A novel kinesin-like protein with a calmodulin-binding domain

Calcium regulates diverse developmental processes in plants through the action of calmodulin. A cDNA expression library from developing anthers of tobacco was screened with ³⁵S-labeled calmodulin to isolate cDNAs encoding calmodulin-binding proteins. Among several clones isolated, a kinesin-like gene (TCK1) that encodes a calmodulin-binding kinesin-like protein was obtained. The TCK1 cDNA encodes a protein with 1265 amino acid residues. Its structural features are very similar to those of known kinesin heavy chains and kinesin-like proteins from plants and animals, with one distinct exception. Unlike other known kinesin-like proteins, TCK1 contains a calmodulin-binding domain which distinguishes it from all other known kinesin genes. Escherichia coli-expressed TCK1 binds calmodulin in a Ca²⁺-dependent manner. In addition to the presence of a calmodulin-binding domain at the carboxyl terminal, it also has a leucine zipper motif in the stalk region. The amino acid sequence at the carboxyl terminal of TCK1 has striking homology with the mechanochemical motor domain of kinesins. The motor domain has ATPase activity that is stimulated by microtubules. Southern blot analysis revealed that TCK1 is coded by a single gene. Expression studies indicated that TCK1 is expressed in all of the tissues tested. Its expression is highest in the stigma and anther, especially during the early stages of anther development. Our results suggest that Ca²⁺/calmodulin may play an important role in the function of this microtubule-associated motor protein and may be involved in the regulation of microtubule-based intracellular transport.     

Nascent structure in the kinase anchoring domain of microtubule-associated protein 2

Biological processes are often viewed as highly ordered interactions between well-folded protein domains. The specific interactions exhibited by certain highly abundant neuronal proteins such as microtubule-associated protein 2 (MAP2) and tau stand in stark contrast because these proteins do not show evidence of structure by standard biophysical assays, yet they do bind to specific targets. It is conceivable that there are regions of MAP2 and tau with propensity to form structural domains upon binding a target. To search for evidence of such regions, limited proteolysis experiments were carried out on MAP2c, the smallest MAP2 isoform. Increased protease resistance was observed around the binding site for the RII subunit of cAMP-dependent protein kinase. Protein constructs spanning this region were produced based on the long-lived tryptic fragments Ser44-Arg93 and Ile94-Arg182, and were probed for structure using spectroscopic methods. The results support the existence of regions of nascent structure in the N-terminal region of MAP2c, which are believed to contribute to its regulatory function.     

Identification and characterization of the calmodulin-binding domain of neuromodulin, a neurospecific calmodulin-binding protein

Neuromodulin (formerly designated P-57) is an abundant, neural specific, calmodulin-binding protein which exhibits higher affinity for calmodulin in the absence of free Ca2+ than in the presence of free Ca2+. In this study a series of proteolytic fragments of neuromodulin were systematically screened for calmodulin-Sepharose binding activity. A 9-amino acid fragment, designated M1-C1 and having the sequence RGHITRKKL, was identified as the putative CaM-binding domain of neuromodulin. Two heptadecapeptides, designated FP57-Phe and FP57-Trp, were synthesized, each containing the M1-C1 sequence and the four flanking amino acids from each site. The FP57-Trp peptide contained a tryptophan residue in place of the native phenylalanine. Anti-FP57-Phe antibody binding to neuromodulin was inhibited by preincubation of antibodies with excess FP57-Phe. 125I-CaM gel overlay of neuromodulin was inhibited by anti-FP57-Phe antibodies. Addition of CaM to FP57-Trp increased peptide tryptophanyl fluorescence. In the presence of Ca2+, the stoichiometry of the FP57-Trp.CaM complex was 1:1, FP57-Trp binding to CaM was competitive with neuromodulin. The Ca2+-independent dissociation constant of the FP57-Phe.CaM complex was 0.41 microM. The Ca2+-dependent affinity of the complex could not be measured directly but appeared to be significantly greater than the Ca2+-independent affinity.     

