartyl residues. At least a subset of these abnormal residues are recognized by a widely distributed protein D-aspartyl/L-isoaspartyl methyltransferase (EC 2.1.1.77) that can participate in their conversion to normal L-aspartyl residues. We have used this methyltransferase as a probe to identify modified aspartyl and asparaginyl residues in peptides and proteins. In purified calmodulin from bovine brain, major sites of methylation were found to originate from the Asp-2 residue near the amino terminus and the Asp-78 residue in the alpha-helix that connects the two globular calcium-binding domains. When purified calmodulin was incubated at physiological temperature and pH in the absence of calcium, additional methylation sites were found in three of the four calcium-binding sites. In this work we have analyzed the methyl esterification of human calmodulin catalyzed by this enzyme in intact erythrocytes. On the basis of results from peptide mapping studies, Asp-2, Asp-78/80, and residues in calcium-binding domains III and IV appear to be methylated. Methylation of sites in the calcium-binding regions appears to reflect the low concentration of free calcium in human erythrocytes. We also found that calmodulin isolated from erythrocytes and methylated in vitro contains major methylation sites at Asp-2 and Asp-78/80 but not in the calcium-binding sites. Comparison of the number of available methylation sites of calmodulin in intact cells and in material aged in vitro supports the hypothesis that repair processes can occur in erythrocytes.

Mode of action of lithium in affective disorders. An influence on intracellular calcium functions

The inference that lithium acts by altering intracellular calcium functions is supported by the three areas considered above. First, recent work in other laboratories has broadened the range of lithium actions on calcium-dependent physiological functions. Second, a theoretical analysis of the coupling of calcium transport to the triphosphoinositide response presents a plausible mechanism by which lithium could limit the damage caused by deficient calcium transport. Third, we have recently reported that there is a direct enhancement of the calmodulin-activated membrane-bound calcium pump in lithium-treated bipolar subjects.

Regulation of neuronal function by calcium

In the classical picture of brain function, electrical impulses are initiated in sensory organs and spread rapidly down axons, jumping synaptic clefts by neurochemical transmission. Patterns of electrical activity generated in this way integrate information throughout the brain and result in coordinated motor output. Even as this picture of the central role of electrical transmission was emerging in the mid-20th century, the more speculative neuroscientists reasoned that there must be more to it. In order to store information and adapt to a changing environment, neurons must be able to alter their own properties or those of their neighbors, in highly controlled ways, sometimes permanently.

Calcium transport by a calmodulin-regulated Ca-ATPase in the enamel organ

Using aldehyde-fixed rat incisor enamel organ, we localized Ca-ATPase activity ultracytochemically in the plasma membranes, the mitochondrial inner membranes, and the Golgi membranes of secretory ameloblasts and the cells of stratum intermedium at the secretory stage and papillary layer cells at the maturation stage, but not in maturation ameloblasts. This Ca-ATPase activity was totally dependent on substrate ATP, the enzyme activator CaCl2, and also sensitive to the specific calmodulin blocker trifluoperazine (TFP) in the incubation media. Specific antigenic sites of endogenous calmodulin were demonstrated in polyribosomes, the nucleus, mitochondria, and the cytoplasmic matrix along the plasma membranes of secretory ameloblasts, by the protein A-immunogold technique using sheep antiserum against bovine testis calmodulin. All other enamel organ cells-such as stratum intermedium, papillary layer cells, and maturation ameloblasts-were also weakly immunoreactive. In control sections incubated with antiserum pre-absorbed with an excess of calmodulin and protein A-gold complex, only a few gold particles were observed to be randomly associated with the tissues. Daily intraperitoneal injection of TFP (1 and 5 mg per 100 g body weight) for one week resulted in prominent migration of mitochondria from the infranuclear to supranuclear regions of secretory ameloblasts but caused no other morphological alterations in the enamel organ cells. EDX analysis of ultrathin sections revealed significantly lower peaks of Ca and P in the forming enamel of TFP-injected rats than those in controls. However, little reduction in the Ca and P levels in the maturing enamel was observed in TFP-injected rats. When growing enamel surfaces were exposed with NaOCl and examined with SEM, a remarkable defect in the enamel matrix was observed in the forming enamel but not in the maturing enamel. These results suggest that early enamel mineralization is dependent upon an intact calmodulin-regulated Ca-transporting ATPase in secretory ameloblasts and that enamel maturation is controlled by different mechanism(s).

