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.     

Limited chymotryptic digestion of bovine adrenal 190,000-Mr microtubule-associated protein and preparation of a 27,000-Mr fragment which stimulates microtubule assembly

A heat stable microtubule-associated protein of Mr 190,000 (190-kDa MAP) has been purified from bovine adrenal cortex (Murofushi, H., Kotani, S., Aizawa, H., Hisanaga, S., Hirokawa, N., and Sakai, H. (1986) J. Cell Biol. 103, 1911-1919). Limited chymotryptic digestion of 190-kDa MAP produced a fragment of Mr 27,000 (27-kDa fragment), which bound to microtubules reconstituted in the presence of taxol. This fragment was purified with the aid of cosedimentation with microtubules. The purified 27-kDa fragment showed an ability to stimulate tubulin polymerization in the absence of taxol. Electron microscopic observation of microtubules reconstituted from purified 27-kDa fragment and tubulin revealed that the microtubules were in the form of thick bundles and that lateral projections which can be seen in microtubules reconstituted from intact 190-kDa MAP and tubulin were not observed. These results indicate that 27-kDa fragment includes or is a part of microtubule-binding domain of 190-kDa MAP and that this fragment is active in stimulating microtubule assembly. Amino acid analysis revealed that the 27-kDa fragment was rich in lysine, proline, and alanine, the sum of these three being about 45% of the total amino acids and that the contents of methionine, tyrosine, phenylalanine, and histidine were very low. These data suggest that the microtubule binding domain of the 190-kDa MAP comprises an unique structure.     

A common amino acid sequence in 190-kDa microtubule-associated protein and tau for the promotion of microtubule assembly

Previously we reported that chymotryptic fragments of bovine adrenal 190-kDa microtubule-associated proteins (27-kDa fragment) and bovine brain tau (14-kDa fragment) contained microtubule-binding domain (Aizawa, H., Murofushi, H., Kotani, Hisanaga, S., Hirokawa, N., and Sakai, H. (1987) J. Biol. Chem. 262, 3782-3787; Aizawa, H., Kawasaki, H., Murofushi, H., Kotani, S., Suzuki, K., and Sakai, H. (1988) J. Biol. Chem. 263, 7703-7707). In order to study the structure of microtubule-binding domain of the two microtubule-associated proteins, we analyzed the amino acid sequence of the 27-kDa fragment and compared the sequence with that of the 14-kDa fragment. This revealed that 190-kDa microtubule-associated protein and tau contained at least one common sequence of 20 amino acid residues in their microtubule-binding domains. A synthetic polypeptide corresponding to the common sequence (Lys-Asn-Val-Arg-Ser-Lys-Val-Gly-Ser-Thr-Glu-Asn-Ile-Lys- His-Gln-Pro-Gly-Gly-Gly-Arg-Ala-Lys) was bound to microtubules competitively with the 190-kDa MAP. The apparent dissociation constant (KD) for the binding of the polypeptide to microtubules was estimated to be 1.8 x 10(-4) M, and the maximum binding reached 1.2 mol of the synthetic polypeptide/mol of tubulin dimer. This synthetic polypeptide increased the rate and extent of tubulin polymerization and decreased the critical concentration of tubulin for polymerization. The polypeptide-induced tubulin polymers were morphologically normal microtubules and were disassembled by cold treatment. The common sequence (termed assembly-promoting sequence) was thus identified as the active site of 190-kDa microtubule-associated protein and tau for the promotion of microtubule assembly. The reconstitution system of microtubules with this synthetic polypeptide with assembly-promoting sequence may be useful to elucidate detailed molecular mechanism of the promotion of microtubule assembly by microtubule-associated proteins.     

