Calmodulin-microtubule association in cultured mammalian cells

Microtubules as Regulators of Neural Network Shape and Function: Focus on Excitability, Plasticity and Memory

Neuronal microtubules (MTs) are complex cytoskeletal protein arrays that undergo activity-dependent changes in their structure and function as a response to physiological demands throughout the lifespan of neurons. Many factors shape the allostatic dynamics of MTs and tubulin dimers in the cytosolic microenvironment, such as protein–protein interactions and activity-dependent shifts in these interactions that are responsible for their plastic capabilities. Recently, several findings have reinforced the role of MTs in behavioral and cognitive processes in normal and pathological conditions. In this review, we summarize the bidirectional relationships between MTs dynamics, neuronal processes, and brain and behavioral states. The outcomes of manipulating the dynamicity of MTs by genetic or pharmacological approaches on neuronal morphology, intrinsic and synaptic excitability, the state of the network, and behaviors are heterogeneous. We discuss the critical position of MTs as responders and adaptative elements of basic neuronal function whose impact on brain function is not fully understood, and we highlight the dilemma of artificially modulating MT dynamics for therapeutic purposes.

Review: Microtubules monitor calcium and reactive oxygen species signatures in signal transduction

Signal transductions require calcium (Ca²⁺) or reactive oxygen species (ROS) signatures, which act as chemical and electrical signals in response to various biotic and abiotic stresses. Calcium as an ion or second messenger affects the membrane potential and microtubules (MTs) dynamicity, while MTs can modulate auto-propagating waves of calcium and ROS signatures in collaboration with ion channels depending on the stimulus type. Thus, in the current review, we highlight advances in research focused on the relationship between dynamic MTs and calcium and ROS signatures in short-distance transmission. The challenges of Ca²⁺-MTs-ROS crosstalk in cold sensing are addressed, which could suggest the prioritization of ROS or Ca²⁺ in signalling.

NMDA receptor activation suppresses microtubule growth and spine entry

Dynamic microtubules are important to maintain neuronal morphology and function, but whether neuronal activity affects the organization of dynamic microtubules is unknown. Here, we show that a protocol to induce NMDA-dependent long-term depression (LTD) rapidly attenuates microtubule dynamics in primary rat hippocampal neurons, removing the microtubule-binding protein EB3 from the growing microtubule plus-ends in dendrites. This effect requires the entry of calcium and is mediated by activation of NR2B-containing NMDA-type glutamate receptor. The rapid NMDA effect is followed by a second, more prolonged response, during which EB3 accumulates along MAP2-positive microtubule bundles in the dendritic shaft. MAP2 is both required and sufficient for this activity-dependent redistribution of EB3. Importantly, NMDA receptor activation suppresses microtubule entry in dendritic spines, whereas overexpression of EB3-GFP prevents NMDA-induced spine shrinkage. These results suggest that short-lasting and long-lasting changes in dendritic microtubule dynamics are important determinants for NMDA-induced LTD.

Calmodulin bound to stress fibers but not microtubules involves regulation of cell morphology and motility

Calmodulin (CaM) is a major cytoplasmic calcium receptor that performs multiple functions including cell motility. To investigate the mechanism of the regulation of CaM on cell morphology and motility, first we checked the distribution of CaM in the living cells using GFP-CaM as an indicator. We found that GFP-CaM showed a fiber-like distribution pattern in the cytosol of living Potorous tridactylis kidney (PtK2) cells but not in living HeLa cells. The endogenous CaM in heavily permeabilized HeLa was also found to display a fiber-like distribution pattern. Further examination showed that the distribution pattern of GFP-CaM was same as that of stress fibers, but not microtubules. Co-immunoprecipitation also showed that CaM can interact with actin directly or indirectly. The microinjection of trp peptide, a specific inhibitor of CaM, attenuated the polymerization of stress fibers and induced the alteration of cell morphology. A wound-healing assay and a single cell tracking experiment showed that CaM in PtK2 cells could increase cell motility. The data we have got from living cells suggested that CaM affect cell morphology and motility through binding to stress fibers and regulate f-actin polymerization.

The L-Type Calcium Channel Blocker Nimodipine Mitigates Cytoskeletal Proteins Phosphorylation in Dichlorvos-Induced Delayed Neurotoxicity in Rats

The present investigation was carried out to assess the protective efficacy of nimodipine against dichlorvos-induced organophosphate induced delayed neurotoxicity (OPIDN). Single subcutaneous dose of dichlorvos (200 mg/kg body weight) led to a consistent increase in the activity of both microtubule associated protein kinases viz. Ca2+/Calmodulin-dependent and cAMP dependent protein kinases, at all post exposure intervals (day 7, 15 and 21) as compared to that of controls. Autoradiography followed by microdensitometric studies demonstrated enhanced phosphorylation of 55 kDa and 280 kDa proteins in dichlorvos-exposed animals. These two proteins were confirmed to be tubulin and microtubule associated protein-2 (MAP-2) by western blotting. The hyperphosphorylation of these two proteins was shown to interfere with the assembly of neuronal microtubules as shown by electron microscopic studies that may eventually lead to possible disruption of neuronal cytoarchtecture resulting in axonal degeneration. Administration of nimodipine along with dichlorvos brought about a significant reduction in the activities of both the kinases as well as the extent of microtubule associated protein phosphorylation. This indicates that nimodipine, a centrally acting calcium channel blocker, may contribute to the amelioration of dichlorvos induced neurotoxicity by attenuation of calcium mediated disruption of cytoskeletal proteins and hence, calcium channel blockers like nimodipine have great future as new therapeutic agents for OPIDN.

Possible role of enhanced microtubule phosphorylation in dichlorvos induced delayed neurotoxicity in rat

The effect of a single subcutaneous dose of 200 mg/kg body weight dichlorvos on neuronal microtubule phosphorylation has been studied in rat following the development of organophosphate induced delayed neurotoxicity (OPIDN). Microtubule associated Ca2+/calmodulin dependent as well as cAMP dependent protein kinases were assayed. Dichlorvos administration led to a consistent increase in the activity of both the kinases at all post exposure intervals (7th, 15th and 21st day) as compared to that of controls. After in vitro phosphorylation using [gamma-32P]ATP, various proteins were resolved on one-dimensional 8% SDS-PAGE, stained with Coomassie Blue and autoradiographed. The amount of 32P incorporated was quantified by microdensitometry. Dichlorvos enhanced the phosphorylation of 55- and 280-kDa proteins. These two proteins were identified as tubulin and microtubule associated protein-2 (MAP-2) by immunoblotting. This study showed that dichlorvos induced hyperphosphorylation of tubulin and MAP-2 which in turn destabilizes microtubule assembly, and may ultimately result in axonal degeneration leading to dichlorvos induced delayed neurotoxicity.

