Interactions of cyclins with cyclin-dependent kinases: a common interactive mechanism

The formation of cdk-cyclin complexes has been investigated at the molecular level and quantified using spectroscopic approaches. In the absence of phosphorylation, cdk2, cdc2, and cdk7 form highly stable complexes with their "natural" cyclin partners with dissociation constants in the nanomolar range. In contrast, nonphosphorylated cdc2-cyclin H, cdk2-cyclin H, and cdk7-cyclin A complexes present a 25-fold lower stability. On the basis of both the structure of the cdk2-cyclin A complex and on our kinetic results, we suggest that interaction of any cyclin with any cdk involves the same hydrophobic contacts and induces a marked conformational change in the catalytic cleft of the cdks. Although cdks bind ATP strongly, they remain in a catalytically inactive conformation. In contrast, binding of the cyclin induces structural rearrangements which result in the selective reorientation of ATP, a concomitant 3-fold increase in its affinity, and a 5-fold decrease of its release from the active site of cdks.

Dual phosphorylation of the T-loop in cdk7: its role in controlling cyclin H binding and CAK activity

A cyclin-dependent kinase (cdk)-activating kinase (CAK) has been shown previously to catalyze T-loop phosphorylation of cdks in most eukaryotic cells. This enzyme exists in either of two forms: the major one contains cdk7, cyclin H and an assembly factor called MAT-1, whilst the minor one lacks MAT-1. Cdk7 is unusual among cdks because it contains not one but two residues (S170 and T176 in Xenopus cdk7) in its T-loop that are phosphorylated in vivo. We have investigated the role of S170 and T176 phosphorylation in the assembly and activity of cyclin H-cdk7 dimers. In the absence of MAT-1, phosphorylation of the T-loop appears to be required for cdk7 to bind cyclin H. Phosphorylation of both residues does not require cyclin H binding in vitro. Phosphorylation of S170 is sufficient for cdk7 to bind cyclin H with low affinity, but high affinity binding requires T176 phosphorylation. By mutational analysis, we demonstrate that in addition to its role in promotion of cyclin H binding, S170 phosphorylation plays a direct role in the control of CAK activity. Finally, we show that dual phosphorylation of S170 and T176, or substitution of both phosphorylatable residues by aspartic residues, is sufficient to generate CAK activity to one-third of its maximal value in vitro, even in the absence of cyclin H and MAT-1, and may thus provide further clues as to how cyclins activate cdk subunits.

cdk5 expression and association with p35nck5a in early stages of rat cerebellum neurogenesis; tyrosine dephosphorylation and activation in post-mitotic neurons

We have examined the expression of cyclin dependent kinase (cdk) 5 protein kinase and p35nck5a, its activator subunit, during postnatal neurogenesis in rat cerebellum, using mono-specific antibodies. Both cdk5 and p35nck5a are present and associated in proliferative stages, although cdk5-p35 kinase activity is barely detectable. Cdk5-p35 activity, but not the expression of either subunit, increases up to 6-fold during neuronal differentiation. Since we observe that cdk5 is phosphorylated on tyrosine in proliferative, but not in post-mitotic stages, we suggest that post-translational regulatory mechanisms control cdk5-p35 protein kinase activity during neurogenesis.

MAT1 (‘menage à trois’) a new RING finger protein subunit stabilizing cyclin H-cdk7 complexes in starfish and Xenopus CAK

The kinase responsible for Thr161-Thr160 phosphorylation and activation of cdc2/cdk2 (CAK:cdk-activating kinase) has been shown previously to comprise at least two subunits, cdk7 and cyclin H. An additional protein co-purified with CAK in starfish oocytes, but its sequencing did not reveal any similarity with any known protein. In the present work, a cDNA encoding this protein is cloned and sequenced in both starfish and Xenopus oocytes. It is shown to encode a new member of the RING finger family of proteins with a characteristic C3HC4 motif located in the N-terminal domain. We demonstrate that the RING finger protein (MAT1: 'menage à trois') is a new subunit of CAK in both vertebrate and invertebrates. However, CAK may also exist in oocytes as heterodimeric complexes between cyclin H and cdk7 only. Stable heterotrimeric CAK complexes were generated in reticulocyte lysates programmed with mRNAs encoding Xenopus cdk7, cyclin H and MAT1. In contrast, no heterodimeric cyclin H-cdk7 complex could be immunoprecipitated from reticulocyte lysates programmed with cdk7 and cyclin H mRNAs only. Stabilization of CAK complexes by MAT1 does not involve phosphorylation of Thr176, as the Thr176-->Ala mutant of Xenopus cdk7 could engage as efficiently as wild-type cdk7 in ternary complexes. Even though starfish MAT1 is almost identical to Xenopus MAT1 in the RING finger domain, the starfish subunit could not replace the Xenopus subunit and stabilize cyclin H-cdk7 in reticulocyte lysate, suggesting that the MAT1 subunit does not (or not only) interact with cyclin H-cdk7 through the RING finger domain.

Two nuclear localization signals present in the basic-helix 1 domains of MyoD promote its active nuclear translocation and can function independently

MyoD, a member of the family of helix-loop-helix myogenic factors that plays a crucial role in skeletal muscle differentiation, is a nuclear phosphoprotein. Using microinjection of purified MyoD protein into rat fibroblasts, we show that the nuclear import of MyoD is a rapid and active process, being ATP and temperature dependent. Two nuclear localization signals (NLSs), one present in the basic region and the other in the helix 1 domain of MyoD protein, are demonstrated to be functional in promoting the active nuclear transport of MyoD. Synthetic peptides spanning these two NLSs and biochemically coupled to IgGs can promote the nuclear import of microinjected IgG conjugates in muscle and nonmuscle cells. Deletion analysis reveals that each sequence can function independently within the MyoD protein since concomittant deletion of both sequences is required to alter the nuclear import of this myogenic factor. In addition, the complete cytoplasmic retention of a beta-galactosidase-MyoD fusion mutant protein, double deleted at these two NLSs, argues against the existence of another functional NLS motif in MyoD.

