Serum-dependent phosphorylation of human MAP4 at Ser696 in cultured mammalian cells

In the previous paper (Ookata et al., (1997) Biochemistry, 36: 249-259), we identified two mitotic cdc2 kinase phosphorylation sites (Ser696 and Ser787) in the proline-rich region of human MAP4. One (Ser696) of them was also phosphorylated during interphase. A protein kinase responsible for interphase phosphorylation of Ser696 could necessarily be distinct from cdc2/cyclin B kinase. To get insights into a physiological role for Ser696 phosphorylation, we searched for a Ser696 kinase and for cellular conditions under which Ser696 is dephosphorylated. Because Ser696 conforms to the MAP kinase phosphorylation consensus motif (PXSP), MAP kinase was tested as a possible kinase phosphorylating Ser696. MAP kinase, in fact, did phosphorylate Ser696 in MTB3, the carboxy-terminal half of human MAP4 in vitro. Phosphorylation of Ser696 in HeLa cell extract was suppressed by a MAP kinase inhibitor, DBTM-0004. Also consistent with the notion that Ser696 is a MAP kinase site were the fact that serum-starvation induced dephosphorylation of Ser696 in HeLa cells, TIG-3 and MRC-5-30 human fibroblasts, while readdition of serum recovered Ser696 phosphorylation, albeit after a surprisingly long interval. Thus, phosphorylation of Ser696 of MAP4, most likely carried out by MAP kinase, may play a role in modulation of MAP4 activity in proliferating versus quiescent cells.

Functional analysis of microtubule-binding domain of bovine MAP4

Bovine microtubule-associated protein 4 (MAP4) consists of an amino-terminal projection domain and a carboxyl-terminal microtubule-binding domain. The carboxyl-terminal domain of MAP4 is further divided into three subdomains: a region rich in proline and basic residues (Pro-rich region), a region containing four repeats of an assembly-promoting (AP) sequence, which consists of 22 amino acid residues (AP sequence region), and a hydrophobic tail region (Tail region). The subdomain structure of MAP4 microtubule binding domain is similar to those of other MAPs (MAP2 and tau). In order to study the function of each subdomain per se of bovine MAP4 microtubule-binding domain, we purified a series of truncated fragments of MAP4, expressed in Escherichia coil. Binding affinity of the PA4T fragment (containing the Pro-rich region, the AP sequence region and the Tail region) is only four times higher than that of the A4T fragment (containing the AP sequence region and the Tail region), while the microtubule nucleating activity of the PA4T fragment is far greater. We propose that the Pro-rich region promotes the nucleation of microtubule assembly. The A4 fragment (corresponding to the AP sequence region) stimulated the assembly of tubulin into coldstable amorphous aggregates. The AP sequence region of MAP4 failed to promote microtubule assembly. On the other hand, the fragment has an activity to stimulate microtubule elongation. The function of the MAP4 Tail region is not clear at present. The A4T fragment (containing the AP sequence region and the Tail region) promote both microtubule nucleation and elongation step, but the A4 fragment only promotes microtubule elongation, suggesting that the Tail region is indispensable for the nucleation step. However, the fragment containing only the Tail region could not bind to microtubule. Although MAP4 was considered to be long, thin and flexible molecule, never the Tail region may contribute to be the proper folding of MAP4, and/or may interact with other molecules. We concluded that both the Pro-rich region and the AP sequence region take part in the promotion of tubulin polymerization, and that the former is important for the lateral protofilament-protofilament interaction, and the latter is important for the longitudinal affinity between each tubulin dimer in a protofilament.