Kinase and phosphatase inhibitors cause rapid alterations in microtubule dynamic instability in living cells

To examine whether microtubule dynamic instability can be rapidly regulated during interphase, we used video-enhanced differential interference contrast (DIC) microscopy to observe individual microtubules at the periphery of living newt lung epithelial cells. Microtubules were observed before and after perfusion with either the phosphatase inhibitor okadaic acid or the kinase inhibitors staurosporine or olomoucine. Addition of these inhibitors caused rapid changes in dynamic instability. Thirty to sixty seconds after perfusion with 0.2-1 microM okadaic acid, a 1.5-fold increase in elongation velocity and small increases in catastrophe and rescue frequencies were observed. In contrast, treatment with 40-200 nM staurosporine decreased microtubule elongation and shortening velocities approximately 2-fold, and catastrophes were slightly more frequent. Olomoucine, at 100 microM, had similar effects. Transition dynamics were further examined by probabilistic analysis, which showed that microtubules become more likely to undergo catastrophe as they elongated and more likely to undergo rescue as they shortened, an effect previously called microtubule "memory." This memory effect for catastrophes was observed in untreated and okadaic acid-treated cells but was abolished by staurosporine or olomoucine. In contrast, the memory effect for rescue was unaffected by these treatments, suggesting that catastrophe and rescue proceed via distinct, multistep mechanisms. Overall, these results demonstrate that microtubule assembly regulators can be altered rapidly by inhibition of either kinases or phosphatases and suggest that, in the absence of inhibitors, these regulators exist in a dynamic equilibrium between phosphorylated and dephosphorylated states.

Cyclosporine A affects the organization of cytoskeletal fibrillar proteins in rat thymus

We have evaluated whether cyclosporine A affects cell structure and cytoskeletal proteins of the thymus of Wistar rats. Immunohistochemical analysis showed that expression of the cytoskeletal proteins vimentin and desmin was much higher in epithelial cells, dendritic cells and lymphocytes in the thymus of treated rats than in untreated controls. Protein expression was observed as a positive condensation in a distinct area near the nucleus with a capping-like configuration. An ultrastructural study showed that the amount of cytoskeletal fibrillar structures was increased in the treated rats. The structures were assembled in a limited area of the cell with a nuclear capping-like configuration which was in agreement with the light microscopical observations. Immunoblotting analysis demonstrated that vimentin and desmin had a lower molecular weight in treated rats than in controls (57 and 53 kDa versus 55 and 51 kDa, respectively). The results clearly indicate that cyclosporine A affects the structure of the cytoskeleton suggesting that this could be the first step in its immunosuppressive effects by altering nucleus/cytoplasm signaling.

Dynamic redistribution of calmodulin in HeLa cells during cell division as revealed by a GFP-calmodulin fusion protein technique

It has been suggested by many studies that Ca2+ signaling plays an important role in regulating key steps in cell division. In order to study the down stream components of calcium signaling, we have fused the gene of calmodulin (CaM) with that of green fluorescent protein (GFP) and expressed it in HeLa cells. The GFP-CaM protein was found to have similar biochemical properties as the wild-type CaM, and its distribution was also similar to that of the endogenous CaM. Using this GFP-tagged CaM as a probe, we have conducted a detailed examination of the spatial- and temporal-dependent redistribution of calmodulin in living mammalian cells during cell division. Our major findings are: (1) high density of CaM was found to distribute in two sub-cellular locations during mitosis; one fraction was concentrated in the spindle poles, while the other was concentrated in the sub-membrane region around the cell. (2) The sub-membrane fraction of CaM became aggregated at the equatorial region where the cleavage furrow was about to form. The timing of this localized aggregation of CaM was closely associated with the onset of cytokinesis. (3) Using a TA-CaM probe, we found that the sub-membrane fraction of CaM near the cleavage furrow was selectively activated during cell division. (4) When we injected a CaM-specific inhibitory peptide into early anaphase cells, cytokinesis was either blocked or severely delayed. These findings suggest that, in addition to Ca2+ ion, CaM may represent a second signal that can also play an active role in determining the positioning and timing of the cleavage furrow formation.

Puromycin-sensitive aminopeptidase. Sequence analysis, expression, and functional characterization

Among the molecular mechanisms that control the cell division cycle, proteolysis has emerged as a key regulatory process enabling cells to pass critical check points. Such proteolysis involves a cascade of enzymes including a multisubunit complex termed 26S proteasome. Here we report on the analysis of a novel mouse cDNA encoding the puromycin-sensitive aminopeptidase (PSA) and on its expression in COS cells and 3T3 fibroblasts. PSA is 27-40% homologous to several known Zn(2+)-binding aminopeptidases including aminopeptidase N. Immunohistochemical analysis revealed that PSA is localized to the cytoplasm and to the nucleus and associates with microtubules of the spindle apparatus during mitosis. Furthermore, puromycin and bestatin both arrested the cell cycle, leading to an accumulation of cells in G2/M phase, and ultimately induced cells to undergo apoptosis at concentrations that inhibit PSA. Control experiments including cycloheximide further suggested that the induction of apoptosis by puromycin was not attributable to inhibition of protein synthesis. Taken together, these data favor the novel idea that PSA participates in proteolytic events essential for cell growth and viability.

Intrinsic microtubule stability in interphase cells

Interphase microtubule arrays are dynamic in intact cells under normal conditions and for this reason they are currently assumed to be composed of polymers that are intrinsically labile, with dynamics that correspond to the behavior of microtubules assembled in vitro from purified tubulin preparations. Here, we propose that this apparent lability is due to the activity of regulatory effectors that modify otherwise stable polymers in the living cell. We demonstrate that there is an intrinsic stability in the microtubule network in a variety of fibroblast and epithelial cells. In the absence of regulatory factors, fibroblast cell interphase microtubules are for the most part resistant to cold temperature exposure, to dilution-induced disassembly and to nocodazole-induced disassembly. In epithelial cells, microtubules are cold-labile, but otherwise similar in behavior to polymers observed in fibroblast cells. Factors that regulate stability of microtubules appear to include Ca2+ and the p34cdc2 protein kinase. Indeed, this kinase induced complete destabilization of microtubules when applied to lysed cells, while a variety of other protein kinases were ineffective. This suggests that p34cdc2, or a kinase of similar specificity, may phosphorylate and inactivate microtubule-associated proteins, thereby conferring lability to otherwise length-wise stabilized microtubules.