Implications for cAMP-dependent protein kinase in the maintenance of the interphase state

The cAMP dependent protein kinase (A-kinase) is one of the first and best studied kinases in mammalian cells. There is extensive evidence that A-kinase activity acts antagonistically toward mitotic entry both in oocyte and somatic cells. Firstly, A-kinase seems to directly compromise the activation process of the cdc2 cyclin B mitotic kinase. Secondly, as shown by specific in vivo inhibition of A-kinase using microinjection of a stable form of its inhibitor peptide PKI, A-kinase modulates several key interphase cellular processes including cytoskeletal dynamics, transcription, chromatin structure and nuclear localization. We discuss the potential mechanisms involved in the down regulation of A-kinase activity at the interphase/mitosis transition.

p40MO15 associates with a p36 subunit and requires both nuclear translocation and Thr176 phosphorylation to generate cdk-activating kinase activity in Xenopus oocytes

p40MO15, a cdc2-related protein, is the catalytic subunit of the kinase (CAK, cdk-activating kinase) responsible for Thr161/Thr160 phosphorylation and activation of cdk1/cdk2. We have found that strong overexpression of p40MO15 only moderately increases CAK activity in Xenopus oocytes, indicating that a regulatory CAK subunit (possibly a cyclin-like protein) limits the ability to generate CAK activity in p40MO15 overexpressing oocytes. This 36 kDa subunit was microsequenced after extensive purification of CAK activity. Production of Xenopus CAK activity was strongly reduced in enucleated oocytes overexpressing p40MO15 and p40MO15 shown to contain a nuclear localization signal required for nuclear translocation and generation of CAK activity. p40MO15 was found to be phosphorylated on Ser170 and Thr176 by proteolytic degradation, radiosequencing of tryptic peptides and mutagenesis. Thr176 phosphorylation is required and Ser170 phosphorylation is dispensable for p40MO15 to generate CAK activity upon association with the 36 kDa regulatory subunit. Finally, Thr176 and Ser170 phosphorylations are not intramolecular autophosphorylation reactions. Taken together, the above results identify protein-protein interactions, nuclear translocation and phosphorylation (by an unidentified kinase) as features of p40MO15 that are required for the generation of active CAK.

Cloning, expression and subcellular localization of the human homolog of p40MO15 catalytic subunit of cdk-activating kinase

Transitions of the cell cycle are controlled by cyclin-dependent protein kinases (cdks) whose phosphorylation on the Thr residue included in the conserved sequence YTHEVV dramatically increases the activity. A kinase responsible for this specific phosphorylation, called CAK for cdk-activating kinase, has been recently purified from starfish and Xenopus oocytes and shown to contain the MO15 gene product as a catalytic subunit. In the present paper, we have cloned the human homolog of Xenopus p40MO15 by probing a HeLa cell cDNA library with degenerate oligonucleotides deduced from Xenopus and starfish MO15 sequences. Human and Xenopus MO15 displayed a strong homology showing 86% identity with regard to amino acid sequences. Northern blot analysis of RNA extracts from a series of human tissues as well as from cultured rodent fibroblasts revealed a unique 1.4 kb MO15 mRNA. No variation in the amount of MO15 transcript or protein was found along the entire course of the fibroblast cell cycle. Fluorescence in situ hybridization on human lymphocyte metaphases showed two distinct chromosomal locations of human MO15 gene at 5q12-q13 and 2q22-q24. By using gene tagging and mammalian cell transfection, we demonstrate that the KRKR motif located at the carboxy terminal end of MO15 is required for nuclear targeting of the protein. Mutation of KRKR to NGER retains MO15 in the cytoplasmic compartment, whilst the wild-type protein is detected exclusively in the nucleus. Interestingly, we demonstrate that the nuclear targeting of MO15 is necessary to confer the protein its CAK activity. In contrast to the wild-type, the NLS-mutated MO15 expressed in Xenopus oocytes is unable to generate CAK as long as the nuclear envelope is not broken. The nuclear localization of both the MO15 gene product and CAK activity may imply that cdks activation primarily occurs in the cell nucleus.

Elongation factor EF-1 delta, a new target for maturation-promoting factor in Xenopus oocytes

A new physiological target for Cdc2 protein kinase has been identified. It corresponds to a protein EF-1 delta, a constituent of the nucleotide exchange factor EF-1 beta gamma delta, involved in the elongation step of protein synthesis. EF-1 delta is phosphorylated by Cdc2 kinase on threonine and serine residues. Threonine has been identified as Thr122 in the sequence VQVTPAAK. During oocyte maturation, Thr122 is phosphorylated at metaphase, when p34cdc2 is active. Phosphorylation studies revealed the presence of two post-translational regulated forms of EF-1 delta protein. Identification of two isoforms of the delta protein, together with the presence of two guanine-nucleotide exchange proteins (beta and delta) and physiologically regulated phosphorylation sites by Cdc2 kinase on gamma and delta proteins, implicate that EF-1 beta gamma delta exists in the cell under a multitude of macromolecular forms which suggests that EF-1 beta gamma delta is a sophisticated regulatory factor rather than a "housekeeping" element of the cell.

Histone H1 kinase activity, germinal vesicle breakdown and M phase entry in mouse oocytes

Meiotic reinitiation of the mouse oocyte is characterized by a slow entry into metaphase I, beginning with germinal vesicle breakdown and ending with spindle formation. It is accompanied by a cascade of protein kinases and phosphatases increasing protein phosphorylation. The activation of histone H1 kinase and that of the mitogen-activated protein kinase p42 have been compared during spontaneous or okadaic acid-induced meiotic reinitiation. In spontaneously maturing oocytes, histone H1 kinase activity increases before germinal vesicle breakdown (2-fold), in a protein synthesis-independent manner. It is associated with the disappearance of the upper migrating form of p34cdc2, which, in our system, seems to represent the tyrosine phosphorylated form. Following germinal vesicle breakdown, histone H1 kinase activity culminates (8-fold) in metaphase I and requires protein synthesis. Activation by phosphorylation of p42MAPK is observed as a permanent shift upward-migrating form and by its myelin basic protein kinase activity. It occurs after germinal vesicle breakdown and depends on protein synthesis. In contrast, no increase of histone H1 kinase is detectable in oocytes induced to reinitiate meiosis by a transient inhibition of okadaic acid-sensitive phosphatase(s), either before germinal vesicle breakdown or during the following 7 hours of culture. A slight increase is nevertheless evident after 17 hours, when oocytes are arrested with an abnormal metaphase I spindle. The upper migrating form of p34cdc2 is present for 8 hours. The activation of p42MAPK begins before germinal vesicle breakdown.(ABSTRACT TRUNCATED AT 250 WORDS)

Calmodulin-dependent protein kinase II mediates inactivation of MPF and CSF upon fertilization of Xenopus eggs