Phosphorylation of microtubule-associated protein 2 by MAP kinase primarily involves the projection domain

Chymotryptic digestion was used to localize the sites in microtubule-associated protein 2 which are preferentially phosphorylated in vitro by MAP kinase, an insulin-stimulated serine/threonine kinase which efficiently utilizes high molecular weight MAPs as substrates. MAP kinase phosphorylates sites in the projection domain almost exclusively; less than 6% of the phosphate incorporated by MAP kinase was found in the tubulin binding domain. This site specificity is in marked contrast to that of the catalytic subunit of cAMP dependent protein kinase, and most other protein kinases phosphorylating MAP-2, which extensively phosphorylate the tubulin binding domain.     

Mapping of calmodulin-binding domain of Ca2+/calmodulin-dependent protein kinase II from rat brain

Recent molecular cloning experiments have identified a 25 amino-acid region as the calmodulin-binding domain of the alpha-subunit of rat brain Ca2+/calmodulin-dependent multifunctional protein kinase II (CaM-K II). Synthetic peptides, derived from the deduced amino-acid sequence encompassing this region, were examined for their ability to bind calmodulin in a calcium dependent manner and to inhibit the Ca2+/calmodulin-dependent autophosphorylation of CaM-K II. Comparison of these structure-function relationships highlighted a region of 5 amino-acids, which was essential for calmodulin interaction and inhibition of kinase activity. This region demonstrated some homology with other calmodulin-binding peptides, and may represent a key site of interaction of the kinase with calmodulin. These analyses provide additional insight into the molecular mechanism underlying the Ca2+ regulation of CaM-K II.     

Removal of the projection domain of microtubule-associated protein 2 alters its interaction with tubulin

Microtubule-associated proteins (MAPs) can promote microtubule assemblyin vitro. One of these MAPs (MAP2) consists of a short promoter domain which binds to the microtubule and promotes assembly and a long projection domain which projects out from the microtubule and may interact wth other cytoskeletal elements. We have previously shown that MAP2 and another MAP, tau, differ in their interactions with tubulin in that tau, but not MAP2, promotes extensive aggregation of tubulin into spiral clusters in the presence of vinblastine and that microtubules formed with MAP2 are more resistant than those formed with tau to the antimitotic drug maytansine [Luduena, R. F.,et al. (1984),J. Biol. Chem. 259, 12890–12898; Fellous, A.,et al. (1985),Cancer Res. 45, 5004–5010]. Here we have used chymotryptic digestion to remove the projection domain of MAP2 and examined the interaction of the digested MAP2 (ctMAP2) with tubulin in the presence of vinblastine and maytansine. We have found that ctMAP2 behaves very much like tau, but not like undigested MAP2, in the presence of vinblastine, in that ctMAP2 causes tubulin to polymerize into large clusters of spirals. In contrast, microtubule assembly in the presence of ctMAP2 is much more resistant to maytansine inhibition than is assembly in the presence of tau or undigested MAP2. Our results suggest that the projection domain of MAP2 may play a role in the interaction of tubulin with MAP2 during microtubule assembly.Abbreviations MAPs microtubule-associated proteins - ctMAP2 MAP2 digested with-chymotrypsin - nMAP2 untreated MAP2 - PMSF phenylmethylsulfonyl fluoride - GMPCPP guanosine-5-(,-methylene)triphosphate     

Purified protein kinase C phosphorylates microtubule-associated protein 2

We have investigated actions of purified protein kinase C on microtubule- and microfilament-related proteins. Among the cytoskeletal proteins examined, microtubule-associated protein 2 (MAP2) was found to serve as a good substrate. Other cytoskeletal proteins, tubulin, fodrin, cofilin, tropomyosin, and 53,000-Da protein, were very poorly phosphorylated. The amino acid residues of MAP2 that were phosphorylated by the protein kinase C were almost exclusively serine. The peptide mapping analysis indicated that protein kinase C and cAMP-dependent protein kinase phosphorylate MAP2 differently. The ability of MAP2 to interact with actin was markedly reduced by this protein kinase C-mediated phosphorylation. These data raise the possibility that phosphorylation of MAP2 by activated protein kinase C may be involved in cell-surface signal transduction.     