Calcium Levels Affect the Ability to Immunolocalize Calmodulin to Cortical Microtubules

Calcium affects the stability of cortical microtubules (MTs) in lysed protoplasts. This calmodulin (CaM)-mediated interaction may provide a mechanism that serves to integrate cellular behavior with MT function. To test the hypothesis that CaM associates with these MTs, monoclonal antibodies were produced against CaM, and one (designated mAb1D10) was selected for its suitability as an immunocytochemical reagent. It is shown that CaM associates with the cortical MTs of cultured carrot (Daucus carota L.) and tobacco (Nicotiana tabacum L.) cells. Inasmuch as CaM interacts with calcium and affects the behavior of these MTs, we hypothesized that calcium would alter this association. To test this, protoplasts containing taxol-stabilized MTs were lysed in the presence of various concentrations of calcium and examined for the association of CaM with cortical MTs. At 1 [mu]M calcium, many protoplasts did not have CaM in association with the cortical MTs, whereas at 3.6 [mu]M calcium, this association was completely abolished. Control experiments were performed to eliminate alternate explanations including differential antibody binding in the presence of calcium and/or taxol, detergent-induced redistribution of antigen, and epitope masking. The results are discussed in terms of a model in which CaM associates with MTs via two types of interactions, one that occurs in the presence of calcium and another that occurs only in its absence.

Disruption of cytoplasmic microtubules by ultraviolet radiation

Ultraviolet (UV) irradiation of cultured human skin fibroblasts causes the disassembly of their microtubules. Using indirect immunofluorescence microscopy, we have now investigated whether damage to the microtubule precursor pool may contribute to the disruption of microtubules. Exposure to polychromatic UV radiation inhibits the reassembly of microtubules during cellular recovery from cold treatment. In addition, the ability of taxol to promote microtubule polymerization and bundling is inhibited in UV-irradiated cells. However, UV irradiation of taxol-pretreated cells or in situ detergent-extracted microtubules fails to disrupt the microtubule network. These data suggest that damage to dimeric tubulin, or another soluble factor(s) required for polymerization, contributes to the disassembly of microtubules in UV-irradiated human skin fibroblasts.

Disruption of microtubules as an early sign of irreversible ischemic injury. Immunohistochemical study of in situ canine hearts

Structural disruption of the cytoskeleton may be involved in irreversible ischemic injury. In the present study, ischemic changes in microtubules during various periods of myocardial ischemia were studied with an immunohistochemical technique in open-chest dogs. In intact myocardium, microtubules were stained as a filamentous network throughout cytoplasm and a circular network around the nucleus, which disappeared with colchicine treatment. In brief ischemia of less than 15 minutes, microtubule patterns were unaltered. After 20 minutes, however, characteristic microtubule stains were partly lost in patchy lesions. As an increase in ischemic period, lesions of loss of microtubule stains were increased in number and size. After 120 minutes of reperfusion following 60 minutes of ischemia, the lesions with intact actin filaments but with disrupted microtubules were replaced by the severely injured cells in which the regular myofibrillar registrations were distinctly disrupted. After 24 hours of reperfusion following 40 minutes of occlusion of the left circumflex artery, the percent area of disrupted microtubules at 40 minutes of ischemia was replaced by that of irreversibly injured lesions in the posterior papillary muscle. These results indicate that disruption of microtubules during ischemia heralds irreversible ischemic injury. However, in in vitro study, the myocardium incubated in hypoxic solution for 60-120 minutes demonstrated earlier disruption of the microtubules than the vinculin. Electron microscopic study also showed minimal irreversible changes in the lesions with disrupted microtubules. Thus, taken together, we conclude that microtubules that support the structural integration of myofibrils and other organelles are disrupted in severe myocardial ischemia before the irreversible injury, promoting the irreversible change after reperfusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Surface blebbing and cytoskeletal changes induced in vitro by toxin B from Clostridium difficile: an immunochemical and ultrastructural study

Clostridium difficile toxin B is a powerful cytopathic agent without enterotoxic activity which is believed to be involved in the pathogenesis of pseudomembranous colitis. Up until today, the mechanisms of toxin B cytotoxicity have not been elucidated. The results of in vitro studies performed on different cell lines by means of immunocytochemical and ultrastructural methods are reported here. Low doses (0.15 ng/ml) of toxin B cause cell rounding and arborization. Higher doses (up to 15 micrograms/ml) induce cell rounding and appearance of numerous surface protrusions with blister or bulb-like features. These "blebs" belong to the potocytotic type, the bleb matrix being devoid of cytoplasmic organelles and filled with ribosomes only. Furthermore, a peculiar role of cytoskeletal apparatus in this phenomenon has been detected. In fact, morphological rearrangement occurs in cytoskeletal elements, mainly represented by the presence, in the blebs, of tubulin and of the actin-binding proteins alpha-actinin, filamin, and calmodulin, while actin and intermediate filaments, keratin and vimentin, appear to be absent. Moreover, beta 2-microglobulin, considered as a surface protein marker, seems to undergo changes in its expression, being absent over the blebbing surface. The results of this study may support the view that C. difficile toxin B affects one or more subcellular components that regulate the structure and function of the actin cytoskeleton.

Calmodulin-binding proteins and calcium/calmodulin-regulated enzyme activities associated with brain actomyosin

Calcium- and calmodulin-regulated ATPase and protein kinase activities are shown to be strongly associated with brain actomyosin. Similar enzymatic activities and an invariable polypeptide profile on sodium dodecyl sulfate-polyacrylamide gel electrophoresis were obtained for brain actomyosin taken through a solubilization-precipitation cycle (1.0-0.1 M KCl), or precipitated from buffers containing 1% Triton X-100 or 10 mM EDTA and 10 mM EGTA. These data suggest a specific complex of brain actomyosin with a protein kinase similar to calmodulin-dependent kinase II, a 190-kDa calmodulin-binding protein (P190), and a calmodulin-like polypeptide. P190 was the major substrate for endogenous calcium-dependent phosphorylation. 125I-Calmodulin overlay technique revealed four major calmodulin-binding polypeptides associated with brain actomyosin: 50- and 60-kDa subunits of the calmodulin-dependent kinase II, P190, and a high molecular weight polypeptide which is probably fodrin. A fraction enriched in P190 had Ca2(+)- and calmodulin-stimulated MgATPase activity, but not myosin-like K-EDTA ATPase activity. The lack of immunological cross-reactivity between brain myosin heavy chain and P190 confirmed that they are distinct molecules.