In vertebrates, unfertilized eggs are arrested at second meiotic metaphase by a cytostatic factor (CSF), an essential component of which is the product of the c-mos proto-oncogene. CSF prevents ubiquitin-dependent degradation of mitotic cyclins and thus inactivation or the M phase-promoting factor (MPF). Fertilization or parthenogenetic activation triggers a transient increase in the cytoplasmic free Ca2+ (reviewed in refs 5 and 6), inactivates both CSF and MPF, and releases eggs from meiotic metaphase arrest. A calmodulin-dependent process is required for cyclin degradation to occur in cell-free extracts prepared from metaphase II-arrested eggs (CSF extracts) when the free Ca2+ concentration is transiently raised in the physiological micromolar range. Here we show that when a constitutively active mutant of calmodulin-dependent protein kinase II (CaM KII) is added to a CSF extract, cyclin degradation and Cdc2 kinase inactivation occur even in the absence of Ca2+, and the extract loses its ability to cause metaphase arrest when transferred into embryos. Furthermore, specific inhibitors of CaM KII prevent cyclin degradation after calcium addition. Finally, the direct microinjection of constitutively active CaM KII into unfertilized eggs inactivates Cdc2 kinase and CSF, even in the absence of a Ca2+ transient. The target for Ca(2+)-calmodulin is thus CaM KII.

The MO15 gene encodes the catalytic subunit of a protein kinase that activates cdc2 and other cyclin-dependent kinases (CDKs) through phosphorylation of Thr161 and its homologues

Phosphorylation of Thr161, a residue conserved in all members of the cdc2 family, has been reported to be absolutely required for the catalytic activity of cdc2, the major regulator of eukaryotic cell cycle. In the present work, we have purified from starfish oocytes a kinase that specifically activates cdc2 in a cyclin-dependent manner through phosphorylation of its Thr161 residue. Our most highly purified preparation contained only two major proteins of apparent M(r) 37 and 40 kDa (p37 and p40), which could not be separated from each other without loss of activity. The purified kinase was found to phosphorylate not only cdc2, but also cdk2 and a divergent cdc2-like protein from Caenorhabditis, in chimeric complexes including both mitotic and G1/S cyclins. Extensive microsequencing of p40 did not reveal any convincing homology with any known protein. In contrast, p37 is the starfish homologue of the M015 gene product, a kinase previously cloned by homology probing from a Xenopus cDNA library. As expected, immunodepletion of the MO15 protein depleted Xenopus egg extracts of CAK (cdk-activating kinase) activity, which was recovered in immunoprecipitates. Taken together, the above results demonstrate that MO15 is a gene conserved throughout evolution (at least from echinoderms to vertebrates) that encodes the catalytic subunit of a protein kinase that activates cdc2-cdks complexes through phosphorylation of Thr161 (or its homologues).

Phosphorylation of the regulatory subunit of type II beta cAMP-dependent protein kinase by cyclin B/p34cdc2 kinase impairs its binding to microtubule-associated protein 2

Subcellular localization of type II cAMP-dependent protein kinase is determined by the interactions of the regulatory subunit (RII) with specific RII-anchoring proteins. By using truncated NH2-terminal RII beta fusion proteins expressed in Escherichia coli and the mitotic protein kinase p34cdc2 isolated from HeLa cells or starfish oocytes, we investigated the in vitro phosphorylation of RII beta by these kinases. The putative site for phosphorylation by the mitotic kinases is Thr-69 in the NH2-terminal domain of RII beta. This phosphorylation site matches the consensus sequence X(T/S)PX(K/R) for p34cdc2 recognition and belongs to a well-conserved sequence found in all RII beta sequences known to date. In contrast to phosphorylation by casein kinase II or the cAMP-dependent protein kinase catalytic subunit, phosphorylation of RII beta by mitotic kinases impaired its interaction with a well-known RII-anchoring protein, the neuronal microtubule-associated protein 2. The potential regulatory significance of the phosphorylation of this site on the interaction with microtubule-associated protein 2 and other RII-anchoring proteins and the physiological relevance of this cyclin B/p34cdc2 kinase-catalyzed modification of RII beta (or phosphorylation by other proline-directed protein kinases) are discussed.

Protein kinase C-dependent phosphorylation of annexins I and II in mesangial cells

In this study we describe the phosphorylation of annexins from cultured rat mesangial cells by protein kinase C (PKC) both in vitro and in vivo. Annexins I and II were detected either by Western-blot analysis or by immunoprecipitation using specific antibodies. In the presence of [gamma-32P]ATP, cytosolic annexin I and annexin II were phosphorylated in vitro only when Ca2+ and phospholipids were added, but not in the presence of phospholipids alone. Annexin I was shown to be a better substrate than annexin II. In experiments in vivo performed on 32P-labelled mesangial cells, the addition of two well-known activators of PKC, namely angiotensin II (AII) and phorbol myristate acetate (PMA), increased preferentially the phosphorylation of annexin I. Annexin II was phosphorylated to a much lesser extent after AII treatment. Phosphoamino acid analysis of annexins, either by two-dimensional chromatography or by using a specific antiphosphotyrosine antibody, revealed only phosphoserine in these experiments in vivo. The addition of AII to mesangial cells increased serine phosphorylation of annexin I and annexin II, whereas PMA only increased serine phosphorylation of annexin I. V8-protease phosphopeptide mapping of annexin I that was phosphorylated both in vitro and in vivo by PKC from mesangial cells shows similar phosphopeptides.

Okadaic acid and p13suc1 modulate the reinitiation of meiosis in mouse oocytes

Short-term exposure to okadaic acid (OA), a specific inhibitor of protein phosphatases 1 and 2A, induced resumption of meiosis, including metaphase spindle formation, in mouse oocytes treated with a phosphodiesterase inhibitor, while long incubations with OA arrested oocyte maturation at a step prior to spindle formation. To explore the basis for this difference, the overall patterns of protein synthesis and phosphorylation and the production of tissue-type plasminogen activator (tPA), the synthesis of which is induced after germinal vesicle breakdown (GVBD), were analyzed under various OA treatments. Short-term exposure to OA led to tPA production and did not greatly affect the maturation-associated changes in protein phosphorylation. By contrast, a long application of OA did not result in tPA production and induced more marked changes in protein phosphorylation. Microinjection into prophase oocytes of the product of the fission yeast gene p13suc1, known to inhibit p34cdc2 kinase activation and/or activity, prevented meiotic reinitiation. This effect was overcome by microinjection of OA, at concentrations higher than those required for induction of maturation in the absence of p13suc1. These observations suggest that inhibition of phosphatase 1 or 2A or both triggers meiotic resumption by acting at the same site or at a site proximal to the p13suc1-sensitive step of cdc2 kinase activation.