Site-specific phosphorylation by protein kinase C inhibits assembly-promoting activity of microtubule-associated protein 4

We have examined the phosphorylation of bovine microtubule-associated protein 4 (MAP4), formerly named MAP-U, by protein kinase C (PKC). When MAP4 was incubated with PKC, about 1 mol of phosphate was incorporated/mol of MAP4. Phosphorylation of MAP4 caused a remarkable decrease in the ability of the MAP to stimulate microtubule assembly. MAP4 consists of an amino-terminal projection domain and a carboxyl-terminal microtubule-binding domain. The carboxyl-terminal domain is subdivided into a Pro-rich region and an assembly-promoting (AP) sequence region containing four tandem repeats of AP sequence that is conserved in MAP4, MAP2, and tau [Aizawa et al. (1990) J. Biol. Chem. 265, 13849-13855]. In order to identify the site of MAP4 phosphorylated by PKC, a series of expressed MAP4 fragments was prepared and treated with the kinase. A fragment corresponding to the Pro-rich region (P fragment) was phosphorylated, while fragments corresponding to the projection domain and the AP sequence region were not. In addition, chymotryptic digestion of an authentic MAP4 prephosphorylated by PKC revealed that phosphate was incorporated almost exclusively into a 27-kDa fragment containing the carboxyl-terminal half of the Pro-rich region. We investigated the phosphorylation site in MAP4 using the P fragment and found that Ser815 was phosphorylated almost exclusively. We conclude that the phosphorylation of a single Ser residue in the Pro-rich region negatively regulates the assembly-promoting activity of MAP4.     

Binding of microtubule-associated proteins (MAPs) to rat brain mitochondria: a comparative study of the binding of MAP2, its microtubule-binding and projection domains, and tau proteins

Two major brain microtubule-associated proteins (MAPs), MAP2 and tau, were found to be able to bind to purified rat brain mitochondria. The apparent dissociation constants of the binding of thermostable 32P-labeled MAP2 and tau are 0.9 +/- 0.04 x 10(-7) and 3.8 +/- 0.7 x 10(-7) M, respectively. 32P-labeled MAP2 and tau bound to the mitochondria can be displaced by phosphorylated, nonradioactive MAP2. The binding parameters of MAP2 prepared without heat treatment and those of the thermostable MAP2 were of the same order of magnitude. Microtubule-binding and projection domains of MAP2 were obtained by chymotryptic digestion of rat brain microtubules (Vallee, Proc. Natl. Acad. Sci. USA, 77:3206-3210, 1980). Displacement studies with these two domains show that MAP2 bound to mitochondria can be displaced by the microtubule-binding domain, whereas the projection domain does not displace MAP2. The two domains of MAP2 bind to the mitochondria with similar affinity constants; however, the Bmax for the projection domain was 10 times and 35 times lower than the Bmax of the binding of the intact MAP2 and the microtubule-binding domain, respectively. Chymotryptic digestion of MAP2 bound to the mitochondria yielded peptide fragments with molecular masses similar to those obtained by the digestion of MAP2 bound to the microtubules. The fragments corresponding to the projection domain were released into the extramitochondrial supernatant, whereas the fragments originating from the microtubule-binding domain remained bound to the mitochondria. These results suggest that MAP2 binds to mitochondria preferentially via its microtubule-binding domain.     

The (Na,K)-ATPase of Friend erythroleukemia cells is phosphorylated near the ATP hydrolysis by an endogenous membrane-bound kinase

Friend murine erythroleukemia cells (MEL cells) contain a cAMP-independent protein kinase which phosphorylates the 100,000-Da catalytic subunit of the (Na,K)-ATPase both in living cells and in the purified plasma membrane (Yeh, L.-A., Ling, L., English, L., and Cantley, L. (1983) J. Biol. Chem. 258, 6567-6574). We have taken advantage of the selective phosphorylation of the 100,000-Da subunit in purified plasma membranes and the similarity between the proteolysis patterns of the MEL cell and dog kidney (Na,K)-ATPase to map the site of kinase phosphorylation on the MEL cell enzyme. The chymotryptic and tryptic cleavage sites of the dog kidney (Na,K)-ATPase have previously been located (Castro, J., and Farley, R. A. (1979) J. Biol. Chem. 254, 2221-2228). The 100,000-Da catalytic subunits of the dog kidney and MEL cell enzymes were specifically labeled at the active site aspartate residue by incubation with (32P)orthophosphate in the presence of Mg2+ and ouabain. Digestion of these two enzymes with chymotrypsin or trypsin revealed similar active site aspartate containing proteolytic fragments indicating a similar structure for the two enzymes. Chymotryptic digestions of MEL cell (Na,K)-ATPase labeled in vitro with [gamma-32P]ATP localize the region of kinase phosphorylation to within a 35,000-Da peptide derived from the middle of the 100,000-Da subunit. Tryptic digestion of the MEL cell plasma membranes degraded the 100,000-Da subunit to an NH2-terminal 43,000-Da peptide which contained the active site aspartate but which did not contain the kinase-labeled region. These results further locate the region of kinase phosphorylation to the COOH-terminal half of the 35,000-Da chymotryptic peptide. This location places the site of phosphorylation between the active site aspartate residue which accepts the phosphate of ATP during turnover and an ATP-binding site which has previously been located by labeling with fluorescein 5'-isothiocyanate (Carilli, C. T., Farley, R. A., Perlman, D. M., and Cantley, L. C. (1982) J. Biol. Chem. 257, 5601-5606). Phosphorylation of the (Na,K)-ATPase in this region may serve to regulate the activity of this enzyme.     