Calmodulin blocker inhibits Ca++-ATPase activity in secretory ameloblast of rat incisor

The effects of the calmodulin blocker, trifluoperazine (TEP), on membrane-bound Ca++-ATPase, Na+-K+-ATPase (EC 3.6.1.3.) and the ultrastructure of the enamel organ were investigated in the lower incisors of normal and TFP-injected rats. The rats, of about 100 g body weight, were given either 0.2 ml physiological saline or 100 micrograms TFP dissolved in 0.2 ml physiological saline through a jugular vein and fixed by transcardiac perfusion with a formaldehyde-glutaraldehyde mixture at 1 and 2 h after TFP administration. Non-decalcified sections of the enamel organ less than 50 micron in thickness, prepared from dissected lower incisors, were processed for the ultracytochemical demonstration of Ca++-ATPase and Na+-K+-ATPase by the one-step lead method at alkaline pH. In control saline-injected animals the most intense enzymatic reaction of Ca++-ATPase was demonstrated along the plasma membranes of the entire cell surfaces of secretory ameloblasts. Moderate enzymatic reaction was also observed in the plasma membranes of the cells of stratum intermedium and papillary layer. Reaction precipitates of Na+-K+-ATPase activity were localized clearly along the plasma membranes of only the cells of stratum intermedium and papillary layer. The most drastic effect of TFP was a marked disappearance of enzymatic reaction of Ca++-ATPase from the plasma membranes of secretory ameloblasts, except for a weak persistent reaction in the basolateral cell surfaces of the infranuclear region facing the stratum intermedium. The cells of stratum intermedium and papillary layer, however, continued to react for Ca++-ATPase even after TFP treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Delayed activation of calcium pump during transient increases in cellular Ca2+ concentration and K+ conductance in hyperpolarizing human red cells

The net Ca2+ influx was increased in human red cells in suspension by adding moderate concentrations of the Ca2+ ionophore A23187, and due to the increased cellular Ca2+ concentration [( Ca]i) the K+ channels opened (the 'Gardos effect'). At low K+ concentration and with the protonophore CCCP in the buffer-free medium the cells hyperpolarized and the extracellular pH (pH0) increased, enhancing the A23187-mediated net Ca2+ influx. This elicited a prolonged response, viz. a primary transient increase of pH0 and [Ca]i followed by one or more spontaneous pH0 and [Ca]i transients. We explored the pump-mediated Ca2+ efflux by blocking the A23187-mediated Ca2+ flux with CoCl2 at appropriate times during the prolonged response. The Ca2+ pumping was higher during the descendent than during the ascendent phase of the primary transient at equal values of [Ca]i. The data were analyzed using a mathematical model that accounts for the prolonged oscillatory response, including pH0 and [Ca]i. In conclusion, the activation of the Ca2+ pump is delayed due to slow binding of cellular calmodulin, which is a hysteretic response to a rapid increase of the cellular Ca2+ concentration. This mechanism may be important for generation and execution of transient signals in other types of cell.

A calmodulin and alpha-subunit binding domain in human erythrocyte spectrin

Human erythrocyte spectrin binds calmodulin weakly under native conditions. This binding is enhanced in the presence of urea. The site responsible for this enhanced binding in urea has now been shown to reside in a specific region of the spectrin beta-subunit. Cleavage of spectrin with trypsin, cyanogen bromide or 2-nitro-5-thiocyanobenzoic acid generates fragments of the molecule which retain the ability to bind calmodulin under denaturing conditions. The origin of these fragments, identified by two-dimensional peptide mapping, is the terminal region of the spectrin beta-IV domain. The smallest peptide active in calmodulin binding is a 10 000 Mr fragment generated by cyanogen bromide cleavage. Only the intact 74 000 Mr fragment generated by trypsin (the complete beta-IV domain) retains the capacity to reassociate with the isolated alpha-subunit of spectrin. The position of a putative calmodulin binding site near a site for subunit-subunit association and protein 4.1 and actin binding suggests a possible role in vivo for calmodulin regulation of the spectrin-actin membrane skeleton or for regulation of subunit-subunit associations. This beta-subunit binding site in erythrocyte spectrin is found in a region near the NH2-terminus at a position analogous to the alpha-subunit calmodulin binding site previously identified in a non-erythroid spectrin by ultrastructural studies.