Phosphorylation of Xenopus elongation factor-1 gamma by cdc2 protein kinase: identification of the phosphorylation site

The cdc2 protein kinase phosphorylates elongation factor-1 gamma (EF-1 gamma) during meiotic maturation of Xenopus oocytes. A synthetic peptide P2: PKKETPKKEKPA matching the cDNA-deduced sequence of EF-1 gamma was an in vitro substrate for cdc2 protein kinase and inhibited phosphorylation of EF-1 gamma. Tryptic hydrolysis of EF-1 gamma and the P2 peptide, both phosphorylated by cdc2 protein kinase, resulted in multiple partial digestion products generated by the presence of barely hydrolysable bonds. The two peptides obtained from the hydrolysis of EF-1 gamma comigrated exactly in two-dimensional separation with two of the P2 peptide hydrolysates. EF-1 gamma therefore contains one unique phosphoacceptor for cdc2 protein kinase, identified as threonine-230.

Cyclin A potentiates maturation-promoting factor activation in the early Xenopus embryo via inhibition of the tyrosine kinase that phosphorylates cdc2

We have produced human cyclin A in Escherichia coli and investigated how it generates H1 kistone kinase activity when added to cyclin-free extracts prepared from parthenogenetically activated Xenopus eggs. Cyclin A was found to form a major complex with cdc2, and to bind cdk2/Eg1 only poorly. No lag phase was detected between the time when cyclin A was added and the time when H1 histone kinase activity was produced in frog extracts, even in the presence of 2 mM vanadate, which blocks cdc25 activity. Essentially identical results were obtained using extracts prepared from starfish oocytes. We conclude that formation of an active cyclin A-cdc2 kinase during early development escapes an inhibitory mechanism that delays formation of an active cyclin B-cdc2 kinase. This inhibitory mechanism involves phosphorylation of cdc2 on tyrosine 15. Okadaic acid (OA) activated cyclin B-cdc2 kinase and strongly reduced tyrosine phosphorylation of cyclin B-associated cdc2, even in the presence of vanadate. 6-dimethylamino-purine, a reported inhibitor of serine-threonine kinases, suppressed OA-dependent activation of cyclin B-cdc2 complexes. This indicates that the kinase(s) which phosphorylate(s) cdc2 on inhibitory sites can be inactivated by a phosphorylation event, itself antagonized by an OA-sensitive, most likely type 2A phosphatase. We also found that cyclin B- or cyclin A-cdc2 kinases can induce or accelerate conversion of the cyclin B-cdc2 complex from an inactive into an active kinase. Cyclin B-associated cdc2 does not undergo detectable phosphorylation on tyrosine in egg extracts containing active cyclin A-cdc2 kinase, even in the presence of vanadate. We propose that the active cyclin A-cdc2 kinase generated without a lag phase from neo-synthesized cyclin A and cdc2 may cause a rapid switch in the equilibrium of cyclin B-cdc2 complexes to the tyrosine-dephosphorylated and active form of cdc2 during early development, owing to strong inhibition of the cdc2-specific tyrosine kinase(s). This may explain why early cell cycles are so rapid in many species.

cdc25 is a nuclear protein expressed constitutively throughout the cell cycle in nontransformed mammalian cells

A family of proteins homologous to the cdc25 gene product of the fission yeast bear specific protein tyrosine phosphatase activity involved in the activation of the p34cdc2-cyclin B kinase. Using affinity-purified antibodies raised against a synthetic peptide corresponding to the catalytic site of the cdc25 phosphatase, we show that cdc25 protein is constitutively expressed throughout the cell cycle of nontransformed mammalian fibroblasts and does not undergo major changes in protein level. By indirect immunofluorescence, cdc25 protein is found essentially localized in the nucleus throughout interphase and during early prophase. Just before the complete nuclear envelope breakdown at the prophase-prometaphase boundary, cdc25 proteins are redistributed throughout the cytoplasm. During metaphase and anaphase, cdc25 staining remains distributed throughout the cell and excludes the condensed chromosomes. The nuclear locale reappears during telophase. In light of the recent data describing the cytoplasmic localization of cyclin B protein (Pines, J., and T. Hunter. 1991. J. Cell Biol. 115:1-17), the data presented here suggest that separation in two distinct cellular compartments of the cdc25 phosphatase and its substrate p34cdc2-cyclin B may be of importance in the regulation of the cdc2 kinase activity.

Dephosphorylation of cdc2 on threonine 161 is required for cdc2 kinase inactivation and normal anaphase

Exit from metaphase of the cell cycle requires inactivation of MPF, a stoichiometric complex between the cdc2 catalytic and the cyclin B regulatory subunits, as well as that of cyclin A-cdc2 kinase. Inactivation of both complexes depends on proteolytic degradation of the cyclin subunit, yet cyclin proteolysis is not sufficient to inactivate the H1 kinase activity of cdc2. Genetic evidence strongly suggests that type 1 phosphatase plays a key role in the metaphase-anaphase transition of the cell cycle. Here we report that inhibition of both type 1 and type 2A phosphatases by okadaic acid allows cyclin degradation to occur, but prevents cdc2 kinase inactivation. Complete inhibition of type 2A phosphatase alone is not sufficient to prevent cdc2 kinase inactivation following cyclin proteolysis. We show further that residue 161 of cdc2 is phosphorylated in active cyclin A or cyclin B complexes at metaphase, whilst unassociated cdc2 is not phosphorylated. Proteolysis of cyclin releases a free cdc2 subunit, which subsequently undergoes dephosphorylation and then migrates more slowly than its Thr161 phosphorylated counterpart in Laemmli gels. Removal of phosphothreonine 161 requires cyclin proteolysis. However, it does not occur even after cyclin proteolysis, when both type 1 and type 2A phosphatases are inhibited. We conclude that both cyclin degradation and dephosphorylation of Thr161 on cdc2, catalysed at least in part by type 1 phosphatase, are required to inactivate either cyclin B- or cyclin A-cdc2 kinases and thus for cells to exit from M phase.