Phosphorylation of thylakoid proteins by a purified kinase

A simplified method is given for the purification of a 64-kilodalton protein kinase from spinach or pea thylakoid membranes (Coughlan, S., and Hind, G. (1986) J. Biol. Chem. 261, 11378-11385). In a heterogeneous reconstitution system comprised of purified kinase and washed thylakoids (having their intrinsic kinase inactivated or removed), endogenous light-harvesting pigment protein of photosystem II could serve as a substrate. Its phosphorylation did not require rebinding of kinase to the thylakoid membrane and, like the phosphorylation of solubilized pigment protein, was not under redox control. No reconstitution was observed upon replacing 64-kilodalton protein kinase with 25-kilodalton protein kinase (Coughlan, S., and Hind, G. (1986) J. Biol. Chem. 261, 14062-14068). Tryptic digestion of phosphorylated membranes removed the site of phosphorylation; the phosphorylated amino acid present in light-harvesting pigment protein and its tryptic peptide was threonine. Immunoglobulin from a polyclonal antiserum, raised against the purified enzyme, fully inhibited kinase activity toward solubilized and endogenous pigment protein. At higher titers, the antibody was effective in totally inhibiting the redox-sensitive phosphorylation of thylakoid proteins by endogenous kinase; inhibition profiles for phosphorylation of pigment protein and thylakoid proteins of 32, 16, and 9 kilodaltons were essentially identical. The 64-kilodalton protein kinase would thus appear to be responsible for all of the observed phosphorylation of thylakoid phosphoproteins.     

Functional analyses of the domain structure of microtubule-associated protein-4 (MAP-U)

Bovine microtubule-associated protein-4 (MAP-4), which was previously named MAP-U, consists of an amino-terminal projection domain (N-domain) and a carboxyl-terminal microtubule-binding domain (C-domain) (Aizawa, H., Emori, Y., Murofushi, H., Kawasaki, H., Sakai, H., and Suzuki, K. (1990) J. Biol. Chem. 265, 13849-13855). The C-domain contains a region rich in proline (Pro-rich region) and a region containing four assembly-promoting sequences (AP sequence region) which is shared by MAP-2 and tau. We purified a series of truncated fragments of MAP-4 expressed in Escherichia coli. An N-domain fragment did not bind to microtubules, while a C-domain fragment promoted microtubule assembly. Both of the fragments corresponding to the Pro-rich region (P fragment) and the AP sequence region (A4 fragment) promoted tubulin polymerization, although the A4 fragment had lower activity than intact MAP-4 and P fragment. A4 fragment produced morphologically normal microtubules whereas P fragment produced abnormal microtubules such as duplex microtubules and tight bundles of microtubules with diverse diameters. We concluded that both Pro-rich and AP sequence regions take part in the promotion of tubulin polymerization, and that the former is important for the MAP to bind to microtubules with high efficiency and the latter is essential for the formation of microtubules with normal morphology.     

Substrate specificities of tyrosine-specific protein kinases toward cytoskeletal proteins in vitro