Calcium release from intact calmodulin and calmodulin fragment 78-148 measured by stopped-flow fluorescence with 2-p-toluidinylnaphthalene sulfonate. Effect of calmodulin fragments on cardiac sarcoplasmic reticulum

Calcium release from high and low-affinity calcium-binding sites of intact bovine brain calmodulin (CaM) and from the tryptic fragment 78-148, purified by high-pressure liquid chromatography, containing only the high-affinity calcium-binding sites, was determined by fluorescence stopped-flow with 2-p-toluidinylnaphthalene sulfonate (TNS). The tryptic fragments 1-77 and 78-148 each contain a calcium-dependent TNS-binding site, as shown by the calcium-dependent increase in TNS fluorescence. The rate of the monophasic fluorescence decrease in endogenous tyrosine on calcium dissociation from intact calcium-saturated calmodulin (kobs 10.8 s-1 and 3.2 s-1 at 25 degrees C and 10 degrees C respectively) as well as the rate of equivalent slow phase of the biphasic decrease in TNS fluorescence (kobsslow 10.6 s-1 and 3.0 s-1 at 25 degrees C and 10 degrees C respectively) and the rate of the solely monophasic decrease in TNS fluorescence, obtained with fragment 78-148 (kobs 10.7 s-1 and 3.5 s-1 at 25 degrees C and 10 degrees C respectively), were identical, indicating that the rate of the conformational change associated with calcium release from the high-affinity calcium-binding sites on the C-terminal half of calmodulin is not influenced by the N-terminal half of the molecule. The fast phase of the biphasic decrease of TNS fluorescence, observed by the N-terminal half of the molecule. The fast phase of the biphasic decrease of TNS fluorescence, observed with intact calmodulin only (kobsfast 280 s-1 at 10 degrees C) but not with fragment 78-148, is most probably due to the conformational change associated with calcium release from low-affinity sites on the N-terminal half. The calmodulin fragments 1-77 and 78-148 neither activated calcium/calmodulin-dependent protein kinase of cardiac sarcoplasmic reticulum nor inhibited calmodulin-dependent activation at a concentration approximately 1000-fold greater (5 microM) than that of the calmodulin required for half-maximum activation (5.9 nM at 0.8 mM Ca2+ and 5 mM Mg2+) of calmodulin-dependent phosphoester formation.

Site-selective laser spectroscopy of lanthanide-binding sites in calmodulin

Site-selective laser spectroscopy has been used to resolve the spectral features of lanthanide fluorescence probe ions in calcium-binding proteins. The capabilities and characteristics of this technique are studied using bovine brain calmodulin where the calcium-binding sites are very similar. Two distinct spectral features are identified. These features were followed during a Eu3+ titration and were found to fill successively, showing they correspond to the high- and low-affinity sites. One set of spectral features is assigned to domains I and III, which are the high-affinity domains, while the other set is assigned to domains II and IV. Additional nonspecific binding is observed after the domains are filled. Tb3+ titrations confirmed earlier results that the tyrosine-containing domains fill second and third (R. W. Wallace, E. A. Tallant, M. E. Duckter, and W. Y. Cheung, 1982, J. Biol. Chem. 257(4), 1845-1854). Site-selective laser spectroscopy was also used to identify the presence of ethylene glycol bis(beta-aminoethyl ether) N,N'-tetraacetic acid contamination that could cause interference in titrations.

Erythrocyte cytosolic free Ca2+ and plasma membrane Ca2+-ATPase activity in cystic fibrosis

The properties of the Ca2+, Mg2+-ATPase of erythrocyte membranes from patients with cystic fibrosis (CF) were extensively compared to that of healthy controls. Following removal of an endogenous membrane inhibitor of the ATPase, activation of the enzyme by Ca2+, calmodulin, limited tryptic digestion or oleic acid, as well as inhibition by trifluoperazine, were studied. The only properties found to be significantly different (CF cells vs controls) were calmodulin-stimulated peak activity (90 vs 101, P less than 0.02) and trypsin-activated peak activity (92 vs 102, P less than 0.02). No significant difference could be measured in the steady-state Ca2+-dependent phosphorylation of CF and control erythrocyte membranes indicating similar numbers of enzyme molecules per cell. The functional state of Ca2+ homeostasis in intact erythrocytes was investigated by measuring the resting cytosolic free Ca2+ levels using quin-2. Both CF and control erythrocytes maintained cytosolic free Ca2+ between 20 to 30 nM. Addition of 50 uM trifluoperazine resulted in an increase in erythrocyte cytosolic free Ca2+ to about 50 nM in both CF and control cells. Estimates of erythrocyte membrane permeability using the steady-state uptake of 45Ca into intact erythrocytes revealed no differences between CF and control cells. These results confirm that there is a small decrease in the calmodulin-stimulated activity of the erythrocyte Ca2+, Mg2+-ATPase in CF. However, this deficit is apparently not large enough to impair the ability of the CF erythrocyte to maintain normal resting levels of cytosolic free Ca2+.