Interaction between S-100 proteins and steady-state and taxol-stabilized microtubules in vitro

S-100 proteins are a group of three 21-kilodalton, acidic, Ca2+-binding proteins of the "E-F hand" type shown to regulate several cell activities, including microtubule (MT) assembly-disassembly. We show here that S-100 proteins interact with MTs assembled from either whole microtubule protein or purified tubulin, both in the absence and in the presence of the MT-stabilizing drug taxol. Evidence for the binding of S-100 to MTs comes from both kinetic (turbidimetric) and binding studies. Kinetically, S-100 enhances the disassembly of steady-state MTs in the presence of high concentrations of colchicine or vinblastine at 10 microM free Ca2+ and disassembles taxol-stabilized MTs at high Ca2+ concentrations. Experiments performed using 125I-labeled S-100 show that S-100 binds Ca2+ independently to a single set of sites on taxol-stabilized MTs assembled from pure tubulin with an affinity of 6 x 10(-5) M and a stoichiometry of 0.15 mol of S-100/mol of polymerized tubulin. Under certain conditions, S-100 proteins also cosediment with MTs prepared by coassembly of S-100 with MTs, probably in the form of an S-100-tubulin complex. Because S-100 binds to MTs under conditions where this protein fraction does not produce observable effects on the kinetics of assembly-disassembly, e.g., in the absence of Ca2+ at pH 6.7, we conclude that the S-100 binding to MTs does not affect the stability of MTs per se, but rather creates conditions for increased sensitivity of MTs to Ca2+.

Mechanism of endothelial cell shape change in oxidant injury

Changes in endothelial cell morphology induced by neutrophil-generated hydrogen peroxide (H2O2) may account for the capillary leak of the adult respiratory distress syndrome (ARDS). The relationship of H2O2 effects on the concentration of intracellular Ca2+ [( Ca2+]i) and ATP to changes in microfilaments and microtubules, important determinants of cell shape, was examined. Bovine pulmonary artery endothelial cells were injured over a 2-hr time course with a range of H2O2 doses (0-20 mM). The higher concentrations of H2O2 consistently produced contraction and rounding of greater than 50-75% of cells by 1-2 hr. The range of 1-20 mM H2O2 produced rapid, significant reductions in endothelial ATP levels over the time course of injury. Although there were significant increases in mean endothelial [Ca2+]i in response to 5, 10, and 20 mM H2O2, 1 mM H2O2 did not affect the [Ca2+]i. Fluorescence microscopy revealed that microfilament disruption occurred as ATP levels fell and preceded depolymerization of microtubules which developed after [Ca2+]i approached 1 X 10(-6) M. H2O2 at 1 mM injury caused microfilament disruption but did not depolymerize microtubules. Microfilament disruption occurred without oxidant exposure, when ATP levels were reduced by glucose depletion and mitochondrial inhibition with oligomycin (650 nM). If a Ca2+ ionophore, ionomycin (5 microM), was then added, [Ca2+]i rose to greater than 1 X 10(-6) M, microtubules fragmented and depolymerized, and cell contraction and rounding very similar to that induced by H2O2 occurred. These results suggest that endothelial cell dysfunction and capillary leak in ARDS may be due to H2O2-mediated changes in cellular ATP and [Ca2+]i.

Calmodulin is associated with microtubules forming in PTK1 cells upon release from nocodazole treatment

To investigate the association of calmodulin (CaM) with microtubules (MTs) in the mitotic apparatus (MA), the distributions of CaM and tubulin were examined in cells in which the normal spindle organization had been altered. A fluorescent CaM conjugate with tetramethylrhodamine isothiocyanate (CaM-TRITC) and a dichlorotriazinyl aminofluorescein conjugate with tubulin (tubulin-DTAF) were injected into cells that had been treated with the MT inhibitor nocodazole. With moderate nocodazole concentration (0.3 micrograms/ml, 37 degrees C, 4 h) in live cells, CaM-TRITC and tubulin-DTAF concentrated identically on or near the centrosomes and kinetochores. In serial sections of these cells, small MT segments were observed by transmission electron microscopy (TEM) in the regions where fluorescent protein had concentrated. When a higher drug concentration was used (3.0 micrograms/ml, 37 degrees C, 4 h), no regions of CaM-TRITC or tubulin-DTAF localization were observed, and no MTs were observed when serial sections were examined by TEM. However, following release from the high-concentration nocodazole block, CaM-TRITC colocalized with newly formed MTs at the kinetochores and centrosomes. Later in the recovery period, when chromosome-to-pole fibers had formed, CaM association with kinetochores diminished, ultimately attaining its normal pole-proximal association with kinetochore MTs in cells that progressed through mitosis. We interpret these observations as supporting the hypothesis that in the MA, CaM attains a physical association with kinetochore MTs and suggest that CaM-associated MTs may be inherently more stable.

Calmodulin stabilization of kinetochore microtubule structure to the effect of nocodazole

To investigate the function of calmodulin (CaM) in the mitotic apparatus, the effect of microinjected CaM and chemically modified CaMs on nocodazole-induced depolymerization of spindle microtubules was examined. When metaphase PtK1 cells were microinjected with CaM or a CaM-TRITC conjugate, kinetochore microtubules (kMTs) were protected from the effect of nocodazole. The ability of microinjected CaM to subsequently protect kMTs from the depolymerizing effect of nocodazole was dose dependent, and was effective for approximately 45 min, with protection decreasing if nocodazole treatment was delayed for more than 60 min after injection of CaM. The CaM-TRITC conjugate, similar to native CaM, displayed the ability to activate bovine brain CaM-dependent adenylate cyclase in a Ca++-dependent manner and showed a Ca++-dependent mobility shift when subjected to PAGE. A heat-altered CaM-TRITC conjugate also protected kMTs from the effect of nocodazole. However, this modified CaM was not able to activate adenylate cyclase nor did it display a Ca++-dependent mobility shift when electrophoresed. In a permeabilized cell model system, both CaM analogs were observed to bind to the spindle in a Ca++-independent manner. In contrast, a performic acid-oxidized CaM did not have a protective effect on spindle structure when microinjected into metaphase cells before nocodazole treatment. The oxidized CaM did not activate adenylate cyclase and did not exhibit Ca++-dependent mobility on polyacrylamide gels. These results are interpreted as supporting the hypothesis that CaM binds to the mitotic spindle in a Ca++-independent manner and that CaM may serve in the spindle, at least in part, to stabilize kMTs.