We have previously reported that fodrin (beta subunit), tubulin (alpha subunit) and microtubule-associated proteins (MAPs; MAP2 and tau) are good substrates for the purified insulin receptor kinase (Kadowaki, T., Nishida, E., Kasuga, M., Akiyama, T., Takaku, F., Ishikawa, M., Sakai, H., Kathuria, S., and Fujita-Yamaguchi, Y. (1985) Biochem. Biophys. Res. Commun. 127, 493-500 and Kadowaki, T., Fujita-Yamaguchi, Y., Nishida, E., Takaku, F., Akiyama, T., Kathuria, S., Akanuma, Y., and Kasuga, M. (1985) J. Biol. Chem. 260, 4016-4020). In this study, to investigate the substrate specificities of tyrosine kinases, we have examined the actions of the purified epidermal growth factor (EGF) receptor kinase and Rous sarcoma virus src kinase on purified microfilament- and microtubule-related proteins. Among microfilament-related proteins examined, the purified EGF receptor kinase phosphorylated the beta subunit, but not the alpha subunit, of fodrin on tyrosine residues with a Km below the micromolar range. The fodrin phosphorylation by the EGF receptor kinase was markedly inhibited by F-actin. In contrast, the purified src kinase preferentially phosphorylated the alpha subunit of fodrin on tyrosine residues. Fodrin phosphorylation by the src kinase was not inhibited by F-actin. Among microtubule proteins examined, MAP2 was the best substrate for the EGF receptor kinase. By contrast, src kinase favored phosphorylation of tubulin as compared to MAP2. The peptide mapping of MAP2 phosphorylated by the EGF receptor kinase and by the insulin receptor kinase produced very similar patterns of phosphopeptides, while that of MAP2 phosphorylated by the src kinase gave a distinctly different pattern. When the phosphorylation of the tubulin subunits was examined, the EGF receptor kinase preferred beta subunit to alpha subunit, but the src kinase phosphorylated both alpha and beta subunits to a similar extent. These results, together with our previous results, indicate that the substrate specificities of the EGF receptor kinase and the insulin receptor kinase are very similar, but not identical, while that of the src kinase is distinctly different from that of these growth factor receptor kinases.     

Microtubule-binding domain of tau proteins

Limited chymotryptic digestion of whole tau proteins produced a fragment of Mr 14,000 (CT14), which was able to bind to microtubules reconstituted from tubulin alone in the presence of taxol. This fragment was also found to persist in microtubules when microtubules consisting of tau proteins and tubulin were digested by chymotrypsin. Analysis of amino acid composition revealed that CT14 was rich in lysine and proline residues, suggesting unique structure of microtubule-binding domain of tau proteins. Amino-terminal sequence of CT14 was determined to be Ser-Ser-Pro-Gly-Ser-Pro-Gly-Thr-Pro-Gly-Ser-Arg-Ser-Arg-X-Pro-Ser-Leu-Pr o. No heterogeneity was detected in this amino-terminal sequence of 19 residues. Five species of polypeptides consisting of tau proteins were separated from each other by gel electrophoresis and subjected to chymotryptic digestion. CT14 was produced from each of the tau polypeptides by chymotryptic digestion, indicating that all tau polypeptides have a common microtubule-binding domain.     

Modulation of protein 4.1 binding to inside-out membrane vesicles by phosphorylation

The effect of phosphorylation on the binding of protein 4.1 to erythrocyte inside-out vesicles was investigated. Protein 4.1 was phosphorylated with casein kinase A, protein kinase C, and cAMP-dependent protein kinase. An analysis of the phosphopeptides generated by alpha-chymotryptic and tryptic digestion indicates these kinases phosphorylate similar as well as distinct domains within protein 4.1. All three enzymes catalyze the phosphorylation to varying degrees of the 46-, 16-, and 8-10-kDa fragments derived from limited chymotryptic cleavage. In addition, casein kinase A phosphorylates a 24-kDa domain, whereas protein kinase C phosphorylates a 30-kDa domain. Protein 4.1 phosphorylated by casein kinase A and protein kinase C, but not cAMP-dependent protein kinase, exhibits a reduced binding to KI-extracted inside-out vesicles. On the other hand, phosphorylation of inside-out vesicles by casein kinase A does not affect their ability to bind protein 4.1. The inside-out vesicles, however, inhibit the phosphorylation of protein 4.1 by casein kinase A and protein kinase C, but not by cAMP-dependent protein kinase. These results suggest that casein kinase A and protein kinase C may modulate the binding of protein 4.1 to the membrane by phosphorylation of specific domains of the cytoskeletal protein. Since the 30-kDa domain has been suggested as a membrane-binding site, that phosphorylation by protein kinase C reduces the binding of protein 4.1 to inside-out vesicles is perhaps not surprising. On the other hand, the role of the casein kinase A substrate 24-kDa domain in membrane binding has not been established and needs to be examined.(ABSTRACT TRUNCATED AT 250 WORDS)     

Domain structure and antiparallel dimers of microtubule-associated protein 2 (MAP2).