Counteracting regulation of chromatin remodeling at a fission yeast cAMP response element-related recombination hotspot by stress-activated protein kinase, cAMP-dependent kinase and meiosis regulators

In fission yeast, an ATF/CREB-family transcription factor Atf1-Pcr1 plays important roles in the activation of early meiotic processes via the stress-activated protein kinase (SAPK) and the cAMP-dependent protein kinase (PKA) pathways. In addition, Atf1-Pcr1 binds to a cAMP responsive element (CRE)-like sequence at the site of the ade6-M26 mutation, which results in local enhancement of meiotic recombination and chromatin remodeling. Here we studied the roles of meiosis-inducing signal transduction pathways in M26 chromatin remodeling. Chromatin analysis revealed that persistent activation of PKA in meiosis inhibited M26 chromatin remodeling, suggesting that the PKA pathway represses M26 chromatin remodeling. The SAPK pathway activated M26 chromatin remodeling, since mutants lacking a component of this pathway, the Wis1 or Spc1/Sty1 kinases, had no M26 chromatin remodeling. M26 chromatin remodeling also required the meiosis regulators Mei2 and Mei3 but not the subsequently acting regulators Sme2 and Mei4, suggesting that induction of M26 chromatin remodeling needs meiosis-inducing signals before premeiotic DNA replication. Similar meiotic chromatin remodeling occurred meiotically around natural M26 heptamer sequences. These results demonstrate the coordinated action of genetic and physiological factors required to remodel chromatin in preparation for high levels of meiotic recombination and eukaryotic cellular differentiation.

Ser787 in the proline-rich region of human MAP4 is a critical phosphorylation site that reduces its activity to promote tubulin polymerization

p34cdc2 kinase-phosphorylation sites in the microtubule (MT)-binding region of MAP4 were determined by peptide sequence of phosphorylated MTB3, a fragment containing the carboxy-terminal half of human MAP4. In addition to two phosphopeptides containing Ser696 and Ser787 which were previously indicated to be in vivo phosphorylation sites, two novel phosphopeptides, containing Thr892 or Thr901 and Thr917 as possible phosphorylation sites, were isolated, though only in in vitro phosphorylation. The role of phosphorylation at Ser696 and Ser787, which were differently phosphorylated during the cell cycle (Ookata et al., (1997). Biochemistry, 36: 15873-15883), was investigated in MT-polymerization, using MAP4 Ser to Glu mutants, which mimic phosphorylation at each site. Mutation of Ser787 to Glu strikingly reduced the MAP4's MT-polymerization activity, while Glu-mutation at Ser696 did not. These results suggest that Ser787 could be the critical phosphorylation site causing MTs to be dynamic at mitosis.

p97 ATPase, an ATPase involved in membrane fusion, interacts with DNA unwinding factor (DUF) that functions in DNA replication

DNA unwinding factor (DUF) unwinds duplex DNA and is supposed to function in DNA replication in Xenopus egg extracts. Here we report the isolation and analysis of a DUF-interacting factor. By immunoprecipitation, we found that p97 ATPase (p97) interacts with DUF in Xenopus egg extracts. This interaction was confirmed by the in vitro binding of purified p97 with DUF. When sperm chromatin was added to Xenopus egg extracts to construct nuclei active in DNA replication, p97 was incorporated into the nuclei. These data suggest that the complex of DUF and p97 may function in DNA replication.

Inhibition of tumor invasion and metastasis by a novel lysophosphatidic acid (cyclic LPA)

Fetal calf serum (FCS) and 1-oleoyl lysophosphatidic acid (LPA) were previously found to be potent inducers of invasion (transcellular migration) in an in vitro system. A novel LPA, composed of cyclic phosphate and cyclopropane-containing hexadecanoic acid (PHYLPA), first isolated from myxoamoebae of Physarum polycephalum, and its synthetic derivatives (cLPA) were tested for their ability to inhibit tumor cell invasion and metastasis. Amoung these, Pal-cLPA, which has a palmitoyl moiety, was most potent in inhibiting invasion, with 93.8% inhibition at the concentration of 25 microM. Invasion in vitro by mouse melanoma cells (B16), human pancreatic adenocarcinoma cells (PSN-1), human lung cancer cells (OC-10) and human fibrosarcoma cells (HT-1080) was also inhibited by Pal-cLPA. The stimulation of MMI cells with LPA triggered F-actin formation, which was impaired by the addition of Pal-cLPA at invasion-inhibitory concentration. Pal-cLPA induced a rapid increase in adenosine 3',5'-cyclic monophosphate (cAMP) concentration in MMI cells. The addition of dibutyryl cAMP significantly abrogated LPA-induced invasion by MM1 cells and actin polymerization in the cells. The inhibition of MM1 cell invasion by Pal-cLPA may be ascribed to an increased level of cAMP. Pal-cLPA also suppressed invasion in vitro by MM1 cells induced by FCS dose dependently, without affecting proliferation. It also suppressed the pulmonary metastasis of B16 mouse melanoma cells injected into the tail vein of C57BL/6 mice. Thus, Pal-cLPA is effective in inhibiting invasion and metastasis of a variety of tumor cells.