Comparison of the membrane-bound (Ca2+ + Mg2+)-ATPase in erythrocyte ghosts from some mammalian species

The properties of the membrane-bound calcium-pumping protein, the (Ca2+ + Mg2+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) were compared in erythrocyte ghosts isolated from five mammalian species--human (Homo sapiens), bovine (Bos taurus), porcine (Sus scrofa melitensis), ovine (Ovis aries crassicandus) and caprine (Capra hircus syriaca). The specific activity of the enzyme in porcine erythrocytes is one order of magnitude higher than in the other species. It was also stimulated to various extents by the regulator protein, calmodulin, and by phosphatidylinositol in all the species. Analysis of membrane proteins revealed a number of differences which seem to suggest that the molecular architecture of the red cell membrane influences the activity of the enzyme.

An enzymatic assay for calmodulins based on plant NAD kinase activity

NAD kinase with increased sensitivity to calmodulin was purified from pea seedlings (Pisum sativum L., Willet Wonder). Assays for calmodulin based on the activities of NAD kinase, bovine brain cyclic nucleotide phosphodiesterase, and human erythrocyte Ca2+-ATPase were compared for their sensitivities to calmodulin and for their abilities to discriminate between calmodulins from different sources. The activities of the three enzymes were determined in the presence of various concentrations of calmodulins from human erythrocyte, bovine brain, sea pansy (Renilla reniformis), mung bean seed (Vigna radiata L. Wilczek), mushroom (Agaricus bisporus), and Tetrahymena pyriformis. The concentrations of calmodulin required for 50% activation of the NAD kinase (K0.5) ranged from 0.520 ng/ml for Tetrahymena to 2.20 ng/ml for bovine brain. The K0.5's ranged from 19.6 ng/ml for bovine brain calmodulin to 73.5 ng/ml for mushroom calmodulin for phosphodiesterase activation. The K0.5's for the activation of Ca2+-ATPase ranged from 36.3 ng/ml for erythrocyte calmodulin to 61.7 ng/ml for mushroom calmodulin. NAD kinase was not stimulated by phosphatidylcholine, phosphatidylserine, cardiolipin, or palmitoleic acid in the absence or presence of Ca2+. Palmitic acid had a slightly stimulatory effect in the presence of Ca2+ (10% of maximum), but no effect in the absence of Ca2+. Palmitoleic acid inhibited the calmodulin-stimulated activity by 50%. Both the NAD kinase assay and radioimmunoassay were able to detect calmodulin in extracts containing low concentrations of calmodulin. Estimates of calmodulin contents of crude homogenates determined by the NAD kinase assay were consistent with amounts obtained by various purification procedures.

Evidence for the involvement of (Cu-ATP)2- in the inhibition of human erythrocyte (Ca2+ + Mg2+)-ATPase by copper

The effects of copper on the activity of erythrocyte (Ca2+ + Mg2+)-ATPase have been tested on membranes stripped of endogenous calmodulin or recombined with purified calmodulin. The interactions of copper with Ca2+, calmodulin and (Mg-ATP)2- were determined by kinetic studies. The most striking result is the potent competitive inhibition exerted by (Cu-ATP)2- against (Mg-ATP)2- (Ki = 2.8 microM), while free copper gives no characteristic inhibition. Our results also demonstrate that copper does not compete with calcium either on the enzyme or on calmodulin. The fixation of calmodulin on the enzyme is not altered in the presence of copper as shown by the fact that the dissociation constant remains unaffected. It may be speculated that (Cu-ATP)2- is the active form of copper, which could plausibly be at the origin of some of the pathological features of erythrocytes observed in conditions associated with excess copper.