Cyclin A-cdc2 kinase does not trigger but delays cyclin degradation in interphase extracts of amphibian eggs

Purified cyclin B-cdc2 kinase has been shown previously to trigger cyclin degradation in interphase frog extracts by initiating a cascade of reactions that includes cyclin ubiquitinylation and ends with proteolysis. However, cyclin A-cdc2 kinase was not assayed in these early experiments. Here we have shown that full-length recombinant human cyclin A failed to induce cyclin degradation when it was added to frog extracts free of cyclin B, although it formed an active kinase complex with Xenopus cdc2. A highly purified kinase complex containing a truncated human cyclin A and starfish cdc2 also failed to switch on the cyclin degradation pathway. In contrast, both recombinant cyclin B and highly purified cyclin B-cdc2 kinase readily triggered degradation of both cyclins B and A in frog extracts. Whilst free cyclin A had no inhibitory effect, cyclin A-cdc2 kinase delayed degradation of both cyclins A and B induced by cyclin B-cdc2 kinase. The finding that cyclin A-cdc2 kinase cannot turn on, and even delays, cyclin destruction may be essential to prevent premature inactivation of MPF (maturation-promoting factor) before complete condensation of chromosomes and formation of the metaphase spindle.

Okadaic acid mimics a nuclear component required for cyclin B-cdc2 kinase microinjection to drive starfish oocytes into M phase

G2-arrested oocytes contain cdc2 kinase as an inactive cyclin B-cdc2 complex. When a small amount of highly purified and active cdc2 kinase, prepared from starfish oocytes at first meiotic metaphase, is microinjected into Xenopus oocytes, it induces activation of the inactive endogenous complex and, as a consequence, drives the recipient oocytes into M phase. In contrast, the microinjected kinase undergoes rapid inactivation in starfish oocytes, which remain arrested at G2. Endogenous cdc2 kinase becomes activated in both nucleated and enucleated starfish oocytes injected with cytoplasm taken from maturing oocytes at the time of nuclear envelope breakdown, but only cytoplasm taken from nucleated oocytes becomes able thereafter to release second recipient oocytes from G2 arrest, and thus contains M phase-promoting factor (MPF) activity. Both nucleated and enucleated starfish oocytes produce MPF activity when type 2A phosphatase is blocked by okadaic acid. If type 2A phosphatase is only partially inhibited, neither nucleated nor enucleated oocytes produce MPF activity, although both do so if purified cdc2 kinase is subsequently injected as a primer to activate the endogenous kinase. The nucleus of starfish oocytes contains an inhibitor of type 2A phosphatase, but neither active nor inactive cdc2 kinase. Microinjection of the content of a nucleus into the cytoplasm of G2-arrested starfish oocytes activates endogenous cdc2 kinase, produces MPF activity, and drives the recipient oocytes into M phase. Together, these results show that the MPF amplification loop is controlled, both positively and negatively, by cdc2 kinase and type 2A phosphatase, respectively. Activation of the MPF amplification loop in starfish requires a nuclear component to inhibit type 2A phosphatase in cytoplasm.

Casein kinase II induces c-fos expression via the serum response element pathway and p67SRF phosphorylation in living fibroblasts

Elevation of intracellular casein kinase II (CKII) levels through microinjection of purified CKII results in the rapid and transient induction of c-fos in quiescent rat embryo fibroblasts, and activation of quiescent cells by serum is accompanied by the nuclear relocation of endogenous CKII. The induction of c-fos by CKII is inhibited by coinjection of oligonucleotides corresponding to the sequence of the serum response element (SRE) present in the c-fos promoter, indicating that competitive displacement of positive factors from the endogenous c-fos SRE prevents c-fos induction by CKII. Furthermore, the expression of c-fos induced by either CKII injection or serum activation is also inhibited by microinjection of antibodies against the 67 kDa serum response factor (p67SRF) indicating the absolute requirement of p67SRF in this process. Finally, we show the specific phosphorylation of p67SRF in vivo following microinjection of CKII into quiescent cells. Together, these data strongly support that CKII induces c-fos expression through binding/activation of the phosphorylated p67SRF at the SRE sequence.

Effective intracellular inhibition of the cAMP-dependent protein kinase by microinjection of a modified form of the specific inhibitor peptide PKi in living fibroblasts

In order to obtain a peptide retaining its biological activity following microinjection into living cells, we have modified a synthetic peptide [PKi(m)(6-24)], derived from the specific inhibitor protein of the cAMP-dependent protein kinase (A-kinase) in two ways: (1) substitution of the arginine at position 18 for a D-arginine; (2) blockade of the side chain on the C-terminal aspartic acid by a cyclohexyl ester group. In an in vitro assay, PKi(m) has retained a specific inhibitory activity against A-kinase as assessed against six other kinases, with similar efficiency to that of the unmodified PKi(5-24) peptide. Microinjection of PKi(m) into living fibroblasts reveals its capacity to prevent the changes in cell morphology and cytoskeleton induced by drugs which activate endogenous A-kinase, whereas the original PKi peptide failed to do so. This inhibition of A-kinase in vivo by PKi(m) lasts between 4 and 6 h after injection. In light of its effective half-life, this modified peptide opens a route for the use of biologically active peptides in vivo, an approach which has been hampered until now by the exceedingly short half-life of peptides inside living cells. By providing a direct means of inhibiting A-kinase activity for sufficiently long periods to observe effects on cellular functions in living cells, PKi(m) represents a powerful tool in studying the potential role of cAMP-dependent phosphorylation in vivo.

Degradation of the proto-oncogene product p39mos is not necessary for cyclin proteolysis and exit from meiotic metaphase: requirement for a Ca(2+)-calmodulin dependent event

Exit from M phase, which requires cyclin degradation, is prevented from occurring in unfertilized eggs of vertebrates arrested at second meiotic metaphase due to a cytostatic factor recently identified as p39mos, the product of the proto-oncogene c-mos. Calpain can destroy both p39mos and cyclin in vitro in extracts prepared from metaphase-arrested Xenopus eggs, but only when free Ca2+ concentration is raised to the millimolar range. When free Ca2+ concentration is raised for only 30 s to the micromolar range, as occurs in physiological conditions after fertilization, cyclin degradation is induced, byt p39mos is not degraded. Cyclin proteolysis at micromolar free Ca2+, is not inhibited by calpastatin, and therefore does not involve calpain. A cyclin mutant modified in the destruction box is found to be resistant at micromolar, but not millimolar free Ca2+, suggesting that the ubiquitin pathway mediates cyclin degradation at micromolar Ca2+ concentration whereas calpain is involved at the millimolar level. A synthetic peptide which binds Ca(2+)-calmodulin with high affinity suppresses cyclin degradation at micromolar but not millimolar free Ca2+, and this only when it is present in the extract during the first 30 s after raising free Ca2+ concentration. The inhibition of the cyclin degradation pathway by the Ca(2+)-calmodulin binding peptide can be overcome by adding calmodulin. These results strongly suggest that a Ca(2+)-calmodulin process is required as an early event following fertilization to release the cyclin degradation pathway from inhibition in metaphase-arrested eggs. In contrast, p39mos degradation is not required.