A DNA unwinding factor involved in DNA replication in cell-free extracts of Xenopus eggs

BACKGROUND

Alteration of chromatin structure is a key step in various aspects of DNA metabolism. DNA unwinding factors such as the high mobility group (HMG) proteins are thought to play a general role in controlling chromatin structure and a specific role in controlling DNA replication. For instance, in the in vitro simian virus 40 replication system, minichromosomes containing HMG-17 replicate more efficiently than those without it, suggesting that HMG-17 enhances the rate of replication of a chromatin template by unfolding the higher-order chromatin structure. At present, however, only limited data suggest an involvement of DNA unwinding factors in DNA replication.

RESULTS

We purified from Xenopus eggs a novel heterodimeric factor, termed DNA unwinding factor (DUF), that consists of 87 kDa and 140 kDa polypeptides. DUF unwinds closed-circular duplex DNA in the presence of topoisomerase I, but it does not possess a DNA gyrase activity: it does not introduce negative supercoils into DNA at the expense of ATP hydrolysis. Cloning and sequencing of the cDNAs encoding the two polypeptides revealed that the 87 kDa polypeptide is homologous to a mammalian HMG protein, T160/structure-specific recognition protein. The 140 kDa polypeptide is homologous to yeast Cdc68, a protein that controls the expression of several genes during the G1 phase of the cell cycle by modulating chromatin structure. Immunodepletion of DUF from Xenopus egg extracts drastically reduced the ability of the extract to replicate exogenously added sperm chromatin or plasmid DNA.

CONCLUSIONS

We propose that DUF plays a role in DNA replication in Xenopus egg extracts.

GTP-binding-protein-coupled receptor kinase 2 (GRK2) binds and phosphorylates tubulin

Tubulin was found to bind to a glutathione S-transferase fusion protein containing the carboxy-terminal domain of GTP-binding-protein-coupled receptor kinase 2 (GRK2) (residues 467-689), which is known to contain a pleckstrin homology site and to bind GTP-binding protein betagamma subunits. The binding of tubulin to the fusion protein was not affected by GTP-binding protein betagamma subunits, indicating that tubulin and betagamma subunits bind GRK2 independently. Western-blotting analysis with anti-GRK2 Ig indicated that GRK2 was copurified with tubulin through the polymerization-depolymerization procedure. Tubulin was phosphorylated by GRK2, in contrast with the facts that the known substrates of GRK2 are restricted to activated forms of GTP-binding-protein-coupled receptors and that tubulin is a poor substrate for most kinases. GRK2 did not phosphorylate microtubule-associated proteins (MAPs), under conditions where MAPs were well phosphorylated by endogenous kinases copurified with tubulin. The Km for tubulin was estimated to be 3 microM, and 1.3 mol phosphate/tubulin dimer was incorporated. The phosphorylation of tubulin was stimulated by betagamma subunits and agonist-bound muscarinic receptors. Phosphorylated tubulin could be polymerized into microtubules, and polymerized tubulin was also phosphorylated by GRK2.

