Rearrangement of nuclear calmodulin during proliferative liver cell activation

How to measure Ca2+ in cellular organelles?

The review will aim to briefly summarise information on calcium measurements in cellular organelles with emphases on studies conducted in live cells using optical probes. When appropriate we will try to compare the effectiveness of different indicators for intraorganellar calcium measurements. We will consider calcium measurements in endoplasmic reticulum, Golgi apparatus, endosomes/lysosomes, nucleoplasm, nuclear envelope, mitochondria and secretory granules.

Polarized calcium and calmodulin signaling in secretory epithelia

This review examines polarized calcium and calmodulin signaling in exocrine epithelial cells. The calcium ion is a simple, evolutionarily ancient, and universal second messenger. In exocrine epithelial cells, it regulates essential functions such as exocytosis, fluid secretion, and gene expression. Exocrine cells are structurally polarized, with the apical region usually dedicated to secretion. Recent advances in technology, in particular the development of videoimaging and confocal microscopy, have led to the discovery of polarized, subcellular calcium signals in these cell types. The properties of a rich variety of local and global calcium signals have now been described in secretory epithelial cells. Secretagogues stimulate apical-to-basal waves of calcium in many exocrine cell types, but there are some interesting exceptions to this rule. The shapes of intracellular calcium signals are determined by the distribution of calcium-releasing channels and mechanisms that limit calcium elevation. Polarized distribution of calcium-handling mechanisms also leads to transcellular calcium transport in exocrine epithelial cells. This transport can deliver considerable amounts of calcium into secreted fluids. Multicellular polarized calcium signals can coordinate the activity of many individual cells in epithelial secretory tissue. Certain particularly sensitive cells serve as pacemakers for initiation of intercellular calcium waves. Many calcium signaling pathways involve activation of calmodulin. This ubiquitous protein regulates secretion in exocrine cells and also activates interesting feedback interactions with calcium channels and transporters. Very recently it became possible to directly study polarized calcium-calmodulin reactions and to visualize the process of hormone-induced redistribution of calmodulin in live cells. The structural and functional polarity of secretory epithelia alongside the polarity of its calcium and calmodulin signaling present an interesting lesson in tissue organization.

The calmodulin multigene family as a unique case of genetic redundancy: multiple levels of regulation to provide spatial and temporal control of calmodulin pools?

Calmodulin (CaM) is a ubiquitous, highly conserved calcium sensor protein involved in the regulation of a wide variety of cellular events. In vertebrates, an identical CaM protein is encoded by a family of non-allelic genes, raising questions concerning the evolutionary pressure responsible for the maintenance of this apparently redundant family. Here we review the evidence that the control of the spatial and temporal availability of CaM may require multiple regulatory levels to ensure the proper localization, maintenance and size of intracellular CaM pools. Differential transcription of the CaM genes provides one level of regulation to meet tissue-specific, developmental and cell-specific needs for altered CaM levels. Post-transcriptional regulation occurs at the level of mRNA stability, perhaps dependent on alternative polyadenylation and differences in the untranslated sequences of the multiple gene transcripts. Recent evidence indicates that trafficking of specific CaM mRNAs may occur to specialized cellular locales such as the dendrites of neurons. This could allow local CaM synthesis and thereby help generate local pools of CaM. Local CaM activity may be further regulated by post-translational mechanisms such as phosphorylation or storage of CaM in a 'masked' form. The spatial resolution of CaM activity is enhanced by the limited free diffusion of CaM combined with differential affinity for and availability of target proteins. Preserving multiple CaM genes with divergent noncoding sequences may be necessary in complex organisms to ensure that the many CaM-dependent processes occur with the requisite spatial and temporal resolution. Transgenic mouse models and studies on mice carrying single and double gene 'knockouts' promise to shed further light on the role of specificity versus redundancy in the evolutionary maintenance of the vertebrate CaM multigene family.

Expression of intracellular calcium channels and pumps after partial hepatectomy in rat

Ca(2+) signals regulate many cellular functions, including proliferation. They are governed by the inositol 1,4,5-trisphosphate receptor (IP(3)R), the only intracellular hepatic Ca(2+) channel and by the endoplasmic reticulum Ca(2+) pumps, SERCA. To characterise their role in regeneration, expression of their isoforms was studied after 2/3 hepatectomy by real-time quantitative PCR, Western blot and binding studies. We found an early increase in the expression of the IP(3)R isoform 1 which contrasted with the decrease of the expression of the IP(3)R isoforms 2 and 3 and of SERCA3. This results in a transient switch between IP(3)R isoforms 1 and 2, IP(3)R isoform 1 becoming predominant before the first round of mitosis. Binding studies detected a 30% diminution of the IP(3)R population at 24 h. In conclusion, the Ca(2+) signalling machinery is regulated, after hepatectomy, by changes in expression of the IP(3)R and SERCA isoforms to adapt Ca(2+) signals to the regenerative state.

Biochemical characterization of a casein kinase I-like actin kinase responsible for the actin-induced suppression of casein kinase II activity in vitro

By combination of column chromatographies (heparin-agarose, HiTrap heparin and HiTrap SP columns) and gel filtration on a Superdex 200-pg HPLC column, an actin kinase was partially purified from a 1. 5 M NaCl extract of porcine liver. The actin kinase was finally purified, by actin-Sepharose column chromatography (HPLC), as an actin-binding protein kinase. The biochemical properties, such as (1) requirements of divalent cations (10 mM Mg(2+) and 3 mM Mn(2+)) and effective phosphate acceptors (actin and alpha-casein), (2) phosphorylation of both Ser- and Thr-residues on these two phosphate acceptors, (3) autophosphorylation of the catalytic subunit (approximately 37 kDa), and (4) inhibition kinetics by CK-I-7 (a CK-I specific inhibitor), of the purified actin kinase were similar to those reported for CK-I purified from various mammalian cells, but it was distinguishable from three cellular actin kinases (A-kinase, C-kinase and actin-fragmin kinase (approximately 80 kDa)). The 37 kDa actin kinase-mediated phosphorylation of actin did not relate to its polymerizability. Inhibition of CK-II-mediated phosphorylation of functional cellular proteins, including calmodulin (CaM), by actin was significantly stimulated after its full phosphorylation by the purified 37 kDa actin kinase or rCK-I in vitro. These results suggest that: (1) the 37 kDa Ser/Thr actin-binding kinase may be classified as a member of the CK-I family; and (2) specific phosphorylation of actin by the actin kinase may be involved in the suppression mechanism of CK-II-mediated signal transduction at the cellular level.