p67SRF is a constitutive nuclear protein implicated in the modulation of genes required throughout the G1 period

Indirect immunofluorescence analysis, using antibodies directed against peptide sequences outside the DNA-binding domain of the 67-kDa serum response factor (p67SRF), revealed a punctuated nuclear staining, constant throughout the cell cycle and in all different cell lines tested. p67SRF was also tightly associated with chromatin through all stages of mitosis. Inhibition of p67SRF activity in vivo, through microinjection of anti-p67SRF antibodies, specifically suppressed DNA synthesis induced after serum addition or ras microinjection, suggesting that these antibodies were effective in preventing expression of serum response element (SRE)-regulated genes. A similar inhibition was also obtained in cells injected with oligonucleotides corresponding to the DNA binding sequence for p67SRF protein, SRE. Moreover, this inhibition of DNA synthesis by anti-p67SRF or SRE injection was still observed in cells injected during late G1, well after c-fos induction. These data imply that genes regulated by p67SRF are continuously involved in the proliferation pathway throughout G1 and that p67SRF forms an integral component of mammalian cell transcriptional control.

Inhibition of cAMP-dependent protein kinase plays a key role in the induction of mitosis and nuclear envelope breakdown in mammalian cells

Inhibiting cAMP-dependent protein kinase (A-kinase) in mammalian fibroblasts through microinjection of a modified specific inhibitor peptide, PKi(m) or the purified inhibitor protein, PKI, resulted in rapid and pronounced chromatin condensation at all phases of the cell cycle. Together with these changes in chromatin, a marked reorganization of microtubule network occurred, accompanied in G2 cells by extensive alterations in cell shape which have many similarities to the premitotic phenotype previously observed after activation of p34cdc2 kinase, including the lack of spindle formation and the persistence of a nuclear envelope. In order to examine whether A-kinase inhibition and p34cdc2 kinase form part of the same or different inductive pathways, PKI and p34cdc2 kinase were injected together. Co-injection of both components resulted in nuclear envelope disassembly, an event not observed with injection of either component alone. This result implies that p34cdc2 and A-kinase inhibition have complementary and additive effects on the process of nuclear envelope breakdown in living fibroblasts, a conclusion further supported by our observation of a pronounced dephosphorylation of lamins A and C in cells after injection of PKi(m). Taken together, these data suggest that down-regulation of A-kinase is a distinct and essential event in the induction of mammalian cell mitosis which co-operates with the p34cdc2 pathway.

Dephosphorylation and activation of a p34cdc2/cyclin B complex in vitro by human CDC25 protein

Oocytes arrested in the G2 phase of the cell cycle contain a p34cdc2/cyclin B complex which is kept in an inactive form by phosphorylation of its p34cdc2 subunit on tyrosine, threonine and perhaps serine residues. The phosphatase(s) involved in p34cdc2 dephosphorylation is unknown, but the product of the fission yeast cdc25+ gene, and its homologues in budding yeast and Drosophila are probably positive regulators of the transition from G2 to M phase. We have purified the inactive p34cdc2/cyclin B complex from G2-arrested starfish oocytes. Addition of the purified bacterially expressed product of the human homologue of the fission yeast cdc25+ gene (p54CDC25H) triggers p34cdc2 dephosphorylation and activates H1 histone kinase activity in this preparation. We propose that the cdc25+ gene product directly activates the p34cdc2-cyclin B complex.

Microinjection of a conserved peptide sequence of p34cdc2 induces a Ca2+ transient in oocytes

The product of the yeast cell cycle control gene cdc2, and its homologs in higher eukaryotes (p34cdc2), all contain a perfectly conserved sequence of 16 amino acids that has not been found in any other protein sequence. Microinjection of this peptide triggers a specific increase in the concentration of intracellular free Ca2+ that originates from intracellular stores in both starfish and Xenopus oocytes. Thus, p34cdc2 might interact through its conserved peptide domain with some component of the Ca2(+)-regulatory system.

Microinjection of p34cdc2 kinase induces marked changes in cell shape, cytoskeletal organization, and chromatin structure in mammalian fibroblasts

We have examined the effects of elevating the intracellular levels of p34cdc2 kinase by microinjection into living mammalian cells. These studies reveal rapid and dramatic changes in cell shape with cells becoming round and losing the bulk of their cell-substratum contact. Such effects were induced at all times in the cell cycle except at S phase and were fully reversible at S phase or mitosis. Similar results were obtained with the homogeneous catalytic subunit of p34cdc2 kinase or p34cdc2 kinase associated with cyclin B. These alterations were accompanied by a marked reduction in interphase microtubules without the spindle formation, actin microfilament redistribution, and premature chromatin condensation. Although these changes closely mimic the events occurring during early phases of mitosis, p34cdc2 kinase-injected cells were not induced to pass further into division. These data provide detailed evidence that p34cdc2 kinase plays a major prerequisite role in the rearrangement of cellular structures associated with mammalian cell mitosis.

A 32 kDa lipocortin from human mononuclear cells appears to be identical with the placental inhibitor of blood coagulation

A 32 kDa protein isolated from human mononuclear cells is a member of the lipocortin family, a new group of Ca2+-dependent lipid-binding proteins thought to be involved in the regulation of phospholipase A2, in exocytosis and in membrane-cytoskeleton interactions. Purification of this protein was based on its ability to associate with membrane phospholipids in a Ca2+-dependent manner and its capacity to inhibit purified phospholipase A2 from pig pancreas. Using immunological detection, we show that it is present in various cells involved in the inflammatory and coagulation processes. We present extensive amino acid data that strongly suggest that this protein is identical with a recently described inhibitor of blood coagulation, with endonexin II and with lipocortin V. Sequence alignment with other known proteins show a significant degree of homology with lipocortins I and II, the substrates of the epidermal-growth-factor receptor tyrosine kinase and the oncogene pp60src tyrosine kinase respectively, and with protein II. The possible physiological role of this 32 kDa lipocortin is discussed.