Quantitative analysis of the interaction between S-100 proteins and brain tubulin

S-100 was shown to regulate the in vitro assembly of brain microtubule proteins (MTPs) in a Ca2+-mediated way by acting on both the nucleation and the elongation of microtubules (MTs). Here data will be shown suggesting that S-100 binds to tubulin. The binding is time-, temperature-, Ca2+-, and pH-dependent, and saturable with respect to S-100. At pH 6.75, the saturation curve is biphasic, displaying a high affinity component (dissociation constant, Kd1, approximately 0.1 microM) and a low affinity component (Kd2 approximately 3.8 microM). At pH 6.75, as the free Ca2+ concentration raises from 0 to 100 microM, the overall binding capacity increases from 0.065 to 0.66 mol S-100/mol tubulin dimer. This finding, together with the observation that the S-100 effect on MTP assembly is Ca2+-dependent at that pH, suggests that the S-100-induced inhibition of MTP assembly depends on S-100 binding to the low affinity sites on the tubulin molecule. The S-100 binding to tubulin is pH-dependent; as the pH raises from 6.75 to 8.3, both binding components are affected, the major changes consisting of an increase in the binding capacity and a decrease in the overall affinity. Moreover, as the pH raises, Ca2+ is no longer required for S-100 to bind to tubulin. S-100 also interacts with a component of whole MTPs (probably tubulin, on the basis of the above results). No S-100 binding to microtubule-associated proteins (MAPs) could be evidenced by the techniques employed in this study. On the contrary, some competition between S-100 and MAPs for binding sites or tubulin seems to occur.

Calmodulin and calmodulin-binding proteins in the renal brush border

The calmodulin content of renal brush-border membrane vesicles, prepared by Mg2+-precipitation in EGTA-containing solutions, amounts to 1.8 micrograms per mg protein. The amount and the distribution of this EGTA-insensitive calmodulin was determined in membrane and cytoskeletal fractions prepared from the brush-border membrane vesicles by extraction with Triton X-100. The Triton X-100 insoluble pellet contains 21.2% of the protein and 52.2% of the EGTA-insensitive calmodulin, which amounts in this fraction to 4.4 micrograms per mg protein. Treatment of the Triton X-100 insoluble pellet, consisting of the microvillar core residue, with ATP and Mg2+ results in the solubilization of a relatively small number of proteins among which are actin, myosin, calmodulin and several calmodulin-binding proteins. The solubilization is partially reversible and a fraction of the proteins can be precipitated by centrifugation after the enzymatic hydrolysis of ATP. Readdition of ATP to the pellet results in the resolubilization of myosin, part of the actin, an 115-kDa calmodulin-binding protein and calmodulin. The calmodulin content of the final extract was 61.8 micrograms per mg protein. We have found roughly the same distribution pattern of calmodulin and ATP-solubilized, calmodulin-binding proteins in renal and intestinal brush-border preparations. The calmodulin content, however, as well as the relative amount of the calmodulin-binding proteins versus actin are about 4 to 5-times higher in intestinal than in renal microvillar core residues.

Early detection of cerebral ischemic damage and repair process in the gerbil by use of an immunohistochemical technique

After occlusion of the right common carotid artery in the gerbil, we monitored the progression of ischemic damage and postischemic damage and the repair process in the brain immunohistochemically by using tubulin, creatine kinase BB-isoenzyme (CK-BB), and neuron-specific enolase as the neuronal markers and astroprotein, glial fibrillary acidic protein, and CK-BB as the astrocytic markers. The earliest ischemic lesion was detected in the hippocampus and the cerebral cortex after ischemia for 5 minutes as loss of the reaction in the neuropil, nerve cell bodies, and dendrites. The reaction disappeared more promptly in the dendrites than in the nerve cell bodies. The reaction for tubulin was the most sensitive for detection of the neuronal ischemic damage. After an ischemic period of 30 minutes and subsequent reestablishment of cerebral circulation, the immunohistochemical lesions affecting the neuronal structure expanded during the first 3 hours and then slowly afterward for up to 12 hours. Reactive astrocytes were already identified 24 hours after reperfusion. The current investigation demonstrated that early ischemic damage can be clearly visualized by use of the immunohistochemical technique soon after the onset of cerebral ischemia but that considerable heterogeneity exists not only in different anatomic regions but also within the neuronal structure. This technique has potential for further investigation of cerebral ischemia or other pathophysiologic conditions when used in combination with other morphologic, physiologic, or biochemical techniques.

Aluminum and Alzheimer's disease

There is now substantial evidence indicating that an accumulation of aluminum occurs in grey matter in diseases associated with Alzheimer neurofibrillary degeneration. Four principle sites of aluminum accumulation have been identified in Alzheimer's disease: DNA containing structures of the nucleus, the protein moieties of neurofibrillary tangles, the amyloid cores of senile plaques and cerebral ferritin. Consideration of the extensive information now available on the toxic effects of aluminum in these four loci strengthens the hypothesis that aluminum could be important in the pathogenesis of this neurodegenerative process. The evidence, however, does not support an etiological role for aluminum in Alzheimer's disease. The primary pathogenic events responsible for Alzheimer's disease are presumed to have affected the genetically determined barriers to aluminum resulting in increased amounts of this toxic element to vulnerable target sites.

Mechanistic aspects on chemical induction of spindle disturbances and abnormal chromosome numbers

Work on the chemical induction of spindle disturbances and abnormal chromosome numbers, and work on the composition and biochemistry of the spindle are reviewed. Some early investigations have shown that there is an unspecific mechanism for chemical induction of spindle disturbances. This mechanism is based on the interaction of compounds with cellular hydrophobic compartments. Some compounds act differently and are more active than predicted from their lipophilic character. Selected compounds of that kind and their possible mechanisms of action are discussed. Changes in sulfhydryl and ATP levels, oxidative damage of membranes and impaired control of cytoplasmic Ca2+ levels are discussed in this context.