Cetiedil inhibition of calmodulin-stimulated enzyme activity

Cetiedil, an in vitro anti-sickling agent, inhibited calmodulin-stimulated cyclic 3':5'-nucleotide phosphodiesterase (EC 3.1.4.17) and Ca2+-ATPase (ATP phosphohydrolase, EC 3.6.1.3) activities. The drug had no effect on basal enzyme activities in the absence of calmodulin. The inhibition of phosphodiesterase was competitive with respect to the concentrations of both cAMP and calmodulin. Cetiedil did not inhibit calmodulin-stimulated enzyme activities by acting as a calcium chelator, since increasing the concentration of calcium did not reverse the inhibitory effect.

Preparation and properties of calcium-dependent resins with increased selectivity for calmodulin

Calmodulin from both animal and plant sources is known to bind a number of hydrophobic compounds with resultant inhibition of calmodulin function. Some of these compounds, including certain phenothiazine and naphthalene sulfonamide derivatives, have been previously shown to be useful in the chromatographic isolation of calmodulin, when covalently linked to a solid support. With the exception of fluphenazine linked to epoxide-activated Sepharose, these resins have the undesirable characteristics of requiring high salt concentrations in the elution buffer for efficient elution of calmodulin, thus decreasing the selectivity for this protein. The synthesis of nine Sepharose-ligand affinity resins is reported. Some of the ligands are newly synthesized naphthalene sulfonamide and phenothiazine derivatives. The synthetic ligands have been coupled to three types of Sepharose: epoxide-activated, CNBr-activated, and carbodiimide-activated. The properties of these resins are reported and their relative abilities to act selectively in the isolation of calmodulin are compared. 2-Trifluoromethyl-10-aminopropyl phenothiazine (TAPP), when linked to epoxide-activated Sepharose, was found to be the most useful for calmodulin isolation in terms of its combined stability, capacity, and ability to select for calmodulin. This resin was found to behave as a true affinity resin. A quantitative evaluation of its affinity behavior was consistent with the presence of two high-affinity Ca2+-dependent phenothiazine binding sites on calmodulin, in apparent agreement with previous reports which involved the use of different methods.

Inhibition of calmodulin-activated Ca2+-ATPase by propranolol and nadolol

Propranolol, at concentrations ranging from 0.05 to 0.5 mM, inhibits the calmodulin-activated Ca2+-ATPase of human erythrocyte membranes. In the same concentration range it is without effect on the basal Ca2+-ATPase. The inhibition is competitive and appears to be due to membrane binding, rather than to combination with cytoplasmic calmodulin as is the case for phenothiazines. This effect of propranolol may explain its ability to open the calcium-gated potassium channel, and could also be related to its action as a beta-adrenergic blocker. Nadolol, another beta-adrenergic blocker, is also an inhibitor of calmodulin-activated Ca2+-ATPase.

Ion binding to calmodulin. A comparison with other intracellular calcium-binding proteins

Over the past few years calcium has emerged as an important bioregulator. Upon external stimulation, the cell generates a transient Ca2+ increase, which is transformed into a cellular event through a molecular cascade. The first step in this cascade is the binding of calcium to proteins present in the cytosol. These proteins capable of binding Ca2+ under physiological conditions all belong to the same evolutionary family that evolved from a common ancestor. However, they strongly differ in the properties of their calcium binding sites. Calmodulin, the ubiquitous calcium binding protein present in all eukaryotic cells, is very close to the ancestor protein, presents four calcium binding sites which bind calcium, magnesium and monovalent ions competitively and is involved in the triggering of cellular processes. Parvalbumin, another member of the family, is more specialized and found mostly in fast-twitch skeletal muscle. It binds calcium and magnesium with high affinity and seems to be involved in muscle relaxation. On the other hand, troponin C which confers Ca2+ sensitivity to acto-myosin interaction exhibits both triggering and relaxing sites. The study of intracellular Ca2+ binding proteins has shown that calcium binding proteins have evolved from a simple common structure to fulfill different functions.

Calmodulin-binding proteins of bovine thyroid plasma membranes

Calmodulin binding proteins in bovine thyroid plasma membranes were investigated using the 125I-labeled calmodulin gel overlay technique. The purified thyroid plasma membranes contained two calmodulin binding proteins with molecular weights of approx. 220 000 and 150 000 respectively. The binding of 125I-labeled calmodulin to the calmodulin binding proteins was inhibited by excess unlabeled calmodulin, 100 microM trifluoperazine or 1 mM EGTA, indicating that the binding was calmodulin-specific and calcium-dependent. The calmodulin binding proteins appear to be components of the cytoskeleton since they remained in the pellet after treatment of the thyroid plasma membranes with 1% Triton X-100. Similar calmodulin binding proteins were present in rat liver plasma membranes, but not in human red blood cell plasma membranes. These two calmodulin binding proteins may interact with other components of the cytoskeleton and regulate endocytosis, exocytosis and hormone secretion in thyroid cells.