Calcium-calmodulin mediates bradykinin-induced MAPK phosphorylation and c-fos induction in vascular cells

The vasoactive peptide bradykinin (BK) has been implicated in the pathophysiology of a number of vascular wall abnormalities, but the cellular mechanisms by which BK generates second messengers that alter vascular function are as yet undefined. Exposure of vascular smooth muscle cells (VSMC) to BK (10(-7) M) produced a rapid and transient rise in intracellular calcium, which preceded an increase in tyrosine phosphorylation of mitogen-activated protein kinase (MAPK). MAPK activation by BK was observed as early as 1 min, peaked at 5 min, and returned to baseline by 20 min. Treatment of cells with the intracellular calcium chelator EGTA-acetoxymethyl ester inhibited BK-stimulated MAPK activation, suggesting that intracellular calcium mobilization contributes to the activation of MAPK. The calmodulin inhibitor W-7 also markedly inhibited BK-induced MAPK phosphorylation in the cytoplasm as well as in the nucleus. Moreover, the BK-induced increase in c-fos mRNA levels was significantly inhibited by the calmodulin inhibitor, indicating that calmodulin is required for BK signaling leading to c-fos induction. These results implicate the calcium-calmodulin pathway in the mechanisms for regulating MAPK activity and the resultant c-fos expression induced by BK in VSMC.

Hormone-induced secretory and nuclear translocation of calmodulin: oscillations of calmodulin concentration with the nucleus as an integrator

Many important enzyme activities are regulated by Ca2+-dependent interactions with calmodulin (CaM). Some of the most important targets for CaM action are in the nucleus, and Ca2+-dependent CaM translocation into this organelle has been reported. Hormone-evoked cytosolic Ca2+ signals occur physiologically as oscillations, but, so far, oscillations in CaM concentration have not been described. We loaded fluorescent-labeled CaM into pancreatic acinar cells and monitored the fluorescence in various regions by confocal microscopy. Sustained high concentrations of the hormone cholecystokinin or the neurotransmitter acetylcholine evoked a transient movement of cytosolic CaM from the basal nonnuclear area into the secretory granule region and, thereafter, a more substantial and prolonged translocation of CaM into the nucleoplasm. About 50% of the CaM that bound Ca2+ translocated. At a lower hormone concentration, evoking Ca2+ oscillations, regular spikes of increased CaM concentration were seen in the secretory granule region with mirror image spikes of decreased CaM concentration in the basal nonnuclear region. The nucleus was able to integrate the Ca2+ spike-evoked pulses of CaM translocation into a sustained elevation of the nucleoplasmic concentration of this protein.

Differential mitogenic actions of alpha 1- and beta-adrenergic agonists on rat hepatocytes

alpha 1-Adrenergic receptor-mediated responses are overwhelming in adult rat hepatocytes. Inversely, beta-responses are predominant over alpha 1-responses in the hepatocytes that have been cultured at a low cell density (10(4) cells/cm2) for 24 h. The insulin-EGF-induced DNA synthesis in the beta-response-dominant hepatocytes was doubled by beta-agonists or cAMP-generating agents added far behind (16-20 h) the addition of insulin/EGF; i.e., immediately before the entry into the S-phase of the cell cycle. Agonists of alpha 1-adrenergic or other Ca2+, mobilising receptors added to the alpha 1-response-dominant hepatocytes increased DNA synthesis only if they were added within 1-2 h after the addition of insulin/EGF, at the early stage of G1-phase. Agonists of "non-dominant" receptors were rather antagonistic to agonists of "dominant" receptors. Thus, agonists of alpha 1-adrenergic (and other Ca2+ mobilising) receptors and agonists of beta-adrenergic (and other cAMP-generating) receptors acted as comitogens in their own particular manners in the presence of growth factors in hepatocytes in which the respective receptor functions were dominant.

The role of Ca2+/calmodulin-dependent protein kinases within the nucleus

Stimulation of cells by Ca(2+)-linked signaling agents increases Ca2+ levels within both the cell cytosol and nucleus. The multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase) family, consisting of CaM kinases I, II and IV, have all been detected within the nucleus and each may serve as a mediator of nuclear Ca2+ signals. Certain isoforms of the large multimeric CaM kinase II are targeted to the nucleus as a result of an alternatively spliced nuclear localization signal. By contrast, CaM kinases I and IV are monomeric and likely gain nuclear access by passive diffusion through nuclear pores. These kinases have activation properties which may allow them to discriminate between Ca2+ signals which differ in their spike frequency, amplitude and duration. In addition, these kinases have the ability to control gene expression through the phosphorylation of key regulatory sites on nuclear transcription factors. CaM kinases may thus serve to decode Ca2+ signals to the nucleus in order to produce a multitude of cellular responses including control of cell cycle, apoptosis and synaptic efficacy.

The prooncoprotein EWS binds calmodulin and is phosphorylated by protein kinase C through an IQ domain

A growing family of proteins is regulated by protein kinase C and calmodulin through IQ domains, a regulatory motif originally identified in neuromodulin (Alexander, K. A., Wakim, B. T., Doyle, G. S., Walsh, K. A., and Storm, D. R. (1988) J. Biol. Chem. 263, 7544-7549). Here we report that EWS, a nuclear RNA-binding prooncoprotein, contains an IQ domain, is phosphorylated by protein kinase C, and interacts with calmodulin. Interestingly, PKC phosphorylation of EWS inhibits its binding to RNA homopolymers, and conversely, RNA binding to EWS interferes with PKC phosphorylation. Several other RNA-binding proteins, including TLS/FUS and PSF, co-purify with EWS. PKC phosphorylation of these proteins also inhibits their binding to RNA in vitro. These data suggest that PKC may regulate interactions of EWS and other RNA-binding proteins with their RNA targets and that IQ domains may provide a regulatory link between Ca2+ signal transduction pathways and RNA processing.