We have studied the microtubule-associated protein MAP2 from porcine brain and its subfragments by limited proteolysis, antibody labeling, and electron microscopy. Two major chymotryptic fragments start at lys 1528 and arg 1664, generating microtubule-binding fragments of Mr 36 kDa (303 residues, analogous to the "assembly domain" of Vallee, 1980) and 18 kDa (167 residues). These fragments can be labeled with the antibody 2-4 which recognizes the last internal repeat of MAP2 (Dingus et al., 1991). The epitope of another monoclonal antibody, AP18 (Binder et al., 1986), was mapped to the first 151 residues of MAP2. The interaction with AP18 is phosphorylation dependent; dephosphorylated MAP2 is not recognized. Intact MAP2 forms rod-like particles of 97 nm mean length, similar to Gottlieb and Murphy's (1985) observations. Both antibodies bind near an end of the rod, suggesting that the sequence and the structure are approximately colinear. There is a pronounced tendency for MAP2 to form dimers whose components are nearly in register but of opposite polarity. MAP2 can also fold in a hairpin-like fashion, generating 50-nm rods, and it can self-associate into oligomers and fibers. The 36-kDa microtubule-binding fragment also has a rod-like shape; its mean length is 49 nm, half of the intact molecule, even though the fragment contains only one-sixth of the mass. The antibody 2-4 decorates one end of the rod, similar to the intact protein. The fragment also forms antiparallel dimers, but its tendency for higher self-assembly forms is much lower than with intact MAP2.     

A model for microtubule-associated protein 4 structure. Domains defined by comparisons of human, mouse, and bovine sequences.

cDNAs encoding human and mouse microtubule-associated protein 4 (MAP 4) were isolated. MAP 4 is encoded by a single gene. Multiple MAP 4 mRNAs are transcribed that are differentially expressed among mouse tissues. Open reading frames for the human and mouse MAP 4 clones indicate three distinct regions consisting of related sequences with different motifs. Approximately 30% of the protein is tandem related repeats of approximately 14 amino acids. Another region contains clusters of serine and proline. Four 18-mer repeats characteristic of the microtubule-binding domains of MAP 2 and tau are located at the carboxyl-terminal portion of MAP 4. Amino acid sequence analysis revealed that human and mouse MAP 4 are homologs of the bovine 190-kDa MAP/MAP U (Aizawa, H., Emori, Y., Murofushi, H., Kawasakai, H., Sakai, H., and Suzuki, K. (1990) J. Biol. Chem. 265, 13849-13855). Mouse and human MAP 4 and the bovine 190-kDa MAP are approximately 75% similar, indicating that these proteins are all members of the same class. Domains with extremely high conservation (greater than or equal to 88%) are: 1) the extreme amino terminus; 2) a proline-rich region between the KDM and S,P domains; 3) the microtubule-binding domain; and 4) the extreme carboxyl terminus.     

The calmodulin-binding domain on microtubule-associated protein 2

Microtubule-associated protein 2 (MAP2) binds calmodulin with a stoichiometry approaching 1-1.5 mol of calmodulin/mol of MAP2 in the presence of calcium ion. The calmodulin-binding domain(s) of MAP2 were probed by cross-linking 125I-calmodulin with partially digested MAP2, by limited digestion of the preformed 125I-calmodulin-MAP2 adduct, and by cross-linking 125I-calmodulin with the projection- and assembly-promoting portions of MAP2. Cross-linking 125I-calmodulin with partially digested MAP2 resulted in radioactive adducts of approximately 300, approximately 235, approximately 205, approximately 58, and approximately 40 kDa. The radioactive adducts with smaller molecular mass became prominent with increasing time of digestion concomitant with loss of those with higher molecular size. Limited chymotryptic digestion of preformed 125I-calmodulin-MAP2 adducts also produced a approximately 58-kDa radioactive band followed later by a approximately 40-kDa band. Brief chymotryptic digestion and subsequent centrifugation of microtubules preformed with pure tubulin and MAP2 permitted separation of microtubule-bound MAP2 fragments (molecular mass = approximately 215, approximately 180, and approximately 36 kDa) from unbound fragments (molecular mass = approximately 240, approximately 180, and approximately 140 kDa). 125I-Calmodulin cross-linked only with the microtubule-bound MAP2 fragments (forming mainly the approximately 58-kDa adduct) and not with unbound MAP2 fragments. Since the apparent molecular size of calmodulin is approximately 21 kDa on these sodium dodecyl sulfate-polyacrylamide gels, the results indicate that partial digestion of MAP2 by chymotrypsin produces a approximately 37-kDa fragment which can be further degraded to a approximately 20-kDa fragment. The approximately 37-kDa fragment that is labeled corresponds to the previously identified assembly-promoting fragment that attaches to the microtubule.     