MAP4 is the in vivo substrate for CDC2 kinase in HeLa cells: identification of an M-phase specific and a cell cycle-independent phosphorylation site in MAP4

We reported previously that cdc2 kinase decreased the microtubule-stabilizing ability of a major HeLa cell microtubule-associated protein, MAP4, by phosphorylation in vitro [Ookata, K., et al. (1995) J. Cell Biol. 128, 849-862]. An important question raised by this study is whether MAP4 is indeed phosphorylated by cdc2 kinase at mitosis in vivo. We present here evidence that cdc2 kinase is the major M-phase MAP4 kinase, and, further, we identify two phosphorylation sites within the proline-rich domain of MAP4. Metabolic 32P labeling showed the increased phosphorylation of MAP4 at mitosis. A specific inhibitor of cdc2 kinase, butyrolactone I, inhibited phosphorylation of MAP4 both in mitotic HeLa cells and in the mitotic HeLa cell extract. The phosphopeptide map analysis revealed the high similarity of in vivo labeled mitotic MAP4 to that phosphorylated by cdc2 kinase in vitro. Ser-696 and Ser-787, both of which lie within SPXK consensus sequences for cdc2 kinase, were identified as phosphorylation sites in the proline-rich region of MAP4 in vivo and in vitro. Immunoblotting with antibodies that recognize the phosphorylation state of Ser-696 or Ser-787 showed that Ser-787 in the SPSK sequence was specifically phosphorylated at mitosis while Ser-696 in the SPEK sequence was phosphorylated both at mitosis and in interphase. These results suggest that cdc2 kinase directly regulates microtubule dynamics at mitosis through phosphorylation of MAP4 at a number of sites, including Ser-787.

The 'assembly-promoting sequence region' of microtubule-associated protein 4 failed to promote microtubule assembly

In order to study the function of the bovine MAP4 microtubule-binding domain (the assembly-promoting (AP) sequence region), a fragment corresponding to the AP sequence region was prepared using an Escherichia coli expression system. When the fragment was mixed with purified tubulin at 37 degrees C, the fragment caused a time- and dose-dependent turbidity increase, and the fragment bound to tubulin. However, the products were cold-stable, and amorphous aggregates were observed by electron microscopy. Using axonemes as the seeds for microtubule assembly, the microtubule-elongating activity of the fragment was examined. A dose-dependent turbidity increase of the sample was observed, and electron microscopic observation revealed that microtubules were dose-dependently elongated from the axonemes. Consequently, the AP sequence region does not nucleate microtubules, but elongates them.

A possible mechanism for hyperthermic radiosensitization mediated through hyperthermic lability of Ku subunits in DNA-dependent protein kinase

DNA-dependent protein kinase (DNA-PK), composed of p470 catalytic subunit and p85/p70 heterodimer of Ku autoantigen, is considered a critical enzyme in DNA double-strand break repair. We purified DNA-PK from human leukaemic MOLT-4 cells by successive column chromatography and separated into p470 and Ku subunits by ultracentrifugation in glycerol gradient. We studied hyperthermic stability of DNA-PK holoenzyme and its separated subunits to test a possible role of DNA-PK in hyperthermic radiosensitization. DNA-PK was found to lose its activity rapidly at hyperthermic 44 degrees C, and further, Ku subunits instead of p470 catalytic subunits were found to be sensitive to hyperthermia. These results indicate a possibility that hyperthermic radiosensitization is mediated through the heat lability of Ku subunits of DNA-PK, impairing repair of radiation-induced double-strand break of DNA.

Cell cycle-dependent activation of telomerase in naturally synchronized culture of a true slime mold, Physarum polycephalum

Telomeres of Physarum plasmodia did not shorten with numerous repeats of nuclear division, and an apparent activity of telomerase was detected in this organism. In naturally synchronized culture of Physarum plasmodia, an evident activation of telomerase was observed at the late S-phase, just prior to the completion of in vivo DNA replication, and the low telomerase activity was detected throughout the cell cycle. In the nuclei isolated from different phases of synchronized plasmodia, a higher activity of telomerase was also observed at late S-phase. These results clearly show the existence of a cell cycle-dependent regulatory mechanism of telomerase activity in growing, naturally synchronized cells.