Identification of a calcium-dependent calmodulin-binding domain in Xenopus membrane skeleton protein 4.1

Xenopus membrane skeleton protein 4.1 is expressed constitutively during embryonic development and accumulates to high levels within the retina during normal morphogenesis. There exists a high degree of amino acid identity between Xenopus protein 4.1 and human protein 4.1, suggesting that the mechanisms known to modulate the function(s) of human protein 4.1 may also serve to regulate Xenopus protein 4.1. Calmodulin (CaM) is one regulatory protein known to affect membrane-cytoskeletal interactions. An in vitro binding assay was used to test the ability of Xenopus protein 4.1 to bind CaM. Two independent approaches, involving protein 4.1 synthesized in vitro from synthetic RNA or a partial length protein 4.1 fusion protein expressed in Escherichia coli, demonstrate calcium-dependent, CaM binding. Both approaches demonstrate that the CaM-binding site is within the amino-terminal region of Xenopus protein 4.1. Results of this calmodulin binding activity suggest a possible regulatory mechanism by which calcium and calmodulin may affect the function of protein 4.1 during development.     

A discrete repeated sequence defines a tubulin binding domain on microtubule-associated protein tau

The protein domain responsible for the interaction of tau with tubulin has been identified. Biophysical studies indicated that the synthetic peptide Val187-Gly204 (VRSKIG-STENLKHQPGGG) from the repetitive sequence on tau binds to two sites on the tubulin heterodimer and to one site on each of the microtubule-associated protein-interacting C-terminal tubulin peptides alpha(430-441) and beta(422-434). The binding data showed a relatively stronger interaction of Val187-Gly204 with beta(422-434) as compared to that with alpha(430-441). The interaction of this tau peptide with either alpha or beta tubulin peptides appears to be associated with conformational changes in both the tau and the tubulin peptides. The beta tubulin peptide also appears to induce a structural change of tau fragment Val218-Gly235. Interestingly, tau peptides Val187-Gly204 and Val218-Gly235 induced tubulin self-assembly in a cold-reversible fashion, and incorporated into the assembled polymers. The specificity of the interaction of the tau peptide was supported by the competition of tau protein for the interaction with the tubulin polymer. In addition, the tau peptide appears to contain the principal antigenic determinant(s) recognized by anti-idiotypic antibodies that react with the tubulin binding domains on microtubule-associated proteins. The present findings together with the demonstration of the presence of multiple sites for the binding of the alpha(430-441) and beta(422-434) tubulin fragments to tau, and the existence of repetitive sequences on tau, strongly support the hypothesis that the region of tau defined by the repetitive sequences is involved in its interaction with tubulin.     

Mitogen-activated-protein-kinase-catalyzed phosphorylation of microtubule-associated proteins, microtubule-associated protein 2 and microtubule-associated protein 4, induces an alteration in their function.

Mitogen-activated protein kinase (MAPK), a serine/threonine-specific protein kinase which is generally activated by stimulation with various growth factors and phorbol esters, utilizes microtubule-associated protein (MAP) 2 as a good substrate in vitro. We have found that MAPK-catalyzed phosphorylation of MAP2 resulted in a significant loss in its ability to induce tubulin polymerization. The chymotryptic fragments, containing a microtubule-binding domain of MAP2, were phosphorylated by MAPK and the ability of the fragments to induce tubulin polymerization was also greatly decreased by the phosphorylation, suggesting that phosphorylation of the microtubule-binding domain is important for functional alteration of MAP2. In addition to MAP2, a 190-kDa heat-stable MAP (MAP4) found in various tissues and cells, was a good substrate for MAPK in vitro. Phosphorylation of MAP4 inactivated tubulin polymerization. We examined the effect of phosphorylation of MAP2 and MAP4 on the dynamics of microtubules nucleated by purified centrosomes in vitro. The data showed that MAPK-catalyzed phosphorylation of MAP2 and MAP4 reduced their ability to increase the apparent elongation rate and the number of microtubules nucleated by the centrosome. Thus, MAPK is capable of phosphorylating MAPs and negatively regulating their microtubule-stabilizing function.     