Calmodulin content, Ca2+-dependent calmodulin binding proteins, and testis growth: identification of Ca2+-dependent calmodulin binding proteins in primary spermatocytes

In contrast with the transient pre-replicative increase in calmodulin (CaM) level observed in proliferative activated cells, postnatal development of rat testis was paralleled by 3 specific rises in CaM. The first one occurred between 5 and 10 days, coincident with the appearance and proliferation start of spermatogonia and Sertoli cells. Meiosis accomplishment and spermatid differentiation were paralleled by 2 additional rises, at 24 and 32 days, respectively. The plateau phase of testis growth was coincident with the appearance of maturating spermatids and spermatozoa in the germinal epithelium, and with a decrease in CaM content. Testicular DNA:g wet tissue ratio reached the highest level in 15-day-old rats and gradually decreased up to 35 days, when a constant level was reached. A similar level of Ca2+-CaMBPs was observed in 5- and 20-day-old rat testis. Although all subcellular fractions showed the ability to bind CaM in a Ca2+-dependent manner, CaM was mainly recovered in the nuclear and soluble fractions of adult and immature rat testis. Several Ca2+-CaMBPs with an apparent M(r) of 82, 75, 64, 19, and 14 kD were purified by affinity chromatography from pachytene primary spermatocyte nuclear matrix. Ca2+-CaMBPs showing an M(r) of 120, 78, 72, and 66 kD were also purified from the supernatant obtained after DNA and RNA hydrolysis of meiotic nuclei. Major cytosolic Ca2+-CaMBPs of primary spermatocytes showed an M(r) of 120, 84, 44, and 39 kD. The functions that these Ca2+-CaMBPs might have during the first meiotic prophase is discussed.

Association of calmodulin with nuclear structures in starfish oocytes and its role in the resumption of meiosis

The resumption of meiosis in prophase-arrested starfish oocytes is induced by the hormone 1-methyladenine, which has been shown previously to induce a calcium transient in the nucleus which at this stage is called the germinal vesicle. This transient precedes the breakdown of the germinal vesicle (GVBD). Experiments were performed to establish whether nuclear calmodulin (CaM) was involved in the progression of the meiotic cycle. CaM antagonists, antibodies, and an inhibitory peptide corresponding to the CaM-binding domain of myosin-light-chain kinase have been injected into the nucleus of prophase-arrested starfish oocytes. The antagonists failed to affect the final response to 1-methyladenine, i.e. GVBD, although two antagonists delayed it, whereas the peptide inhibitor and the antibodies completely inhibited it. The antibodies suppressed the nuclear Ca2+ spikes that were shown by previous work to be induced by the photoreleasing of caged adenosine 3',5'-(cyclic)diphosphate ribose in the germinal vesicle. Immunofluorescence staining of isolated starfish oocyte nuclei with CaM antibodies showed CaM in the envelope and in the nucleolus. Immunogold labelling of oocytes revealed aggregates of CaM and of a 36-kDa protein, of the heterogeneous ribonucleoprotein particles (hnRNP), in electron-dense hnRNP in the nuclear matrix. 1-Methyladenine induced the disappearance of these hnRNP from the nucleoplasm and the translocation of CaM and the 36-kDa protein previously associated with them to the cytoplasm, prior to the breakdown of the nuclear envelope.

Comparative study of the pattern of expression of calmodulin messenger RNAs in the mouse brain

Calmodulin is a major calcium-binding protein in the mammalian brain, playing an important role in neuronal cell function. Its amino acid sequence is highly conserved and the protein is encoded by multiple genes. In the mouse brain, as well as in the rat and the human brain, three different genes have been detected for calmodulin, CaM I, CaM II and CaM III, all of which encode an identical protein. We studied the pattern of expression of the three calmodulin genes and the pattern of calmodulin distribution in the mouse brain by in situ hybridization histochemistry and immunohistochemistry. We found that calmodulin messenger RNAs from the three calmodulin genes were widely expressed in the mouse brain. Nevertheless, there were differences in their patterns of distribution. In general, all calmodulin messenger RNAs were preferentially distributed in hippocampus, cerebral cortex and cerebellar cortex, and CaM II messenger RNA also in caudate-putamen. However, all messenger RNAs showed clearly differentiated patterns of distribution in the hippocampus and the cerebellar cortex. Calmodulin immunoreactivity was present in all cells so far examined. Immunostaining was observed both in the cell nucleus, where it was especially strong, and in the cytoplasm. Our results suggest that the three calmodulin genes are differentially regulated in the mouse brain and also that, although all calmodulin genes have a basal expression, precise regulation of calmodulin levels might be attained through the different contribution of the three calmodulin genes.

Addition of calmodulin antagonists to NRK cells during G1 inhibits proliferating cell nuclear antigen expression

The mRNAs of most proteins involved in DNA synthesis show an S phase correlated expression when mammalian cells are stimulated to proliferate from G0. This is the case for proliferating cell nuclear antigen (PCNA), a cofactor of DNA polymerase delta that is essential for the synthesis of the leading and lagging strands of DNA. Normal rat kidney cells re-entering the cell cycle from quiescence start DNA synthesis at 12 h and reach a maximum at 20 h. The expression of PCNA parallels the synthesis of DNA. Progression through the S phase was inhibited by addition of the anticalmodulin drug W13 to the cells during G1, 5 h after activation. W13 also inhibited the increase in both PCNA protein and mRNA indicating that calmodulin regulates its expression. Using TK-ts13 cells transfected with a plasmid containing the thymidine kinase gene under the control of the human 2.8 kb PCNA promoter, we demonstrated that this promoter is not regulated by calmodulin. The half-life of PCNA mRNA during G1/S transition was not modified by the treatment with W13, indicating that the decrease in the mRNA found when calmodulin was inhibited is not due to changes in its stability. Run-on assays revealed that control cells produced predominantly complete PCNA transcripts during S phase, while short incomplete transcripts were generated in W13-treated cells at the same time. These results indicate that calmodulin participates in a more direct or indirect way during G1 in the activation of PCNA expression. From data presented here it can be suggested that calmodulin activates the release of a transcriptional block leading to an increase in the amount of PCNA during S phase.

The 68 kDa calmodulin-binding protein is tightly associated with the multiprotein DNA polymerase alpha-primase complex in HeLa cells

Calcium and its receptor protein calmodulin function in the regulation of proliferation of mammalian cells. A 68 kDa calmodulin-specific binding protein was shown previously to be associated with growth factor-dependent progression of a variety of mammalian cells from G1 to S phase and to stimulate DNA synthesis in permeabilized hematopoietic progenitor cells. In this report we show that the 68 kDa calmodulin-specific binding protein in HeLa cells is tightly associated with the DNA polymerase alpha-primase component of the 21S complex of enzymes for DNA synthesis. The 68 kDa calmodulin-binding protein and the DNA polymerase alpha-primase complex cofractionate during Q-Sepharose chromatography to isolate the 21S enzyme complex, native and denatured DNA-cellulose to dissociate the 21S complex, and DEAE-Bio-Gel chromatography to isolate the multiprotein DNA polymerase alpha-primase complex. The 68 kDa calmodulin-specific binding protein and DNA polymerase alpha also bind and coelute during affinity chromatography on calmodulin-agarose. They also coprecipitate with C10-agarose-linked monoclonal antibody SJK 132-20 to human DNA polymerase alpha. The tight association of the 68 kDa calmodulin-binding protein to the DNA polymerase alpha-primase complex supports a function for this protein in the regulation of DNA synthesis in vivo.