MPF from starfish oocytes at first meiotic metaphase is a heterodimer containing one molecule of cdc2 and one molecule of cyclin B

We have purified to near homogeneity the M-phase-specific protein kinase from starfish oocytes at first meiotic metaphase, using an improved procedure based on affinity chromatography on the immobilized yeast protein suc1. As already reported, this is identical to MPF, the cytoplasmic factor that controls entry of eukaryotic cells into M-phase. MPF is a complex formed by the stoichiometric association of a 34-kd polypeptide previously identified as cdc2 with a polypeptide that migrates with the same mobility as starfish cyclin in SDS-PAGE (apparent mol. wt 47 kd). A cDNA clone encoding starfish cyclin B has been isolated and its sequence determined. It contains a single open reading frame encoding a predicted 43 729-dalton protein. Partial microsequencing of the 47-kd polypeptide component of MPF allowed its identification as the starfish cyclin. Since the apparent mol. wt of native starfish MPF was found to be less than 100 kd, it is a heterodimer comprising one molecule of cdc2 and one molecule of cyclin B.

Further characterization of four lipocortins from human peripheral blood mononuclear cells

Four calcium and phospholipid binding proteins purified from mononuclear cells were characterized for PKC and EGF phosphorylation, actin binding capacity, and partial tissue distribution. Those named 35K, 32K, and 73K are equivalent, respectively, to lipocortin III, endonexin II and the 67 kDa calelectrin; 36K is a fragment of 73K. After purification, 35K and 73K were phosphorylated by protein kinase C in vitro but 36K nor 32K were not. None were phosphorylated by the epidermal growth factor receptor kinase in vitro; 73K bound F-actin in a calcium-dependent manner, whereas 35K, 36K, and 32K did not. Using Western blotting analysis, 32K and 73K were detected in high amounts in human lymphocytes, monocytes, liver, and placenta and in rat adrenal medulla; but 32K was not detected in polymorphonuclear cells, and 36K and 35K were detected in high amounts only, respectively, in human blood lymphocytes and polymorphonuclear cells. Thus, 32K and 73K appear to have a wide tissue distribution, whereas 35K has a much more restricted distribution.

Immunocytochemical localization of myosin, tropomyosin and actin in vestibular hair cells of human fetuses and cats

Semithin sections (1 micron) of human fetuses and young cat vestibular epithelium embedded in Epon were reacted with primary antibodies raised against myosin, tropomyosin, and actin. The results were very similar in the two species. Myosin and tropomyosin were colocalized and strictly limited to an apical superficial corona arranged in a punctiform pattern. The stereocilia and cuticular plate were unreactive to myosin and tropomyosin antibodies. Actin antibodies stained the stereocilia, cuticular plate, and a marginal ring surrounding and underlying the cuticular plate. At this level, myosin and tropomyosin were not detected. This result suggests that the circumferential actin ring has a structural role. Colocalization of myosin, tropomyosin, and actin in a superficial area around the stereocilia bundle and along the apical surface of the hair cell imply that active processes could exist at the apex of certain sensory vestibular hair cells. The specificity of myosin antibodies and their cross-reactivity with different types of myosin are discussed.

High-molecular-mass proteins in haemodialysis-associated amyloidosis

1. Protein constituents were determined in eight amyloid deposits from eight patients (five male and three female), 53 +/- 4 years of age, treated by haemodialysis for 9-20 years using only cuprophane membranes and operated for carpal tunnel syndrome. 2. Soluble proteins were removed by solubilization in phosphate-buffered saline after osmotic lysis. The proteins of the insoluble fibrils were characterized by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and two-dimensional gel electrophoresis, and immunologically identified by Western blotting. 3. In addition to beta 2-microglobulin, alpha 2-macroglobulin was identified in the fibrillar material. The presence of these two proteins in amyloid deposits was confirmed by immunofluorescent microscopic studies. 4. Our data confirm the presence of beta 2-microglobulin in haemodialysis-associated amyloidosis, and also suggest a possible role for alpha 2-microglobulin: it may protect beta 2-microglobulin from proteolytic digestion, leading to its accumulation in intact form and to amyloid fibril formation.

Purification of MPF from starfish: identification as the H1 histone kinase p34cdc2 and a possible mechanism for its periodic activation

MPF extracted from starfish oocytes copurifies with an M phase-specific H1 histone kinase encoded by a homolog of the fission yeast cell cycle control gene cdc2+. The most purified preparations contain p34cdc2 as the only major protein. Activation of the p34cdc2 kinase is correlated with appearance of the MPF activity both in vivo and in vitro. The increase in protein kinase activity is associated with p34cdc2 dephosphorylation and the decrease in protein kinase activity on leaving M phase with rephosphorylation. Microinjection of a peptide perfectly conserved in p34cdc2 from yeast to humans induces meiotic maturation, suggesting that an inhibitory component in G2 arrested oocytes interacts with this region of the p34cdc2 kinase. We propose that initiation of M phase is brought about by the dephosphorylation of p34cdc2, leading to increase in its protein kinase activity.

Structural and functional variations in skeletal-muscle and scallop muscle actins

Structural and functional properties in two striated-muscle actins, one from a vertebrate, the other from an invertebrate (scallop), were compared in relation to a smooth-muscle actin isoform (aortic actin). In spite of differences in the variable N-terminal region, the two striated-muscle isoactins showed, in contrast with aortic actin, a large structural homology revealed by proteinase-susceptibility and interaction with the myosin head. Thus the myosin head may bind to the two striated-muscle actins in constant parts of the 18-113 sequence. In contrast, antigenic reactivity of conformational epitopes of these actins strongly differentiated scallop actin from the two others. The behaviour of the scallop actin appears to be related to several amino acid substitutions located near or at functional domains such as monomer-monomer binding site, DNAase-I-dependent actin-actin binding site and actin-severing domain, which modified the polypeptide chain exposure.

Modulation of the accumulation of inositol phosphates and the mobilization of calcium in aortic myocytes

In vascular smooth muscle cells the phorbol ester, 12-O-tetradecanoyl phorbol 13-acetate (TPA), a potent activator of C-kinase, inhibited the accumulation of inositol phosphates and the mobilization of calcium produced by several agonists. In the same way, TPA inhibited the fluoride-induced activation of phosphoinositide metabolism. These results suggest a C-kinase action at a post-receptor level. Moreover, the fluoride-induced accumulation of inositol phosphates shows the presence of one or more guanine nucleotide-binding proteins (G-proteins) in the regulation of receptor-phospholipase C coupling. This was confirmed by the use of N-ethylmaleimide and pertussis toxin. These results support the view that, in addition to the induction of sustained contractions, C-kinase can activate negative feedback mechanisms in aortic myocytes.