Nature of the (Ca2+ + Mg2+)-ATPase activator protein which associates with human erythrocyte membranes

(Ca2+ + Mg2+)-ATPase activator protein associated with human erythrocyte membranes could be extracted with EDTA under isotonic condition at pH 7.6. No activator was released, however, using isotonic buffer alone. Like calmodulin, the activator in the EDTA extract migrated as a fast moving band on polyacrylamide gel electrophoresis. It was also heat-stable, was capable of stimulating active calcium transport and could stimulate (Ca+ + Mg2+)-ATPase to the same extent. When chromatographed on a Sephacryl S-200 column, it was eluted in the same position as calmodulin and a membrane associated (Ca2+ + Mg2+)-ATPase activator prepared according to Mauldin and Roufogalis (Mauldin, D. and Roufogalis, B. D. (1980) Biochem. J. 187, 507-513). Furthermore, both Mauldin and Roufogalis protein and the activator in the EDTA extract exhibited calcium-dependent binding to a fluphenazine-Sepharose affinity column. On the basis of these data, it is concluded that the activator protein released from erythrocyte membranes by EDTA is calmodulin. A further pool of the ATPase activator could be released by boiling but not by Triton X-100 treatment of the EDTA-extracted membranes. This pool amounted to 8.9% of the EDTA-extractable pool.

Characterization of the Ca2+ binding sites of calmodulin from bovine testis using 43Ca and 113Cd NMR

The exchange rates of Ca2+ ions to the two classes of sites on calmodulin have been determined from the temperature dependence of the 43Ca NMR line width. The exchange rates were found to differ by a factor of about 40 at room temperature. The apparent pK value for one of these classes was estimated from the pH dependence of the 43Ca line width. The pK values of the two-high-affinity sites were found to differ about 0.1-1. Zn2+ ions were shown to bind to calmodulin and affect the exchange rates of Ca2+ and Cd2+ for all four sites.

Regulation of the Ca2+-pump by calmodulin in intact cells

ATP-enriched human red cells display high rates of Ca2+-dependent ATP hydrolysis (16 mmol . litre cells-1 . h-1) with a high Ca2+ affinity (K0.5 approximately 0.2 microM). The finding suggests a mechanism for regulation of cell Ca2+ levels, involving highly-cooperative stimulation of active Ca2+ extrusion following binding of calmodulin to the (Ca2+ +Mg2+)-ATPase.

Calmodulin-dependent spectrin kinase activity in resealed human erythrocyte ghosts

Membrane protein phosphorylation has been studied in resealed human erythrocyte ghosts by measuring the incorporation of 32P into spectrin and band 3. Norepinephrine- and Ca2+-stimulated phosphate incorporation was diminished in ghosts depleted of calmodulin. Ghosts prepared with endogenous calmodulin showed Ca2+- and norepinephrine-stimulated protein phosphorylation only when the ghosts had been resealed in the presence of (gamma-32P)ATP. Ghosts resealed with or without calmodulin in the presence of unlabeled ATP showed no net gain or loss of 32P when exposed to norepinephrine or a Ca2+-specific ionophore. These observations suggest that Ca2+ and norepinephrine stimulation of membrane protein phosphorylation is mediated by calmodulin-dependent spectrin kinase activity, and not by increased turnover of spectrin ATPase or by inhibition of phosphospectrin phosphatase.