Use of a heat-stable microtubule-associated protein class-specific antibody to investigate the mechanism of microtubule binding.

Members of the heat-stable family of microtubule-associated proteins (MAPs), MAP 2, tau, and MAP 4, contain three or four tandem imperfect repeated sequences close to their carboxyl termini. These sequences lie within the microtubule-binding domains of the MAPs; they have been proposed to be responsible for microtubule binding and the ability of these MAPs to lower the critical concentration for microtubule assembly. Their spacing may reflect that of the regularly arrayed tubulin subunits on the microtubule surface. We here characterize the 32- and 34-kDa chymotryptic microtubule-binding fragments of MAP 2 identified in earlier work. We identify the primary chymotryptic cleavage site in high molecular weight MAP 2 as between Phe1525 and Lys1526, within 13 amino acids of the known MAP 2 splice junction. We have raised a monoclonal antibody to the 32- and 34-kDa fragments and find that it reacts with all members of the heat-stable MAPs class. To determine where it reacts, we sequenced immunoreactive subfragments of the 32- and 34-kDa fragments, selected several cDNA clones with the antibody, and tested for antibody reactivity against a series of synthetic MAP 2 and tau peptides. We identify the epitope sequence as HHVPGGG (His-His-Val-Pro-Gly-Gly-Gly). The antibody also recognized several other MAP 2 and tau repeats. Despite reacting with this highly conserved element, we find that the antibody does not block microtubule binding, but binds to the MAPs and co-sediments with microtubules. These results suggest that there are other regions besides the repeated elements which are essential for microtubule binding.     

Molecular cloning of a ubiquitously distributed microtubule-associated protein with Mr 190,000

A heat-stable microtubule-associated protein (MAP) with apparent molecular weight of 190,000 is a major non-neural MAP which distributes ubiquitously among bovine tissues (termed here MAP-U). Previously we reported that microtubule-binding chymotryptic fragments of MAP-U and tau contain a common assembly-promoting (AP) sequence of 22 amino acid residues (Aizawa, H., Kawasaki, H., Murofushi, H., Kotani, S., Suzuki, K., and Sakai, H. (1989) J. Biol. Chem. 264, 5885-5890). We isolated cDNA clones for MAP-U containing the whole coding sequence. Northern blot analysis revealed that a major species of MAP-U mRNA is 5 kilobases in length and is expressed ubiquitously among bovine tissues. Nucleotide sequence analysis revealed the complete amino acid sequence of MAP-U which consists of 1,072 amino acid residues. Analysis of the deduced amino acid sequence of MAP-U indicated that this molecule is clearly divided into two domains in terms of electrostatic charge distribution: an amino-terminal acidic domain (residues 1-640) and a carboxyl-terminal basic domain (residues 641-1072). The amino-terminal domain of MAP-U shows no significant sequence homology with other known protein sequences including neural MAPs, tau, and MAP-2. The amino-terminal domain of MAP-U contains unique 18 1/2 repeats of 14-amino acid motif which have not been observed in other MAPs. The carboxyl-terminal domain of MAP-U is further divided into three regions: a Pro-rich region (residues 641-880), an AP sequence region (residues 881-1003), and a short hydrophobic tail (residues 1004-1072). The Pro-rich region is mainly composed of five species of amino acid residues, Pro, Ala, Lys, Ser, and Thr. The AP sequence region contains four tandem repeats of AP sequences, and thus, this region is considered to play a leading role in the interaction of MAP-U with microtubules.     

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.     