Enhanced expression of calcium-binding protein regucalcin mRNA in regenerating rat liver

The expression of hepatic calcium-binding protein regucalcin mRNA was investigated in regenerating rat liver. The change of regucalcin mRNA levels was analyzed by Northern blotting, using liver regucalcin cDNA (0.9 kb with complete open reading frame). The reduced liver weight by partial hepatectomy (about 70%) was completely restored at 3 days after surgery. Regenerating liver significantly increased calcium content. Liver regucalcin mRNA levels clearly increased 1-5 days after hepatectomy, in comparison with that of sham-operated rats, although the increase was not seen 12 hr after the surgery. Increased regucalcin mRNA levels in regenerating liver were appreciably reduced by single intraperitoneal administration of actinomycin D (100 micrograms/100 g body weight), an inhibitor of transcriptional process. Moreover, the increased regucalcin mRNA levels by hepatectomy was weakened by a single intraperitoneal administration of trifluoperazine (2.5 mg/100 g), an inhibitor of Ca2+/calmodulin. These findings demonstrate that the expression of hepatic regucalcin mRNA is enhanced in regenerating rat liver.

Calcium, calmodulin and cell cycle progression

Proliferation of mammalian cells both in vivo and in vitro is dependent upon physiological concentrations of extracellular Ca2+. Growth factor stimulation of quiescent cells at the G0/G1 border usually results in a rapid mobilization of Ca2+ from both intra- and extracellular pools. However, Ca2+ influx is also required for later phases of cell cycle transition, especially in the late G1 phase for initiation of DNA synthesis. Available evidence indicates that calmodulin plays the major and essential roles in the Ca(2+)-dependent regulation of cell proliferation. Ca2+ and calmodulin act at multiple points in the cell cycle, including the initiation of the S phase and both initiation and completion of the M phase. Ca2+ and calmodulin stimulate the expression of genes involved in the cell cycle progression, leading to activation of cyclin-dependent kinases p33cdk2 and p34cdc2. Ca2+ and calmodulin are also involved in activation of enzymes participating in nucleotide metabolism and DNA replication, as well as nuclear envelope breakdown and cytokinesis. Ca2+/calmodulin-dependent protein kinase II and protein phosphatase calcineurin are both involved in the Ca2+ and calmodulin-mediated signalling of growth regulation. As compared to normal cells, growth of transformed cells is independent of extracellular Ca2+ and much less sensitive to calmodulin antagonists, suggesting the existence of derangements in the Ca2+ and calmodulin-mediated growth regulation mechanisms.

Phosphorylation of rat liver heterogeneous nuclear ribonucleoproteins A2 and C can be modulated by calmodulin

It was previously reported that the phosphorylation of three proteins of 36, 40 to 42, and 50 kDa by casein kinase 2 is inhibited by calmodulin in nuclear extracts from rat liver cells (R. Bosser, R. Aligué, D. Guerini, N. Agell, E. Carafoli, and O. Bachs, J. Biol. Chem. 268:15477-15483, 1993). By immunoblotting, peptide mapping, and endogenous phosphorylation experiments, the 36- and 40- to 42-kDa proteins have been identified as the A2 and C proteins, respectively, of the heterogeneous nuclear ribonucleoprotein particles. To better understand the mechanism by which calmodulin inhibits the phosphorylation of these proteins, they were purified by using single-stranded DNA chromatography, and the effect of calmodulin on their phosphorylation by casein kinase 2 was analyzed. Results revealed that whereas calmodulin inhibited the phosphorylation of purified A2 and C proteins in a Ca(2+)-dependent manner, it did not affect the casein kinase 2 phosphorylation of a different protein substrate, i.e., beta-casein. These results indicate that the effect of calmodulin was not on casein kinase 2 activity but on specific protein substrates. The finding that the A2 and C proteins can bind to a calmodulin-Sepharose column in a Ca(2+)-dependent manner suggests that this association could prevent the phosphorylation of the proteins by casein kinase 2. Immunoelectron microscopy studies have revealed that such interactions could also occur in vivo, since calmodulin and A2 and C proteins colocalize on the ribonucleoprotein particles in rat liver cell nuclei.

Calmodulin: effects of cell stimuli and drugs on cellular activation

The activity, localization and cellular content of CaM can be regulated by drugs, hormones and neurotransmitters. Regulation of physiological responses of CaM can depend upon local Ca(2+)-entry domains in the cells and phosphorylation of CaM target proteins, which would either decrease responsiveness of CaM target enzymes or increase CaM availability for binding to other target proteins. Despite the abundance of CaM in many cells, persistent cellular activation by a variety of substances can lead to an increase in CaM, reflected both in the nucleus and other cellular compartments. Increases in CaM-binding proteins can accompany stimuli-induced increases in CaM. A role for CaM in vesicular or protein transport, cell morphology, secretion and other cytoskeletal processes is emerging through its binding to cytoskeletal proteins and myosins in addition to the more often investigated activation of target enzymes. More complete knowledge of the physiological regulation of CaM can lead to a greater understanding of its role in physiological processes and ways to alter its actions through pharmacology.

Evaluation of the expression and intracellular localization of a 44-kDa calmodulin binding protein during exponential growth and quiescence (G0)