Calcium ions, drug action and the red cell membrane

Intracellular calcium regulates a number of membrane functions in the erythrocyte, including control of shape, membrane lipid composition and cation permeability. Measurement of total red cell calcium has yielded values between 5 and 15 nmol/ml cells, and these low values in part reflect the absence of Ca2+ -containing organelles. Most intracellular Ca2+ is bound and the low cell ionized Ca2+ concentration (approximately 0.2 microM) is maintained by a combination of low membrane permeability and a powerful Ca2+ -pump. This pump has been identified with a (Ca2+ + Mg2+)-stimulated ATPase, and both Ca2+ transport and ATP splitting are stimulated by calmodulin, a low molecular weight protein which binds Ca2+ avidly and activates many Ca2+ -dependent enzymes. Both high and low affinity kinetics for Ca2+ pumping have been demonstrated, depending on the extent of binding of calmodulin to the pump. A stoichiometry of either 1 or 2 Ca2+ ions pumped per ATP molecule split has been shown, and the value may vary with the level of intracellular Ca2+. Phenothiazines, such as chlorpromazine inhibit the Ca2+ -pump by antagonizing the increment in activity produced by calmodulin. The passive inward leak of Ca2+ into erythrocytes can be quantitated by 45Ca2+ uptake into red cells whose Ca2+ -pump has been inhibited. Estimates of the Ca2+ permeability, based on unidirectional influx, yield values many orders of magnitude lower than for nucleated cells. Influx of Ca2+ into human erythrocytes occurs by a facilitated diffusion process, which can be inhibited by phenothiazines and the cinchona alkaloids. Calcium affects many membrane functions including cation permeability, lipid composition and some cytoskeletal interactions which may determine cell shape. Any rise in intracellular Ca2+ activates a specific K+ channel which normally makes little contribution to K+ fluxes. Kinetic studies of this process demonstrate either high or low affinity Ca2+ -activation of K+ efflux, with low affinity of the channel to Ca2+ being the probable state in vivo. Propranolol is the best known activator of Ca2+ -stimulated K+ efflux, although the mechanism of stimulation is unclear. Like other tissues, red cells possess a Ca2+ -activated phosphoinositol phosphodiesterase. Although it has been suggested that the echinocytic shape change induced by Ca2+ is due to the hydrolysis of polyphosphoinositides, it seems more likely that this shape change results from an effect of Ca2+ on the macromolecular interactions of the cytoskeleton. Abnormal Ca2+ permeability may contribute to red cell destruction in a variety of diseases. For example, in sickle cell anemia a large Ca2+ influx occurs when cells are sickled under deoxy conditions, and moreover, the ability of the Ca2+ -pump to extrude the increment of cell Ca2+ is impaired. Thus, red cell Ca2+ is increased 3-7-fold above normal and this may contribute to the short survival of sickle red cells...

Decrease of apparent calmodulin affinity of erythrocyte (Ca2+ + Mg2+)-ATPase at low Ca2+ concentrations

The calmodulin activation of the (Ca2+ + Mg2+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) in human erythrocyte membranes was studied in the range of 1 nM to 40 microM of purified calmodulin. The apparent calmodulin-affinity of the ATPase was strongly dependent on Ca2+ and decreased approx. 1000-times when the Ca2+ concentration was reduced from 112 to 0.5 microM. The data of calmodulin (Z) activation were analyzed by the aid of a kinetic enzyme model which suggests that 1 molecule of calmodulin binds per ATPase unit and that the affinities of the calcium-calmodulin complexes (CaiZ) decreases in the order of Ca3Z greater than Ca4Z greater than Ca2Z greater than or equal to CaZ. Furthermore, calmodulin dissociates from the calmodulin-saturated Ca2+-ATPase in the range of 10(-7)-10(-6) M Ca2+, even at a calmodulin concentration of 5 microM. The apparent concentration of calmodulin in the erythrocyte cytosol was determined to be 3 to 5 microM, corresponding to 50-80-times the cellular concentration of Ca2+-ATPase, estimated to be approx. 10 nmol/h membrane protein. We therefore conclude that most of the calmodulin is dissociated from the Ca2+-transport ATPase in erythrocytes at the prevailing Ca2+ concentration (probably 10(-7)-10(-8) M) in vivo, and that the calmodulin-binding and subsequent activation of the Ca2+-ATPase requires that the Ca2+ concentration rises to 10(-6)-10(-5) M.

Stimulation by tubulin of an adenylate cyclase from murine plasmacytoma

Different brain tubulin preparations were shown to stimulate membrane-bound adenylate cyclase from the murine plasmacytoma MOPC 173. Purified tubulin devoid of microtubule-associated proteins and of nucleoside-diphosphate kinase activity was responsible for this stimulation. Activation of the basal adenylate cyclase activity occurred in less than 2 min at 32 degrees C and was amplified by a 4 degree C preincubation of tubulin with plasma membranes. Tubulin affected the Km and the V of the enzyme and was shown to be associated with the membrane during the activation phenomenon. Tubulin was more active on the basal adenylate cyclase activity than that stimulated by fluoride or guanosine 5'-[beta, gamma-imido]triphosphate. GTP has no effect on the tubulin-stimulated enzyme.