Cerebral ischemia in the gerbil: transmission electron microscopic and immunoelectron microscopic investigation

Progression of cerebral ischemia from 5 min to 3 h after occlusion of a common carotid artery was investigated in the subiculum-CA1 region of the hippocampus of the gerbil by transmission electron microscopic and immunoelectron microscopic technique. The earliest change was found after 5 min in the periphery of the apical dendrites in the stratum moleculare, where mitochondrial swelling and disintegration of microtubules were clearly seen inside swollen dendritic processes. After ischemia for 10 min, similar abnormalities were observed in the more proximal part of the apical dendrites, and the basal dendrites also became similarly affected. After ischemia for 30 min to 1 h, the pyramidal cell bodies showed mitochondrial swelling, distension of endoplasmic reticulum and disaggregation of polyribosomes. The immunoelectron microscopic procedure for tubulin revealed irregularity of reaction products associated with microtubules after ischemia for 5 min in the dendritic terminals in the stratum moleculare and in the stratum radiatum after ischemia for 10 min. Reaction products in the pyramidal cell bodies became sparse after ischemia for 30 min to 1 h. The present investigation revealed early onset of ischemic damage in the dendritic terminals and subsequent proximal extension, with disintegration of microtubules and mitochondrial swelling.

Distribution of calmodulin in pea seedlings: Immunocytochemical localization in plumules and root apices

Immunofluorescence techniques have been used to study the distribution of calmodulin in several tissues in young etiolated pea (Pisum sativum L.) seedlings. A fairly uniform staining was seen in the nucleoplasm and background cytoplasm of most cell types. Cell walls and nucleoli were not stained. In addition, patterned staining reactions were seen in many cells. In cells of the plumule, punctate staining of the cytoplasm was common, and in part this stain appeared to be associated with the plastids. A very distinctive staining of amyloplasts was seen in the columella of the root cap. Staining associated with cytoskeletal elements could be shown in division stages. By metaphase, staining of the spindle region was quite evident. In epidermal cells of the stem and along the underside of the leaf there was an intense staining of the vacuolar contents. Guard cells lacked this vacuolar stain. Vacuolar staining was sometimes seen in cells of the stele, but the most distinctive pattern in the stele was associated with young conducting cells of the xylem. These staining patterns are consistent with the idea that the interactions of plastids and the cytoskeletal system may be one of the Ca(2+)-mediated steps in the response of plants to environmental stimuli. Nuclear functions may also be controlled, at least in part, by Ca(2+).

Heterogeneous distribution of the cAMP receptor protein RII in the nervous system: evidence for its intracellular accumulation on microtubules, microtubule-organizing centers, and in the area of the Golgi complex

The cellular and subcellular distribution of the regulatory subunit RII of cAMP-dependent protein kinase was studied by light and electron microscopy immunocytochemistry in tissue sections from rat brain and in primary cultures of brain cells. RII immunoreactivity was present in most neurons, although at variable concentration. In addition, RII was also detectable in other cell types including glia, neuroepithelial cells, and cells of mesenchymal origin. In the cell cytoplasm, RII immunoreactivity was concentrated at certain sites. An accumulation of RII immunoreactivity was found in all RII-positive cells at the Golgi area, precisely at a region directly adjacent to one of the two major faces of the Golgi complex. RII was also highly concentrated in some microtubule-rich cell processes such as cilia and neuronal dendrites, but was below detectability in most axons. In neurons, its concentration in dendrites is consistent with the previously demonstrated high affinity interaction between RII and the dendritic microtubule-associated protein 2. In addition, RII was accumulated at basal bodies of cilia and at centrosomes, i.e., sites known to act as microtubule organizers. RII-labeled centrosomes, however, were visible only in cells where the Golgi complex had a pericentrosomal organization, and not in cells where the Golgi complex was perinuclear such as in neurons and glia in situ. We hypothesize that centrosomal RII is bound to the pericentriolar microtubule-organizing material and that this material remains associated with the trans region of the Golgi complex when the latter is no longer associated with the centrosome. Our results suggest a key but not obligatory role of cAMP in the Golgi-centrosomal area, the headquarters of cell polarity, mobility and intracellular traffic, and in the function of a subpopulation of microtubules.

Association of calcium/calmodulin-dependent kinase with cytoskeletal preparations: phosphorylation of tubulin, neurofilament, and microtubule-associated proteins

Calcium and calmodulin have been implicated in the regulation of cytoskeletal function. In this report, we demonstrate that microtubule preparations from rat brain contain a calcium/calmodulin-dependent protein kinase that phosphorylates endogenous MAP-2, tubulin, synapsin I, and neurofilament proteins. This cytoskeletal-associated kinase has been biochemically characterized and shown to be identical to Type II calcium/calmodulin-dependent protein kinase (CaM kinase II). The subunits of CaM kinase II represented major calmodulin-binding proteins in cytoskeletal preparations. A monoclonal antibody against the 52000 Da subunit of CaM kinase II specifically labeled cytoskeletal elements in cortical neurons. These results indicate that CaM kinase II is associated with the neuronal cytoskeleton and may play a role in mediating some of the effects of calcium on cytoskeletal function.

Dynamics of microtubule depolymerization in monocytes

Human monocytes, which contain few interphase microtubules (35.+/- 7.7), were used to study the dynamics of microtubule depolymerization. Steady-state microtubule assembly was abruptly blocked with either high concentrations of nocodazole (10 micrograms/ml) or exposure to cold temperature (3 degrees C). At various times after inhibition of assembly, cells were processed for anti-tubulin immunofluorescence microscopy. Stained cells were observed with an intensified video camera attached to the fluorescence microscope. A tracing of the entire length of each individual microtubule was made from the image on the television monitor by focusing up and down through the cell. The tracings were then digitized into a computer. All microtubules were seen to originate from the centrosome, with an average length in control cells of 7.1 +/- 2.7 microns (n = 957 microtubules). During depolymerization, the total microtubule polymer and the number of microtubules per cell decreased rapidly. In contrast, there was a slow decrease in the average length of the persisting microtubules. The half-time for both the loss of total microtubule polymer and microtubule number per cell was approximately 40 s for nocodazole-treated cells. The rate-limiting step in the depolymerization process was the rate of initiation of disassembly. Once initiated, depolymerization appeared catastrophic. Further kinetic analysis revealed two classes of microtubules: 70% of the microtubule population was very labile and initiated depolymerization at a rate approximately 23 times faster than a minor population of persistent microtubules. Cold treatment yielded qualitatively similar characteristics of depolymerization, but the initiation rates were slower. In both cases there was a significant asynchrony and heterogeneity in the initiation of depolymerization among the population of microtubules.