We have previously demonstrated that changes in calmodulin (CaM) levels are associated with G1/S transition of the cell cycle and entry into and release from quiescence (G0). CaM mediates its regulation through the specific interaction with different intracellular proteins called calmodulin binding proteins (CaMBPs). This study was designed to evaluate the expression of the CaMBPs during the cell cycle. Mouse C127 cells were synchronized in quiescence (G0) by serum deprivation. Analysis of the CaMBPs by the 125I-labeled CaM ([125I]CaM) overlay procedure on one- and two-dimensional gels revealed many proteins that bind to CaM at any given time during the cell cycle. However, specific expression of a 44-kiloDalton CaMBP (44CaMBP) was observed. As cells entered quiescence (G0) phase, there was a decrease in the CaM binding to the 44CaMBP. During release into the cell cycle from G0 phase, the binding to CaM was maintained at the low level, but reappeared as the cells entered S phase. CaM binding to the 44CaMBP was intense during S phase and decreased as the cells progressed into G2/M. Antibody directed against the 44CaMBP was produced in rabbit. Quantitation of the 44CaMBP by Western blot analysis revealed a similar pattern to that observed by the [125I]CaM overlay procedure during the course of G0 entry and release. The anti-44CaMBP antibody was used to evaluate the intracellular localization of the 44CaMBP by indirect immunofluorescence. A distinctive punctate nuclear staining, Mwas observed. This punctate nuclear staining, observed in all cells during exponential growth, disappeared as the cells entered G0. The nuclear staining remained absent in cells released from G0 until the cells approached and entered the S phase, at which time the punctate nuclear staining reappeared. This staining pattern was then maintained through G2/M progression. Following M phase and entry into G1 phase, the punctate nuclear staining was observed in all G1 cells. Similar analysis for cells synchronized at the G1/S boundary by the double thymidine block procedure revealed that the punctate nuclear staining was present in all cells throughout the entire course of the cell cycle. The immunofluorescence staining pattern for the 44CaMBP was sensitive to the anti-CaM drug W13 at a dose that is known to reversibly block cells at G1/S. No effect was observed by the inactive analog W12. The punctate nuclear staining of the 44CaMBP would appear to be present during all phases of the cell cycle when cells are committed to be in the cell cycle.(ABSTRACT TRUNCATED AT 400 WORDS)

Facilitated nuclear transport of calmodulin in tissue culture cells

Calmodulin (CaM) potentiates Ca(2+)-dependent signaling pathways in both the cytoplasm and nucleus. We have investigated the mechanism of CaM nuclear transport using tissue culture cell microinjection and a permeabilized cell import assay. The inhibition of CaM import by the translocation inhibitor wheat germ agglutinin (WGA) and by chilling, indicates that CaM import is facilitated, but because ATP depletion does not affect CaM import, the mechanism does not appear to be active. Chilling and WGA arrest persist in ATP-depleted cells, indicating that CaM is not retained in the cytoplasm by an ATP-dependent mechanism. In permeabilized cells, both Ca(2+)-CaM and Ca(2+)-free CaM are sensitive to extract-dependent WGA and chilling import inhibition. Titration experiments in microinjected and permeabilized cells indicate that a saturable cytosolic factor(s) mediates chilling and WGA arrest.

Protein kinase C regulates calmodulin expression in NRK cells activated to proliferate from quiescence

We have investigated the levels of calmodulin protein and calmodulin mRNA species during proliferative activation of NRK cells. Cells activated to proliferate from quiescence started to replicate DNA at 15 h, reaching a maximum at 20 h after serum addition. The maximum of mitosis was observed at 24 h. Quiescent cells showed a calmodulin concentration of 1.5 ng/micrograms of protein. At 10 h after serum addition the amount of calmodulin started to increase, reaching values of 3.0 ng/micrograms of protein at 24 h. NRK cells expressed predominantly 3 species of calmodulin transcripts: the 1.7 kb from CaM I, the 1.4 kb from CaM II and the 2.3 kb from CaM III. The amount of all the 3 transcripts was low in quiescent cells and 10 h after activation the levels were already high, reaching a maximum around 20 h. At the latter time the amount of the 3 calmodulin mRNAs was 5-10-fold higher than in serum starved cells. Run-on experiments showed that at 20 h after activation the transcription rates of the 3 calmodulin genes were higher than in quiescent cells. The addition of protein kinase C inhibitors to the cultures blocked the increase of the calmodulin transcripts while inhibitors of protein kinase A did not have any effect. Moreover, the addition of submitogenic doses of phorbol 12-tetradecanoate induced the increase of all 3 calmodulin transcripts. These results indicate that protein kinase C regulates calmodulin expression when NRK cells are activated to proliferate.

Nuclear calmodulin/62 kDa calmodulin-binding protein complexes in interphasic and mitotic cells

We report here that a 62 kDa calmodulin-binding protein (p62), recently identified in the nucleus of rat hepatocytes, neurons and glial cells, consists of four polypeptides showing pI values between 5.9 and 6.1. By using a DNA-binding overlay assay we found that the two most basic of the p62 polypeptides bind both single- and double-stranded DNA. The intranuclear distribution of calmodulin and p62 was analysed in hepatocytes and astrocyte precursor cells, and in proliferating and differentiated astrocytes in primary cultures by immunogold-labeling methods. In non-dividing cells nuclear calmodulin was mostly localized in heterochromatin although it was also present in euchromatin and nucleoli. A similar pattern was observed for p62, with the difference that it was not located in nucleoli. p62/calmodulin complexes, mainly located over heterochromatin domains were also observed in interphasic cells. These complexes remained associated with the nuclear matrix after in situ sequential extraction with nucleases and high-salt containing buffers. In dividing cells, both calmodulin and p62 were found distributed over all the mitotic chromosomes but the p62/calmodulin aggregates were disrupted. These results suggest a role for calmodulin and p62 in the condensation of the chromatin.

Calmodulin expression during rat liver regeneration

We have investigated the messenger RNAs expressed from the three calmodulin genes during rat liver regeneration. The results revealed that all the calmodulin transcripts increased from 8 hr after a partial hepatectomy, although differences in the timing and the level of expression from the three genes were observed. Calmodulin I transcripts peaked at 16 hr, whereas calmodulin II and calmodulin III progressively increased from 8 to 24 hr. At 24 hr after surgery, calmodulin I, calmodulin II and the 2.3 kb calmodulin III transcripts reached values of a 6-fold increase, whereas the 0.8 kb product of calmodulin III increased 25-fold. At 30 hr the levels of all the calmodulin transcripts were similar to those observed at 24 hr. The transcription rates of the three calmodulin genes augmented after hepatectomy (calmodulin I and calmodulin II twofold and calmodulin III fourfold), indicating that the elevation of the calmodulin transcripts could be, at least partially, the result of this increase in the transcription rates. The total calmodulin concentration also increased twofold at 24 hr after hepatectomy. We also report that the administration of the beta-adrenergic blocker, D,L-propranolol inhibited the accumulation of calmodulin protein without significantly affecting the increase of the messenger RNAs. These results indicate that the expression of calmodulin observed during liver regeneration could be regulated by cyclic AMP at the translational or posttranslational level.