Protein-kinase-C-catalyzed phosphorylation of the microtubule-binding domain of microtubule-associated protein 2 inhibits its ability to induce tubulin polymerization

It has previously been demonstrated that microtubule-associated protein 2 (MAP2) is a good substrate for the purified protein kinase C [Tsuyama, S., Bramblett, G. T., Huang, K.-P. & Flavin, M. (1986) J. Biol. Chem. 261, 4110-4116; Akiyama, T., Nishida, E., Ishida, J., Saji, N., Ogawara, H., Hoshi, M., Miyata, Y. & Sakai, H. (1986) J. Biol. Chem. 261, 15648-15651]. We have shown here that phosphorylation of MAP2, catalyzed by protein kinase C, reduces the ability to induce tubulin polymerization. MAP2 is divided into two domains by digestion with alpha-chymotrypsin; the microtubule-binding and the non-binding (projection) domains. The limited chymotryptic digestion following phosphorylation of MAP2 by protein kinase C has shown that both the domains of MAP2 were phosphorylated by protein kinase C, 50-60% of the incorporated phosphates being detected in the microtubule-binding domain. Polypeptide fragments, containing the microtubule-binding domain of MAP2, were purified by DEAE-cellulose column chromatography after chymotryptic digestion of MAP2. The purified microtubule-binding fragments were competent to polymerize tubulin, and served as good substrates for protein kinase C. The phosphorylation of the microtubule-binding fragments by protein kinase C reduced their ability to induce tubulin polymerization. These results suggest that the ability of MAP2 to induce tubulin polymerization is inhibited by phosphorylation of the microtubule-binding domain catalyzed by protein kinase C.     

The calmodulin-binding domain mediates the self-association of the plasma membrane Ca2+ pump.

Dimerization (oligomerization) of the plasma membrane Ca2+ pump increases its activity (Kosk-Kosicka, D., Bzdega, T., and Wawrzynow, A. (1989) J. Biol. Chem. 264, 19495-19499). Fluorescence titration on preparations of the purified eosin-labeled human erythrocyte ATPase has been used to monitor the oligomerization process. Calmodulin inhibits oligomerization, although it can bind to the oligomerized enzyme. Synthetic peptides corresponding to the calmodulin-binding domain of the pump stimulate its ATPase activity, indicating the formation of heterooligomers of the peptides with the pump. The oligomerization is prevented by the preincubation of the ATPase with calmodulin. Polyclonal antibodies against the synthetic calmodulin-binding domain inhibit its basal and its calmodulin-stimulated ATPase activity and prevent the formation of the oligomers. ATPase preparations truncated at the COOH terminus with calpain to a fragment of 124 kDa which does not contain the calmodulin-binding domain fail to oligomerize with the intact ATPase. The results show that the calmodulin-binding domain mediates the oligomerization of the Ca2+ pump.     

Characterization of a pathogen-induced calmodulin-binding protein: mapping of four Ca2+-dependent calmodulin-binding domains

Ca²⁺ and calmodulin (CaM), a key Ca²⁺ sensor in all eukaryotes, have been implicated in defense responses in plants. To elucidate the role of Ca²⁺ and CaM in defense signaling, we used ³⁵S-labeled CaM to screen expression libraries prepared from tissues that were either treated with an elicitor derived from Phytophthora megasperma or infected with Pseudomonas syringae pv. tabaci. Nineteen cDNAs that encode the same protein, pathogen-induced CaM-binding protein (PICBP), were isolated. The PICBP fusion proteins bound ³⁵S-CaM, horseradish peroxidase-labeled CaM and CaM-Sepharose in the presence of Ca²⁺ whereas EGTA, a Ca²⁺ chelator, abolished binding, confirming that PICBP binds CaM in a Ca²⁺-dependent manner. Using a series of bacterially expressed truncated versions of PICBP, four CaM-binding domains, with a potential CaM-binding consensus sequence of WSNLKKVILLKRFVKSL, were identified. The deduced PICBP protein sequence is rich in leucine residues and contains three classes of repeats. The PICBP gene is differentially expressed in tissues with the highest expression in stem. The expression of PICBP in Arabidopsis was induced in response to avirulent Pseudomonas syringae pv. tomato carrying avrRpm1. Furthermore, PICBP is constitutively expressed in the Arabidopsis accelerated cell death2-2 mutant. The expression of PICBP in bean leaves was also induced after inoculation with avirulent and non-pathogenic bacterial strains. In addition, the hrp1 mutant of Pseudomonas syringae pv. tabaci and inducers of plant defense such as salicylic acid, hydrogen peroxide and a fungal elicitor induced PICBP expression in bean. Our data suggest a role for PICBP in Ca²⁺-mediated defense signaling and cell-death. Furthermore, PICBP is the first identified CBP in eukaryotes with four Ca²⁺-dependent CaM-binding domains.     