Heat stress induces a glycosylation of membrane sterol in myxoamoebae of a true slime mold, Physarum polycephalum

To know the very early events occurring after heat shock, the changes of membrane lipids were examined. Heat stress induced the production of a certain glycolipid in the myxoamoebae of Physarum polycephalum in a few minutes. The purified glycolipid was determined to be a poriferasterol monoglucoside by structural studies that was previously reported to be expressed during the differentiation of Physarum cells from haploid myxoamoebae to diploid plasmodia (Murakami-Murofushi, K., Nakamura, K., Ohta, J., Suzuki, M., Suzuki, A., Murofushi, H., and Yokota, T. (1987) J. Biol. Chem. 262, 16719-16723). The activity of UDP-glucose:poriferasterol glucosyltransferase (Murakami-Murofushi, K., and Ohta, J. (1989) Biochim. Biophys. Acta 992, 412-415) was also expressed rapidly after heat shock. Thus, the activation of sterol glucosyltransferase and the production of sterol-glucoside were considered to be important events that were involved in the signal transduction system to induce some succeeding heat-shock responses, such as the synthesis of heat-shock proteins.

Xenopus oocytes express multiple receptors for LPA-like lipid mediators

In Xenopus laevis oocytes, both lysophosphatidic acid (LPA) and a cyclic phosphate-containing analogue 1-acyl-sn-glycero-2,3-cyclic phosphate (cLPA) isolated from Physarum polycephalum activated oscillatory Cl- currents. cLPA elicited oscillatory currents only when applied extracellularly and, similarly to LPA, evoked homologous desensitization. cLPA applied to oocytes previously desensitized b y LPA failed to elicit a current, indicating that LPA completely desensitized the cLPA receptors. In contrast, when oocytes were desensitized by cLPA, LPA still evoked large currents. The lack of heterologous desensitization between cLPA and LPA indicates that the former acts on a distinct receptor subpopulation(s), which is also activated by LPA. The alkyl-ether analogue 1-hexadecyl-2-lyso-sn-glycero-3-phosphate (16:0-GP) and dioleoyl-phosphatidic acid (18:1-PA) showed heterologous desensitization patterns similar to that of LPA with regard to cLPA. Complete heterologous desensitization was obtained between LPA and 16:0-GP or 18:1-PA. These observations demonstrate the simultaneous expression of at least two different types of receptors for LPA-like lipid mediators on Xenopus oocytes and that these receptors show different pharmacological properties in their selectivity to cLPA.

Microinjection of intact MAP-4 and fragments induces changes of the cytoskeleton in PtK2 cells

The molecular cloning and sequencing of microtubule-associated protein (MAP)-4 identified microtubule-binding repeats near the C-terminus and a projection domain near the N-terminus. Although it is well known that MAP-4 stimulates the assembly of and stabilizes microtubules (MT) in vitro, the function of MAP-4 in vivo is still unclear. In this study, we examined the function of MAP-4 in the cytoskeleton both in vitro and in vivo. Intact MAP-4 was prepared from bovine adrenal cortex, and the truncated fragments of the N- and the C-terminal halves (named NR and PA4 fragments, respectively) were expressed in Escherichia coli and isolated. In vitro studies demonstrated that in a solution containing a physiological concentration of NaCl, intact MAP-4 and the PA4 fragment were bound to MT, but not to F-actin. The NR fragment was not bound to MT or to F-actin. We also examined the MT changes in PtK2 cells after they had been microinjected with intact MAP-4 and the truncated fragments of PA4 and NR. The injection of intact MAP-4 or PA4 into the cells induced an increase in the number of cytoplasmic MT, as well as MT bundling. The NR fragment did not affect the MT array. Injected MAP-4 and PA4 were associated with the increased MT. In addition, injection with MAP-4 and PA4 stabilized MT in relation to treatment with the MT-disrupting drug, nocodazole. These results indicated that intact MAP-4 and the PA4 fragment promoted MT assembly and stabilized MT, by binding to MT, in vivo as well as in vitro. Further, the injection of the PA4 fragment induced an increase in stress fibers. However, these proteins did not show any association with the stress fibers. Our results suggest that there is an indirect effect of MAP-4 on stress fibers rather than a direct interaction between MAP-4 and stress fibers.