Calmodulin concentrated at the osteoclast ruffled border modulates acid secretion

Osteoclasts mediate acid dissolution of bone for maintenance of serum [Ca2+] and for replacement of old bone in terrestrial vertebrates. Recent findings point to the importance of intracellular signals, particularly Ca2+, in osteoclast regulation. However, acid degradation of bone mineral subjects the osteoclast to uniquely high extracellular [Ca2+]. We hypothesized that this high calcium environment would affect calcium signalling mechanisms, and studied the calcium binding regulatory protein, calmodulin, in the osteoclast. Avian osteoclast bone resorption was inhibited 30% at 1 microM and 90% at 7 microM by the calmodulin antagonist trifluoperazine. Osteoclast bone attachment was not affected by 10 microM trifluoperazine. Quantitative immunofluorescence using fluorescein-labelled calmodulin monoclonal antibody showed a severalfold increase of calmodulin concentration in bone attached relative to plastic attached osteoclasts. Western blots confirmed this, showing two to threefold increased osteoclast calmodulin per milligram of cell protein in 3-day bone-attached vs. nonattached cells. Scanning confocal microscopy showed calmodulin polarization to areas of bone attachment. Electron micrographs with 9 nm colloidal gold labelling showed calmodulin in the acid secreting ruffled membrane. ATP-dependent acid transport in osteoclast membrane vesicles was inhibited by the calmodulin antagonist calmidazolium. This effect was reversed by addition of excess calmodulin, showing that the inhibition is specific. Vesicle acid transport inhibition reflects an approximately fourfold shift in the apparent Km for ATP of vesicular acid transport in the presence of the calmodulin antagonist. We conclude that calmodulin concentration and distribution is modified by bone attachment, and that osteoclastic acid secretion is calmodulin regulated.

Expression of calmodulin and calmodulin binding proteins in lymphoblastoid cells

Calmodulin is encoded in vertebrates by three different genes: CALM1, CALM2, and CALM3. We have examined the mRNAs expressed from these three genes in eight lines of human lymphoblastoid cells (Namalwa, Raji, Ramos, JY, Molt-4, Jurkat, CEM, and HPB-ALL). We found that all these cell lines (except Ramos) overexpressed CALM3 transcripts, which led to an increase of total CaM protein with respect to quiescent normal T lymphocytes. The nuclear concentration of calmodulin was measured in two of these lymphoblastoid cell lines (JY and HPB-ALL) and compared to quiescent and phytohemagglutinin-activated T lymphocytes. Activated lymphocytes showed a 2-fold increase of nuclear calmodulin with respect to quiescent cells, whereas in the two lymphoblastoid cell lines, nuclear calmodulin remained similar to that of quiescent cells. The levels of a calmodulin-binding protein of 150 kDa in the homogenates of the eight lymphoblastoid lines was found to be higher than those of quiescent and activated lymphocytes. Likewise, the amount of three calmodulin-binding proteins of 240, 200, and 170 kDa was also increased in several of the cell lines, but not in all of them. The 170-kDa protein was only expressed by activated lymphocytes and lymphoblastoid cells, suggesting that it could be specific for proliferating cells. In the nuclei of activated lymphocytes and lymphoblastoid cells, a decrease of a calmodulin-binding protein of 110 kDa and increases of three other of 240, 180 and 170 kDa were also detected.

A comparative study of the calcium-binding proteins calbindin-D28K, calretinin, calmodulin and parvalbumin in the rat spinal cord

Comparison of the immunocytochemical localizations revealed distinct patterns of differential distribution and overlapping of calbindin-D28K (CB-D28K), calretinin (CR), calmodulin (CM) and parvalbumin (PV) in the rat spinal cord. In some areas, one of the four calcium-binding proteins (CBPs) appears to be predominant, for example, CB-D28K in lamina I and ependymal cells, PV at the inner part of laminae II, CR in laminae V and VI and CM in motoneurons of lamina IX. In other regions of the spinal cord, more than one CBPs was abundant. CB-D28K and CR were similarly distributed in lamina II and the lateral spinal and cervical nucleus; CM and PV were similarly abundant in the ventromedial dorsal horn, internal basilar and central cervical nucleus; CR and PV were similarly abundant in the ventromedial dorsal horn, internal basilar and central cervical nucleus; CR and PV were similarly heterogeneous in the gracile fasciculus from caudal to rostral spinal cord. In the sacral dorsal gray commissure, the distribution patterns of CR and PV were clearly complementary. The unilateral ganglionectomies resulted in a substantial reduction of CBP-like immunoreactivity (CBP-LI) in the dorsal columns and a reduction of CM- and PV-LI in the ventromedial dorsal horn. In the motor system, only CM labeled large motoneurons in lamina IX and CB-D28K lightly stained pyramidal tract. The apparent absence of CM-LI in the superficial dorsal horn is contradictory to the presence of a CM-dependent nitric oxide synthase in the region. These data indicate that most CBP-LI in the dorsal column pathway had primary afferent origin, while the superficial dorsal horn exhibited intrinsic CBP immunoreactivity. The differential and selective localizations of CBPs in the spinal cord suggest a role for these proteins in spinal nociceptive processing, visceral regulation and dorsal column sensory pathways.

Cell cycle: regulatory events in G1-->S transition of mammalian cells

A cell divides into two daughter cells by progressing serially through the precisely controlled G1, S, G2, and M phases of the cell cycle. The crossing of the G1/S border, which is marked by the initiation of DNA synthesis, represents commitment to division into two complete cells. Beyond this critical point no further external signals are required. We now have more comprehensive knowledge of the temporal sequence of systems at this key transition from G1 to S--growth factor responses, a cascade of kinase reactions, activation of cyclins and their associated kinases, and oncogene and tumor suppressor gene products. Furthermore, we know that the absolute requirement for calcium and the timing of events associated with calmodulin and the 68 kDa calmodulin-binding protein are consistent with overall Ca++/calmodulin control of all steps from the response to growth factors in G1 to DNA replication in S phase. We now have to sort out the inter-relationships of myriad control proteins and their relation to the Ca++/calmodulin-dependent controls--Which are causes? Which are effects? And which are parallel processes? The answers will be important, as they represent both a much deeper understanding of this key process of life and an important opportunity for improving therapeutic medicine.