The 28,000 Mr microtubule-binding domain of microtubule-associated protein-2 also contains a neurofilament-binding site

We have developed a thrombin proteolytic cleavage procedure to obtain higher yields of the Mr 28,000 microtubule-binding and Mr 240,000 microtubule-projection components of MAP-2. The former is a highly basic component, whereas the latter and intact MAP-2 are acidic polypeptides. Most notably, our studies reveal that this Mr 28,000 fragment binds to neurofilaments, but the Mr 240,000 projection domain fails to interact. These data indicate that microtubules and neurofilaments share a common binding site on high-molecular-weight MAP-2.     

Interaction of microtubule-associated protein 2 with actin filaments

The interaction of unphosphorylated and phosphorylated microtubule-associated protein 2 (MAP-2) with actin filaments was examined by electron microscopic, electrophoretic, and dark-field light microscopic techniques. Unphosphorylated MAP-2 was observed to cross-link and bundle individual actin filaments. Chymotryptic fragments of MAP-2 protein were produced which bound to, but could not cross-link, actin polymer; these fragments encompassed the tubulin binding domain of MAP-2. The phosphorylation of intact MAP-2, by means of endogenous protein kinases, inhibited the ability of this molecule to cross-link and bundle actin filaments. Phosphorylation did not, however, inhibit the binding of MAP-2 to F-actin. The chymotryptic fragments of phosphorylated MAP-2 that retained their ability to bind to actin and promote microtubule assembly also encompassed the tubulin binding domain of this molecule. An analysis of MAP-2 fragments by nonequilibrium pH gradient electrophoresis indicated that most of the polypeptide backbone is relatively acidic with the exception of the tubulin binding domain. This region was determined to be the most basic (positively charged) region of the MAP-2 molecule. Biochemical and morphological evidence is presented to demonstrate that both unphosphorylated MAP-2 and phosphorylated MAP-2 have the capacity to use actin, in addition to microtubules, as a separate anchoring substrate. The presence of tubulin, however, strongly inhibits the interaction of MAP-2 with actin filaments.     

The RII subunit of cAMP-dependent protein kinase binds to a common amino-terminal domain in microtubule-associated proteins 2A, 2B, and 2C

Three products of the MAP2 gene are known: MAP2A and MAP2B (Mr approximately 200,000) and MAP2C (Mr 70,000). The structural relationship between these MAPs and the basis for their diversity in size are unknown. Previously, we found that a significant fraction of type II cAMP-dependent protein kinase was associated via its regulatory subunits with MAP2A and MAP2B. We now use an antibody prepared against the microtubule binding domain of MAP2A and MAP2B to identify MAP2C. All three forms of MAP2 bound to cAMP affinity columns and reacted with 32P-labeled RII in a blot overlay assay. By assaying proteolytic fragments of MAP2A and MAP2B as well as segments of MAP2 expressed in E. coli, the binding site for RII was localized to an 83 amino acid stretch at the distal (amino-terminal) end of the MAP2 arm domain. Therefore, the microtubule binding and RII binding domains are located at extreme opposite ends of MAP2A and MAP2B, and both are conserved in the much shorter MAP2C.     

Extensive cAMP-dependent and cAMP-independent phosphorylation of microtubule-associated protein 2

Microtubule-associated protein 2 (MAP 2) is the major substrate for phosphorylation in purified preparations of brain microtubules. In earlier work, we showed that phosphorylation is catalyzed by a type II cAMP-dependent protein kinase tightly associated with MAP 2 itself. In the present study, we have examined the extent of MAP 2 phosphorylation by its associated protein kinase. Using an inorganic phosphate assay, we found that MAP 2 contained from 8 to 13 mol of phosphate/mol of protein as isolated. The catalytic subunit of the MAP 2-associated kinase catalyzed the incorporation of additional phosphate to a final level of 20-22 mol/mol of MAP 2. Potato acid phosphatase was used to remove phosphate from MAP 2. Rephosphorylation of acid phosphatase-treated MAP 2 resulted in maximal incorporation of 13 mol of phosphate/mol of MAP 2. The rates and extent of [32P] phosphate incorporation into as isolated and dephosphorylated MAP 2 were found to be identical, and phosphate was incorporated into identical peptides in the two preparations. These results were interpreted to indicate that MAP 2 contains as many as 13 cAMP-dependent phosphorylation sites, and approximately eight phosphates of as yet undetermined origin.     