Selective inhibition of DNA polymerase-alpha family with chemically synthesized derivatives of PHYLPA, a unique Physarum lysophosphatidic acid

PHYLPA, a unique Physarum lysophosphatidic acid (LPA), showed selective inhibition of a family of DNA polymerase alpha, including DNA polymerases alpha, delta and epsilon; but no inhibition of DNA polymerase beta or gamma was observed. To reveal the molecular mechanism of inhibition of DNA polymerases by PHYLPA, four stereoisomers and some other derivatives were synthesized and their effects on DNA polymerases were studied. Among eight derivatives synthesized, PHYLPA-1 (the natural PHYLPA; sodium 1-O-[(9'S,10'R)-9',10'-methanohexadecanoyl]-sn-glycerol 2,3-cyclic phosphate) and PHYLPA-2 (sodium 3-O-[9'S,10'R)-9',10'-methanohexadecanoyl]-sn-glycerol 1,2-cyclic phosphate) were strong and specific inhibitors of a family of DNA polymerase alpha. But their stereoisomers PHYLPA-3 (sodium 1-O-[9'R,10'S)-9',10'-methanohexadecanoyl]-sn-glycerol 2,3-cyclic phosphate) and PHYLPA-4 (sodium 3-O-[9'R,10'S)-9',10'-methanohexadecanoyl-sn-glycerol 1,2 cyclic phosphate) were weak inhibitors, showing the critical importance of stereochemistry of a cyclopropane-containing fatty acid for the inhibitory activity. Some derivatives having no cyclopropane-containing fatty acids--palmitoyl-, oleoyl-, and palmitoleoyl-PHYLPA--showed inhibition to some extent; but 1-palmytoyl and 1-oleoyl lysophosphatidic acid, which has no cyclic phosphate, did not show an apparent inhibitor activity on DNA polymerases. Hence, the extent of the inhibition apparently depends on the stereochemistry of both the fatty acid moiety and the cyclic phosphate.

Cyclin B interaction with microtubule-associated protein 4 (MAP4) targets p34cdc2 kinase to microtubules and is a potential regulator of M-phase microtubule dynamics

We previously demonstrated (Ookata et al., 1992, 1993) that the p34cdc2/cyclin B complex associates with microtubules in the mitotic spindle and premeiotic aster in starfish oocytes, and that microtubule-associated proteins (MAPs) might be responsible for this interaction. In this study, we have investigated the mechanism by which p34cdc2 kinase associates with the microtubule cytoskeleton in primate tissue culture cells whose major MAP is known to be MAP4. Double staining of primate cells with anti-cyclin B and anti-MAP4 antibodies demonstrated these two antigens were colocalized on microtubules and copartitioned following two treatments that altered MAP4 distribution. Detergent extraction before fixation removed cyclin B as well as MAP4 from the microtubules. Depolymerization of some of the cellular microtubules with nocodazole preferentially retained the microtubule localization of both cyclin B and MAP4. The association of p34cdc2/cyclin B kinase with microtubules was also shown biochemically to be mediated by MAP4. Cosedimentation of purified p34cdc2/cyclin B with purified microtubule proteins containing MAP4, but not with MAP-free microtubules, as well as binding of MAP4 to GST-cyclin B fusion proteins, demonstrated an interaction between cyclin B and MAP4. Using recombinant MAP4 fragments, we demonstrated that the Pro-rich C-terminal region of MAP4 is sufficient to mediate the cyclin B-MAP4 interaction. Since p34cdc2/cyclin B physically associated with MAP4, we examined the ability of the kinase complex to phosphorylate MAP4. Incubation of a ternary complex of p34cdc2, cyclin B, and the COOH-terminal domain of MAP4, PA4, with ATP resulted in intracomplex phosphorylation of PA4. Finally, we tested the effects of MAP4 phosphorylation on microtubule dynamics. Phosphorylation of MAP4 by p34cdc2 kinase did not prevent its binding to microtubules, but abolished its microtubule stabilizing activity. Thus, the cyclin B/MAP4 interaction we have described may be important in targeting the mitotic kinase to appropriate cytoskeletal substrates, for the regulation of spindle assembly and dynamics.