Calmodulin expression during proliferative activation of human T lymphocytes

We have investigated the levels of calmodulin mRNA species and calmodulin protein during proliferation of human T lymphocytes. Quiescent lymphocytes expressed the 1.7 kb transcript of CaM I, the 1.4 kb of CaM II and the 2.3 kb of CaM III. Phytohaemagglutinin added to peripheral blood lymphocytes induced DNA replication which started at 48 h and reached a maximum at 72 h after activation. All the species of calmodulin mRNAs, including the 4.0 kb transcript of CaM I and the 0.8 kb of CaM III which were not detected in quiescent cells, increased during lymphocyte proliferation. At 72 h after activation, the increase of CaM I and CaM II transcripts were found to be 2-fold whereas CaM III mRNAs increased 9-fold. The cellular content of calmodulin protein was also found to increase during proliferation and calmodulin accumulations in cytosol and nuclei of activated cells were observed. Two calmodulin binding proteins of 180 and 170 kD were found to increase in the nuclei of proliferating lymphocytes, whereas on the contrary 3 other calmodulin binding proteins of 110, 62 and 60 kD decreased during proliferation.

Identification and characterization of nuclear calmodulin-binding proteins of Saccharomyces cerevisiae

Nuclear calmodulin-binding proteins of the yeast Saccharomyces cerevisiae were investigated. The soluble fractions after serial treatments of the isolated nuclei with buffers containing the nonionic detergent NP-40 (F1), 0.5 M KCl (F2) and 2.0 M KCl (F3) in this order, and the residual proteins (F4) were obtained. The calmodulin-binding proteins of the nucleus and nuclear subfractions were identified using the gel overlay method using 125I-calmodulin. Each subnuclear fraction contained a large number of components that bound calmodulin in a Ca(2+)-dependent or -independent manners. The calmodulin-binding proteins were isolated from F1 and F2 subnuclear fractions by affinity chromatography. The affinity-purified proteins bound calmodulin in a Ca(2+)-dependent manner when analyzed using the gel overlay method. The major calmodulin-binding components of F1 were 44, 42, 36, 32 and 29 kDa proteins, and those of F2 were 200, 100, 40, 42, 36, 34 and 32 kDa proteins. The isolated proteins also contained several Coomassie-blue stained proteins that did not bind calmodulin and, therefore, may represent the proteins associated with the calmodulin-binding proteins. Antisera raised against the affinity-purified preparation of F1 and F2 recognized almost all of the calmodulin-binding proteins present in the fraction and several other proteins of the nucleus. The presence of Ca(2+)-dependent protein phosphatase (type 2B) in the nucleus was demonstrated by Western blotting. The enzyme was localized predominantly in F1 and F4.

Nuclear calmodulin-binding proteins in rat neurons

By using a 125I-calmodulin overlay assay, three major high-affinity calmodulin-binding proteins, showing apparent molecular masses of 135, 60, and 50 kDa, have been detected in purified nuclear fractions isolated from rat neurons. It has been shown that after extraction of the nuclei with nucleases and high salt, all these proteins remain strongly associated with the nuclear matrix. The 60- and 50-kDa proteins have been previously identified as subunits of the calmodulin-dependent protein kinase II. We report here the immunoblot identification of the 135-kDa calmodulin-binding protein as myosin light chain kinase. We also show that the calmodulin-dependent protein phosphatase calcineurin is present in the neuronal nuclei and associated with the nuclear matrix. The nuclear localization of both calcineurin and myosin light chain kinase has been confirmed by immunocytochemical studies.

Increase of cytokeratin D during liver regeneration: association with the nuclear matrix

An increase of a 45 kD protein (p45) in the nuclear matrix has been observed when rat liver cells were proliferatively activated in vivo by a partial hepatectomy. The maximal levels of the association of p45 with the nuclear matrix have been detected 24 hr after hepatectomy just at the time when DNA replication is also maximal. By amino acid sequence analysis, immunoblotting and immunocytochemical methods, it has been demonstrated that p45 is identical to rat cytokeratin D. Immunogold staining of nuclear matrix-intermediate filament preparations from cultured hepatocytes indicated that p45 is associated with cytoskeletal filaments that are strongly interconnected to the lamina, whereas no intranuclear localization of the protein has been detected. With an overlay assay a specific binding of labeled p45 to two nonidentified high-molecular weight proteins and also to lamin B has been observed. Northern blot analysis revealed a biphasic pattern of expression of the messenger RNA for cytokeratin D during liver regeneration. A sharp increase in the messenger RNA levels occurred in the prereplicative phase of liver regeneration a few hours before the accumulation of the protein in the nuclear matrix fraction, and a second peak occurred 48 hr after partial hepatectomy.

Effect of different convulsants on calmodulin levels and proto-oncogene c-fos expression in the central nervous system

In the present study, a relationship between convulsant activity and two cellular events, changes in calmodulin (CaM) concentration and proto-oncogene c-fos expression has been considered. c-fos has been found activated after the administration of the organochlorine insecticide lindane, the Ca2+ channel agonist Bay K, and N-methyl-D-aspartate (NMDA). The administration of the voltage-dependent Ca2+ channel antagonist nifedipine was able to block the expression elicited by lindane. The effect of lindane on c-fos expression could not be blocked by prior administration of MK-801, a non-competitive antagonist of the NMDA receptor. These results suggest a possible role for the voltage-dependent Ca2+ channels in the mechanism of action of lindane. By means of in situ hybridization, the different patterns of c-fos expression after the administration of the mentioned compounds have been described. A possible modification of the levels of CaM has also been investigated. Among all the subcellular fractions considered, only levels of nuclear CaM appeared to be affected after the different treatments. The changes observed seemed to follow a similar pattern to that described for c-fos induction. Calcium entry through these voltage-dependent calcium channels would be the link between membrane depolarizing events and expression of c-fos and/or increase in nuclear CaM.

Calmodulin regulates DNA polymerase alpha activity during proliferative activation of NRK cells

When Normal Rat Kidney cells are allowed to reenter the cell cycle after quiescence they start to replicate DNA around 12 h, reaching a maximum at 20 h. Activation of DNA polymerase alpha parallels the increase in DNA synthesis. The addition of two different anti-calmodulin drugs, trifluoroperazine (7.5 microM) or W13 (10 micrograms/ml), to the media at 4 h after proliferative activation, inhibits DNA synthesis by 55% and 80%, respectively. The blockade of calmodulin produced by trifluoroperazine allows the cells to progress through G1 phase but stops progression through S phase as determined by 5-Bromo deoxyuridine labeling. Both anti-calmodulin drugs also inhibit by more than 50% the increase in DNA polymerase alpha activity observed at 20 h. These results indicate that a calmodulin-dependent event, essential for the activation of DNA polymerase alpha and subsequently for DNA replication, is produced during G1. Therefore, the control of DNA polymerase alpha activation is one of the ways by which calmodulin is regulating the progression of NRK cells through S phase.