Structural variability of rabbit secretory components. Allotype-associated differences in the third, fourth, and fifth domains

We have previously shown (Frutiger, S., Hughes, G. J., Hanly, W. C., Kingzette, M., and Jaton, J.-C. (1986) J. Biol. Chem. 261, 16673-16681) that limited tryptic digestion of the high Mr form of rabbit secretory component of allotypes t61, t62, and t63 generates two major fragments, the NH2-terminal domain and a 40-kDa fragment encompassing domains 3, 4, and 5. Similarly, from the low Mr form of secretory component, (SC) the NH2-terminal domain, together with a 30-kDa fragment containing domains 4 and 5, were released. These fragments were used as inhibitors in a sensitive competitive binding radioimmunoassay with noncross-reactive rabbit alloantisera to study the distribution and localization of the major allotype-specific allotopes within the SC polypeptide. The 40-kDa fragments were shown to inhibit the 125I-labeled intact SC/anti-SC allotype reaction to the extent of 90%, i.e. nearly as well as the intact homologous high Mr SC form. In contrast, the NH2-terminal fragments (domain 1) were not inhibitory. The low Mr SC of each allotype was less inhibitory on a molar basis than the homologous high Mr SC polypeptide, an observation compatible with the deletion of domains 2 and 3 in the smaller polypeptide (Deitcher, D. L., and Mostov, K. E. (1986) Mol. Cell. Biol. 6, 2712-2715; Frutiger, S., Hughes, G. J., Fonck, Ch., and Jaton, J.-C. (1987) J. Biol. Chem. 262, 1712-1715). The structural correlates of the allotypic specificities were evaluated by comparative peptide mapping of the 40-kDa fragments (allotypes t61, t62, and t63). The data suggest that the t61 allotype structure differs significantly from the t62 and t63 structures, the latter two being much more related to each other than to t61. These findings are in full agreement with the serological data. The inhibition results suggest that the major allotype-specific, noncross-reactive allotopes of SC are distributed throughout domains 3, 4, and 5, even though domain 4 appears to be more conserved than domains 3 and 5 between the allotypes t61 and t63. Seven amino acid substitutions between t61 and t63 have been detected within domains 3, 4, and 5.     

Different protofilament-dependence of the microtubule binding between MAP2 and MAP4

To see a molecular basis of the difference in the microtubule binding between MAP2 and MAP4, we compared the binding of them onto microtubule and Zinc-sheet in the presence of various concentrations of NaCl. The Zinc-sheet is the lateral association of protofilaments arranged in an antiparallel fashion with alternatively exposed opposite surfaces, so that binding requiring adjacent protofilaments is restricted. While the salt-dependence of the MAP2 desorption was not altered between these tubulin polymers, MAP4 dissociated from Zinc-sheet at lower concentrations of NaCl than from microtubule. These results suggest that single protofilament is sufficient for microtubule binding of MAP2 as observed by Al-Bassam et al. [J. Cell Biol. 157 (2002) 1187], but MAP4 appeared to interact with adjacent protofilaments during microtubule-binding. Weakened binding on Zinc-sheets was also observed in the projection domain-deletion mutants of MAP4, so that the difference in the protofilament-dependence would lie in the relatively conserved microtubule-binding domain.     

Isolation and sequence of a tropomyosin-binding fragment of turkey gizzard calponin

Limited chymotryptic cleavage of turkey gizzard calponin yields a 13 kDa fragment which could be purified by its ability to bind to Sepharose-immobilized tropomyosin. This 13 kD polypeptide is shown to be derived from a 22 kDa fragment. Complete amino acid sequence analysis of the 13 kD and 22 kD fragments reveals high homology with the formerly characterized smooth muscle-specific protein SM22 alpha (Pearlstone, J.R., Weber, M., Lees-Miller, J.P., Carpenter, M.R. and Smillie L.B., 1987, J. Biol. Chem. 262, 5985-5991) and the product of gene mp20 of Drosophila (Ayme-Southqate, A., Lasko, P., French, C, and Pardue, M.L. [(1989) J. Cell Biol. 108, 521-531]. Futhermore we recognize sequence elements of a putative actin-binding domain of alpha-actinin, the calpactin I or p 36 sequence, and a consensus motif present in the repeats of the gene product of the candidate unc-87 gene of C. elegans (S.D. Goetinck and R.H. Waterston, personal communication).