Immunohistochemical investigation of regional cerebral ischemia in the gerbil: occlusion of the posterior communicating artery

Evolution, progression and recovery of neural damage during and following cerebral ischemia were investigated in the gerbil after occlusion of a posterior communicating artery and by using the immunohistochemical reaction for tubulin and creatine kinase BB-isoenzyme which are enriched in the neuronal structure and the reaction for astroprotein which is specific for astrocytes. The transcardiac perfusion study with India ink revealed marked hypoperfusion diffusely in the hippocampus and moderately in the thalamus on the occluded side. The earliest immunohistochemical lesion, manifested as loss of the reaction for tubulin and creatine kinase BB-isoenzyme in dendrites and nerve cell bodies, was found in the CA1 and CA2 region of the hippocampus after ischemia for 4 min, while it took 10 min before the earliest lesion became visible in the ventral nucleus of the thalamus and it took over 1 h before scattered lesions evolved in granular cells of the dentate gyrus. The staining with hematoxylin-eosin was much less sensitive in detection of early ischemic lesions. After re-establishment of blood flow to the posterior communicating artery, the ischemic lesions which were visualized with the reaction for tubulin or creatine kinase BB-isoenzyme disappeared or reduced the size, if the ischemic period was brief. Beyond a certain ischemic period, the lesion expanded further during the early postischemic period. The reaction for astroprotein visualized reactive astrocytes even in the area without any abnormalities with other reactions, an evidence of subtle ischemic insults.

The interaction of calmodulin with rat liver plasma membrane

Rat liver plasma membranes contain relatively high amounts of EGTA-insensitive calmodulin which seems to interact with cytoskeletal proteins. Calmodulin is particularly enriched in a subplasmamembrane fraction containing basolateral membranes. Two calmodulin-binding proteins with apparent Mr of 240 KDa and 145 KDa have been found associated with the purified plasmamembranes.

Cyclosporine A, an in vitro calmodulin antagonist, induces nuclear lobulations in human T cell lymphocytes and monocytes

Cyclosporine A is a noncytotoxic, natural, 11 amino acid cyclic peptide used clinically as an immunosuppressant to prevent organ rejection after transplantation. Cyclosporine A is an in vitro calmodulin antagonist. At the low concentrations required to inhibit calmodulin-dependent phosphodiesterase in vitro, cyclosporine A causes a dramatic alteration in the nuclear morphology of 23% of human peripheral blood mononuclear leukocytes in vitro without loss of viability. The shape of the nucleus changes from ovoid to a distinctive, radially splayed lobulated structure. The changes occur in a dose-dependent manner in 60 min at 37 degrees C. Specific monoclonal antibodies to human leukocytes identify the cells susceptible to nuclear lobulation by cyclosporine A as OKT4 antigen-positive T cell lymphocytes and monocytes. The lobulated nuclei are 2N as determined by flow cytometric measurement of ethidium bromide fluorescence of DNA. The cyclosporine A-induced lobulation of T cell nuclei requires both physiologic temperature and metabolic energy. Although structurally different than cyclosporine A, the calmodulin antagonists R24571 and W-7 [N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide] also produce T cell nuclear lobulations that are indistinguishable from the nuclear lobulations caused by cyclosporine A. These data indicate that nonmitotic structural elements that govern normal nuclear morphology in a subset of mononuclear leukocytes appear to require a calmodulin-mediated process. Cyclosporine A may be a useful noncytotoxic inhibitor of calmodulin-dependent systems that influence nuclear structure and function.

Immunohistochemical investigation of cerebral ischemia during recirculation

Immunohistochemical methods for the determination of tubulin, creatine kinase BB-isoenzyme, and astroprotein-glial fibrillary acidic protein were used to investigate recovery of the ischemic lesion after temporary occlusion of a common carotid artery in the gerbil and the evolution of the postischemic lesion following reperfusion. One group of gerbils was followed from 15 minutes to one month after an ischemic period of 30 minutes, and another group was examined after 7 days following an ischemic period of 5 to 30 minutes. It was found that the postischemic lesion, visualized as loss of the immunohistochemical reaction for tubulin and creatine kinase BB-isoenzyme, evolved within 60 minutes after reperfusion in the hippocampus and cerebral cortex and within 3 hours in the caudoputamen and thalamus. Resolution of the preexisting ischemic lesion was possible only after an ischemic period of less than 10 minutes in the cerebral cortex and caudoputamen and less than 15 minutes in the thalamus. In the CA1-CA2 region of the hippocampus, the ischemic lesion already existed after an ischemic period of 5 minutes and was mostly irreversible. The immunohistochemical method of testing for different cellular and subcellular components was very useful for investigation of cerebral ischemia and may also be advantageous for investigation of other pathophysiological conditions of the nervous system.

Behavior of a fluorescent analogue of calmodulin in living 3T3 cells

We have prepared and partially characterized a lissamine-rhodamine B fluorescent analogue of calmodulin, LRB-CM. The analogue had a dye/protein ratio of approximately 1.0 and contained no free dye or contaminating labeled proteins. LRB-CM was indistinguishable from native calmodulin upon SDS PAGE and in assays of phosphodiesterase and myosin light chain kinase. The emission spectrum of LRB-CM was insensitive to changes in pH, ionic strength, and temperature over the physiological range, but the apparent quantum yield was influenced somewhat by divalent cation concentration. LRB-CM injected into living Swiss 3T3 fibroblasts became associated with nitrobenzoxadiazole-phallacidin staining stress fibers in some interphase cells. LRB-CM and acetamidofluorescein-labeled actin co-injected into the same cell both became associated with fibers in some cells, but in most cases association of the two analogues with fibers was mutually exclusive. This suggests that calmodulin may differ from actin in the timing of incorporation into stress fibers or that we have distinguished distinct populations of stress fibers. We were able to detect no direct interaction of LRB-CM with actin by fluorescence photobleaching recovery (FRAP) of aqueous solutions. Interaction of LRB-CM with myosin light chain kinase also was not detected by FRAP. This suggests that the mean lifetime of the calmodulin-myosin light chain kinase complex is too short to affect the diffusion coefficient of calmodulin. We examined various fluorescent derivatives of proteins and dextrans as suitable control molecules for quantitative fluorescent analogue cytochemistry in living cells. Fluorescein isothiocyanate-dextrans were found to be preferable to all the proteins tested, since their mobilities in cytoplasm were inversely dependent on molecular size and there was no evidence of binding to intracellular components. In contrast, FRAP of LRB-CM in the cytoplasm of living 3T3 cells suggested that the analogue interacts with intracellular components with a range of affinities. The mobility of LRB-CM in the cytoplasm was sensitive to treatment of the cells with trifluoperazine, which suggests that at least some of the intracellular binding sites are specific for calmodulin in the calcium-bound form. FRAP of LRB-CM in the nuclei of living 3T3 cells indicated that the analogue was highly mobile within the nucleus but entered the nucleus from the cytoplasm much more slowly than fluorescein isothiocyanate-dextran of comparable molecular size and much more slowly than predicted from its mobility in cytoplasm.