The microtubule-associated protein tau is phosphorylated by Syk

Aberrant phosphorylation of tau protein on serine and threonine residues has been shown to be critical in neurodegenerative disorders called tauopathies. An increasing amount of data suggest that tyrosine phosphorylation of tau might play an equally important role in pathology, with at least three putative tyrosine kinases of tau identified to date. It was recently shown that the tyrosine kinase Syk could efficiently phosphorylate alpha-synuclein, the aggregated protein found in Parkinson's disease and other synucleinopathies. We report herein that Syk is also a tau kinase, phosphorylating tau in vitro and in CHO cells when both proteins are expressed exogenously. In CHO cells, we have also demonstrated by co-immunoprecipitation that Syk binds to tau. Finally, by site-directed mutagenesis substituting the tyrosine residues of tau with phenylalanine, we established that tyrosine 18 was the primary residue in tau phosphorylated by Syk. The identification of Syk as a common tyrosine kinase of both tau and alpha-synuclein may be of potential significance in neurodegenerative disorders and also in neuronal physiology. These results bring another clue to the intriguing overlaps between tauopathies and synucleinopathies and provide new insights into the role of Syk in neuronal physiology.     

The plasma membrane Ca2+ pump contains a site that interacts with its calmodulin-binding domain.

A synthetic, 28-residue peptide derived from the calmodulin-binding sequence of the plasma membrane Ca2+ pump (C28W) inhibits the ATPase activity of a calpain-produced, truncated fragment of the enzyme. The fragment, which has lost the calmodulin-binding domain, has a molecular mass of 124 kDa and is fully active in the absence of calmodulin. Replacement of Trp-8 in the peptide by an Ala decreases the overall inhibitory activity, while replacement with a Tyr increases it. However, at very low peptide concentrations the effect of Tyr replacement disappears. The synthetic peptide has been made photoactivatable by replacing Phe in position 9 with a synthetic phenylalanine analogue containing a diazirine group and was radioactively labeled by coupling a [3H]acetyl function to its N terminus. After cross-linking with the derivatized peptide, the 124-kDa fragment has been proteolyzed with either Lys-C, Asp-N, or V8 proteases, and the fragment(s) have been separated. Partial sequencing of the cross-linked, radioactive peptides has identified a site of the pump located C terminally to the phosphoenzyme-forming aspartic acid, spanning residues 537-544 of the hPMCA4 isoform of the enzyme. It is concluded that this sequence is part of a site which binds the calmodulin-binding domain of the pump.     

Mapping of distinct structural domains on microtubule-associated protein 2 by monoclonal antibodies

Monoclonal antibodies against microtubule-associated protein 2 (MAP2) were prepared and their specificity was verified by visualization of the antigens using the antibody overlay technique and by radioimmunoassay. MAP2 was cleaved by alpha-chymotrypsin to generate a series of high-molecular-mass fragments ranging between 270 and 140 kDa. The precursor-product relationship of these fragments was suggested from the rate of their appearance and from the analysis of the tryptic peptide map of each fragment. A group of monoclonal antibodies was found to react predominantly with the intact 270-kDa MAP2 molecule and a fragment having a mass of 240 kDa and to some extent with a 215-kDa fragment. Another group of monoclonal antibodies reacted with an antigenic determinant which was located on the 270-kDa molecule as well as on fragments as small as 140 kDa. None of the two groups of monoclonal antibodies reacted with the microtubule-binding domain of MAP2. These results suggest that one group of antibodies reacts with sites located at or dependent upon a terminal 60-kDa domain(s) distal to the microtubule-binding site of MAP2. The second group of antibodies, which can still bind to smaller proteolytic products, appear to be associated with the central region of the MAP2 molecule. Indirect immunofluorescence experiments with the antibody preparations indicated that at least some of the antigenic determinants are exposed when MAP2 is associated with microtubules in the cell body and neurite outgrowths of differentiated rat brain neuroblastoma B104 cells.