Reversible effect of calcium-binding protein regucalcin on the Ca(2+)-induced inhibition of deoxyuridine 5'-triphosphatase activity in rat liver cytosol

The effect of regucalcin, a calcium-binding protein isolated from rat liver cytosol, on deoxyuridine 5'-triphosphatase (dUTPase) in the cytosol of rat liver was investigated. Addition of Ca2+ up to 5.0 microM to the enzyme reaction mixture caused a significant decrease of dUTPase activity, while Zn2+, Cd2+, Co2+, Al3+, Mn2+ and Ni2+ (10 microM) did not have an appreciable effect. The Ca(2+)-induced decrease of dUTPase activity was reversed by the presence of regucalcin; the effect was complete at 1.0 microM of the protein. Regucalcin had no effect on the basal activity of the enzyme. Meanwhile, the reversible effect of regucalcin on the Ca2+ (10 microM)-induced decrease of dUTPase activity was not altered by the coexistence of Cd2+ or Zn2+ (10 microM). The present data suggest that liver cytosolic dUTPase is uniquely regulated by Ca2+ of various metals, and that the Ca2+ effect is reversed by regucalcin.

Association of calmodulin with isolated nuclei from rat hepatocytes

Calmodulin plays an important role in regulating cell proliferation and intranuclear processes (J. Biol. Chem. 265: 18595, 1990). Therefore we studied the association of 125I-calmodulin with highly purified rat hepatocyte nuclear preparations which were characterized by marker enzymes and electron microscopy. Steady-state association of 125I-calmodulin was reached within 5 minutes. Half-maximal binding was achieved at approximately 7.1 microM. This association was partially Ca(2+)-dependent, but was not influenced by ATP, GTP or wheat germ agglutinin. Ultrastructural autoradiography showed specific association of 125I-calmodulin with peripheral and non-peripheral heterochromatin, nuclear membranes, and nucleoli. Specific binding (ratio of the grain density of 125I-calmodulin to Na125I) was greatest in the regions of the nucleoli and non-peripheral heterochromatin. The data indicate that exogenous calmodulin can associate with specific nuclear components in an energy-independent and Ca(2+)-dependent manner.

Presence of calmodulin and calmodulin-binding proteins in the nuclei of brain cells

The nuclear calmodulin levels have been measured in rat neurons and glial cells. The values are 1.0 and 1.1 micrograms/mg of protein, respectively. These levels are about threefold higher than those in the nuclei of rat liver cells. We have also investigated the presence of several calmodulin-binding proteins in the nuclei of both brain cellular types. As similarly observed in the nuclei of liver cells, we detected the presence of alpha-spectrin and a 62-kDa calmodulin-binding protein (p62) in the nuclei of neurons and glial cells by immunoblotting and immunocytochemical methods. Both proteins are enriched in the purified nuclear matrix samples from both cellular types. In contrast to that occurring in rat hepatocytes, we have not been able to detect, by immunoblotting methods, caldesmon in the nuclear matrices of neurons and glial cells. The immunocytochemical studies suggest, however, that caldesmon can be present in the nuclei but in a fraction distinct from the nuclear matrices.

Decrease of calmodulin and actin in the plasma membrane of rat liver cells during proliferative activation

After proliferative activation of rat liver cells in vivo by a partial hepatectomy a decrease of the calmodulin content in the three plasma membrane domains (blood sinusoidal, canalicular and lateral) was observed. At 24 hours after partial hepatectomy calmodulin was found to be 3 fold lower in the sinusoidal and lateral fractions whereas a 2 fold decrease was detected in the canalicular domain. Decreases on the actin levels have been also detected at 24 hours after a partial hepatectomy. Since at this time after surgery increases on nuclear actin and calmodulin have been reported, these results suggest the possibility that the actin and calmodulin dissociated from the plasma membrane after a partial hepatectomy could subsequently be translocated into the nuclei.

Nuclear localization of 68 kDa calmodulin-binding protein is associated with the onset of DNA replication

In Chinese hamster embryo fibroblast cells, an increase in intracellular calmodulin levels coincided with the nuclear localization of a calmodulin-binding protein of about 68 kDa as the cells progressed from G1 to S phase. When cells were limited from entering into S phase, by omitting insulin a defined medium, intracellular CaM levels did not increase and the 68 kDa calmodulin-binding protein was completely absent from the nuclei. Corresponding to the nuclear localization of calmodulin and the 68 kDa calmodulin-binding protein in S phase cells, there was a dramatic increase in DNA polymerase and thymidine kinase activities in the nuclei of S phase cells as compared to G1 phase cells. In addition, the 68 kDa calmodulin-binding protein, along with calmodulin, is observed to be an integral component of replitase complex responsible for nuclear DNA replication in S phase cells. These observations point to the association of calmodulin and calmodulin-binding protein(s) with the replication machinery responsible for nuclear DNA replication during S phase. A possible regulatory role of these proteins in the onset of DNA replication and cell proliferation is discussed.

Identification and partial purification of a chromatin bound calmodulin activated histone 3 kinase from calf thymus

A calcium-calmodulin (Ca2(+)-CaM) stimulated histone H3 phosphorylating activity was identified as a component of a nuclear protein complex purified from a 150 mM NaCl extract of calf thymus chromatin. This activity bound to a CaM-Sepharose affinity column in a Ca2+ dependent manner and was eluted off the column in the presence of EGTA. Equilibrium centrifugation of the EGTA eluate on a sucrose density gradient revealed that the activity is a component of a larger complex identified at 25% sucrose. This complex consisted of two major proteins, having Mr of 65 and 75 kDa. Using [125I] CaM and the gel overlay technique it was shown that the 75 kDa protein is the major CaM binding protein in the complex.

Increase in a 55-kDa keratin-like protein in the nuclear matrix of rat liver cells during proliferative activation

We have identified a protein (p55) with a molecular weight of 55 kDa and a pI of 6.2, which was strongly increased in the nuclear matrix of rat liver cells during proliferative activation. This protein is highly insoluble since it could not be solubilized either by detergents or by alkaline extraction. We have obtained three partial amino acid sequences which revealed that p55 has a high homology with cytokeratins. Polyclonal antibodies raised against p55 were used to carry out Western blot and immunocytochemical studies which indicated that p55 was localized only in the nuclei, specifically in the nuclear matrix. Autoradiographic experiments revealed that not all the cells presenting an increase in p55 incorporated [3H]thymidine, indicating that this protein is not related to DNA replication. Immunocytochemical studies also revealed that during mitosis p55 is localized surrounding the chromosomes and associated with the mitotic apparatus, suggesting that p55 is involved in the separation of chromosomes during cell division.