Phorbol ester modulation of cyclic AMP accumulation in a primary culture of rat aortic smooth muscle cells

Over the past few years, the importance of calcium and cyclic AMP in the regulation of vascular smooth muscle tone has been well documented. We used a primary culture of rat aortic myocytes to study the effect of protein kinase C on isoproterenol- and forskolin-stimulated cyclic AMP production. Addition of the protein kinase C activator 12-O-tetradecanoylphorbol-13-acetate (TPA) to these cells, but not an inactive analog, increased the stimulation of cyclic AMP production induced with isoproterenol or forskolin without changes in the apparent affinity of these compounds but did not affect the basal cAMP level. TPA also enhanced the cholera toxin-stimulated cyclic AMP accumulation. Isoproterenol and cholera toxin increased the forskolin apparent potency suggesting that interaction of activatory GTP-dependent protein with the catalytic subunit of adenylate cyclase facilitates forskolin interaction to the catalytic subunit. Treatment of myocytes with pertussis toxin had no effect on the basal level of cyclic AMP production and did not significantly modify isoproterenol- and forskolin-induced stimulation. Pertussis toxin treatment of cells did not affect the TPA-enhanced isoproterenol or forskolin stimulations suggesting that pertussis toxin and TPA actions would not share a common target of myocyte adenylate cyclase system. Our data would be in agreement with a possible direct interaction of protein kinase C with the catalytic subunit of adenylate cyclase system.

The effects of maitotoxin on phosphoinositides and calcium metabolism in a primary culture of aortic smooth muscle cells

Maitotoxin, a potent marine toxin isolated from toxic tropical dinoflagellates and poisonous fishes induces contraction of different smooth muscle preparations. Actions of maitotoxin on phosphoinositides and calcium metabolism were studied using a primary culture of aortic smooth muscle cells. Maitotoxin induced a very large increase of cytosolic calcium concentration as evaluated by fura-2 acetoxymethyl ester fluorescence. This increase was concomitant with stimulation of inositol-phosphate accumulation and loss of viability of aortic smooth muscle cells. These responses to maitotoxin were abolished in Ca2+-free medium, and were mimicked by saponin. Calcium ionophores or K+ depolarisation did not induce inositol-phosphate formation. These results suggest that maitotoxin acts by altering smooth muscle cells permeability allowing a sustained calcium influx which is able to activate inositol-phosphate formation and which is lethal for the cells.

Acute adaptative changes to unilateral nephrectomy in humans

Renal function was monitored in 20, living-related kidney donors before and after uninephrectomy. Urinary protein excretion and retinoid metabolism respectively were studied in 10 and 6 of these donors. The functional adaptation was characterized by an increase in glomerular filtration rate and tubular function, which began in the first two days after uninephrectomy. Changes in tubular function were also demonstrated by significant increases in the urinary excretion of beta 2 microglobulin (beta 2M), retinol binding protein (RBP), kappa and lambda light chains of immunoglobulins. In addition, a protein identical to or homologous to cellular retinoic acid binding protein (CRABP), appeared in the urine after nephrectomy. We did not find CRABP in serum samples either before or after nephrectomy, suggesting that urinary CRABP was synthesized by the remaining kidney. Increases in serum levels of Vitamin A and RBP were also observed in the post-nephrectomy period. These modifications in retinol metabolism suggest that these substances could have a role as renotropic growth factors in compensatory hypertrophy.

Haemodialysis-associated amyloidosis: beta 2-microglobulin alone or associated with globin chains?

1. The protein constituents of amyloid fibrils were characterized in amyloid deposits extracted from surgical material obtained from a 66-year-old patient undergoing maintenance haemodialysis and operated for a carpal tunnel syndrome. 2. Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis disclosed the presence of bands at 12 and 14 kDa. Two-dimensional electrophoresis and Western blotting confirmed that the proteins were beta 2-microglobulin (beta 2M) and globin chains. 3. When the effluent of high-performance gel filtration chromatography corresponding to molecular masses of 10-15 kDa was subjected to Edman degradation, only one amino acid residue was found at each step. The 18 residues determined corresponded to the N-terminal sequence of beta 2M. 4. Although globin chains were clearly present in the amyloid material, they were not accessible for sequence determination. The identification of the other protein constituents present in the amyloid material, along with beta 2M, should provide a better understanding of haemodialysis-associated amyloidosis, the mechanisms of formation of which have not yet been completely determined.

Sequence homologies between p36, the substrate of pp60src tyrosine kinase and a 67 kDa protein isolated from bovine aorta

A 67 kDa actin-binding protein was isolated from bovine aorta. Partial amino acid sequence determination of two large thermolysin peptides were used to compare 67 kDa bovine aorta protein and p36 the substrate of pp60src tyrosine kinase. Sequence analysis shows that 67 kDa bovine aorta protein shares common domains with p36 and possesses the consensus aminoacid sequences of mammalian Ca2+-dependent membrane-binding protein and p36/gelsolin.

ATP stimulates inositol phosphates accumulation and calcium mobilization in a primary culture of rat aortic myocytes

The effects of extracellular ATP on phosphoinositide metabolism and intracellular Ca2+ concentration were studied in a primary culture of rat aortic myocytes. ATP increases the level of inositol phosphates, the putative second messenger for Ca2+ mobilization. No saturation of inositol phosphates accumulation is obtained (up to 10(-2) M ATP). Under the same conditions, ATP rapidly mobilizes intracellular Ca2+ in fura-2 loaded myocytes. The mobilization of intracellular Ca2+ is dose-dependent (maximal at 10(-4) M ATP), and is not affected by addition of EGTA. It is concluded that the receptors mediating the cytosolic increase of Ca2+ are of the P2-purinoceptor subtype. The physiological functions of these receptors are not presently known.

Actin antibodies. Preparation and characterization of antibodies specific for smooth-muscle actin isoforms

We have determined the specificity of sera elicited by glutaraldehyde-stabilized bovine aortic actin. This modification induces a high titre of antibodies directed against the N-terminal (residues 1-39) and the C-terminal region of smooth-muscle actins. The crude antisera were purified on peptide (corresponding to the 1-9 or 1-8 N-terminal sequences of smooth-muscle isoactins)-polyacrylic-resin columns. By fractionating the antisera we obtained oligoclonal antibody populations specific for each isoactin.