Characterization and immunocytochemical distribution of calmodulin in higher plant endosperm cells: localization in the mitotic apparatus

In this study we have examined the immunocytochemical distribution of calmodulin during mitosis of higher plant endosperm cells. Spindle development in these cells occurs without centrioles. Instead, asterlike microtubule converging centers appear to be involved in establishing spindle polarity. By indirect immunofluorescence and immunogold staining methods with anti-calmodulin antibodies, we found endosperm calmodulin to be associated with the mitotic apparatus, particularly with asterlike and/or polar microtubule converging centers and kinetochore microtubules, in an immunocytochemical pattern distinct from that of tubulin. In addition, endosperm calmodulin and calcium showed analogous distribution profiles during mitosis. Previous reports have demonstrated that calmodulin is associated with the mitotic apparatus in animal cells. The present observation that calmodulin is also associated with the mitotic apparatus in acentriolar, higher plant endosperm cells suggests that some of the regulatory mechanisms involved in spindle formation, microtubule disassembly, and chromosome movement in plant cells may be similar to those in animal cells. However, unlike animal cell calmodulin, endosperm calmodulin appears to associate with kinetochore microtubules throughout mitosis, which suggests a specialized role for higher plant calmodulin in the regulation of kinetochore microtubule dynamics.

A calcium requirement for electric field-induced cell shape changes and preferential orientation

C3H/10T1/2 mouse embryo fibroblasts stimulated by a steady electric field (10 V/cm) for 30 min exhibited lamellar retraction on the sides facing the electrodes. Some cells elongated and preferentially oriented with their long axis perpendicular to the field direction. Depletion of external calcium or blockage of calcium influx with lanthanum or the calcium channel blocker D-600 resulted in a reduction of the field-induced response. When external calcium was elevated stepwise from 0 to 10 mM, the field-induced response increased correspondingly. Electric stimulation in the presence of the calcium ionophore A23187 resulted in an increase of spindle-shaped cells with no preferential orientation. This response was blocked by calcium depletion and lanthanum, but not by D-600. The anticalmodulin drug W-13 inhibited the field-induced responses observed in normal buffer as well as in the presence of A23187. Some cell death resulted from prolonged electric field exposure, and the mortality was reduced by calcium depletion, lanthanum or D-600, but was not affected by W-13. We postulate that local calcium influx through channels opened by the electric field produces areas of high intracellular calcium which stimulate the cytoskeletal network to induce lamellar retraction. Prolonged field-induced calcium influx may eventually overcome the cell's mitochondrial calcium-buffer system, leading to necrotic calcification.

Separation of endogenous calmodulin- and cAMP-dependent kinases from microtubule preparations

Both cAMP- and calmodulin-dependent kinases are proposed regulators of microtubule function by means of their ability to phosphorylate microtubule-associated protein 2(MAP 2). A cAMP-dependent kinase/MAP 2 complex is endogenous to microtubules. In this report, we demonstrate that an endogenous calmodulin-dependent kinase that phosphorylates MAP 2 as a major substrate is also present in microtubules prepared under conditions that preserve kinase activity. This enzyme is identical to a calmodulin-dependent kinase purified previously from rat brain cytosol. A fraction containing calmodulin-dependent kinase and MAP 2 was separated from the cAMP-dependent kinase/MAP 2 complex by gel filtration chromatography of microtubule protein in high ionic strength buffer. All of the recovered calmodulin-dependent kinase activity in microtubules eluted in a single protein peak. The specific activity of the enzyme for MAP 2 was enriched 31-fold in this fraction compared to cytosol. Two-dimensional tryptic phosphopeptide mapping revealed that the endogenous cAMP- and calmodulin-dependent kinases phosphorylated distinct sites on MAP 2. These data demonstrate that both kinases are present in microtubule preparations and that they may differentially regulate MAP 2 function by phosphorylating separate sites on MAP 2.

Identification of endogenous calmodulin-dependent kinase and calmodulin-binding proteins in cold-stable microtubule preparations from rat brain

Calmodulin-dependent kinase activity was investigated in cold-stable microtubule fractions. Calmodulin-dependent kinase activity was enriched approximately 20-fold over cytosol in cold-stable microtubule preparations. Calmodulin-dependent kinase activity in cold-stable microtubule preparations phosphorylated microtubule-associated protein-2, alpha- and beta-tubulin, an 80,000-dalton doublet, and several minor phosphoproteins. The endogenous calmodulin-dependent kinase in cold-stable microtubule fractions was identical to a previously purified calmodulin-dependent kinase from rat brain by several criteria including (1) subunit molecular weights, (2) subunit isoelectric points, (3) calmodulin-binding properties, (4) subunit autophosphorylation, (5) calmodulin-binding subunit composition on high-resolution sodium dodecyl sulfate-polyacrylamide gel electrophoresis, (6) isolation of kinase on calmodulin affinity resin, (7) kinetic parameters, (8) phosphoamino acid phosphorylation sites on beta-tubulin, and (9) phosphopeptide mapping. Endogenous cold-stable calmodulin-dependent kinase activity was isolated from the microtubule fraction by calmodulin affinity resin column chromatography and specifically eluted with EGTA. This kinase fraction contained the calmodulin-binding, autophosphorylating rho and sigma subunits of the previously purified kinase. The rho and sigma subunits of this kinase represented the major calmodulin-binding proteins in the cold-stable microtubule fractions as assessed by denaturing and non-denaturing procedures. These results indicate that calmodulin-dependent kinase is a major calmodulin-binding enzyme system in cold-stable microtubule fractions and may play an important role in mediating some of the effects of calcium on microtubule and cytoskeletal dynamics.