Inhibitory effect of calmodulin on phosphorylation of NAP-22 with protein kinase C

NAP-22, a recently identified neural tissue-enriched acidic protein, was shown to be a substrate of protein kinase C in vitro. Its phosphorylation site was assigned as Ser6 using deleted mutants expressed in Escherichia coli. Calmodulin inhibited this phosphorylation reaction. This inhibitory effect of calmodulin was dose-dependent and much stronger than its inhibitory effect to the phosphorylation of neuromodulin (GAP-43) with protein kinase C. The dissociation constant of NAP-22 and calmodulin obtained using the fluorescence change of dansyl-labeled calmodulin was much lower than that of neuromodulin and calmodulin. The phosphorylation of NAP-22 inhibited the association with calmodulin.

Modulation of DNA synthesis by microtubule-associated protein 2 in the nuclear matrix isolated from Physarum polycephalum

The mechanism of stimulation of DNA synthesis by microtubule-associated protein 2 (MAP2) was examined in the nuclear matrix isolated from Physarum polycephalum. Porcine brain MAP2 stimulated DNA synthesis by the matrix with exogenous templates, but not with endogenous templates. Kinetic analyses showed that MAP2 decreases the Km of the matrix for deoxyribonucleoside triphosphates. Comparison of the Km values of active- and latent-type DNA replication machineries of Physarum suggested a possible role for MAPs or MAP-like proteins in DNA replication.

Inhibition of cell proliferation by a unique lysophosphatidic acid, PHYLPA, isolated from Physarum polycephalum: signaling events of antiproliferative action by PHYLPA

The unique Physarum lysophosphatidic acid, PHYLPA, having a cyclopropane in the fatty acid moiety and a cyclic phosphate at C-2 and C-3 positions of the glycerol, inhibited proliferation of human fibroblast cells, TIG-3 and TIG-7, which were cultured in a chemically defined (serum-free) medium. The cells at S- and M-phases proceeded to G2- and G1-phases, respectively, and most of cells were arrested at G1- or G2-phase during PHYLPA treatment. The growth was recovered when PHYLPA was removed from the medium. In the presence of serum, PHYLPA did not show obvious inhibitory effects, indicating the existence of a factor(s) which neutralizes the antiproliferative activity of PHYLPA. PHYLPA elicited an increase in 3',5'-cyclic adenosine monophosphate (cAMP) in a biphasic fashion in fibroblast cells. It also elicited inositol phosphate accumulation, as well as a transient rise in cytoplasmic free Ca2+ ion.

Isolation of a new species of Physarum lysophosphatidic acid, PHYLPA, and its effect on DNA polymerase activity

A new species of lysophosphatidic acid was isolated from myxoamoebae of a true slime mold, Physarum polycephalum, and structural studies were performed. The purified substance was subjected to nuclear magnetic resonance spectroscopy (NMR), infrared spectroscopy (IR), fast atom bombardment mass spectroscopy (FAB/MS), alkaline hydrolysis and tandem mass spectroscopy (MS/MS), and the results suggested this substance to be lysophosphatidic acid composed of a cyclic phosphate and cis-11,12-methylene octadecanoic acid. The effects of the LPA on DNA polymerases were studied and compared with the effects of PHYLPA, which had been isolated as a specific inhibitor of eukaryotic DNA polymerase alpha (6). It showed a specific inhibitory activity on eukaryotic DNA polymerase alpha, but no activity on the repair-type, or mitochondrial DNA polymerases.

Initiation of DNA synthesis in a high molecular weight fraction of Xenopus egg extract

Xenopus egg extract was fractionated by gel-filtration column chromatography and DNA synthetic activity was examined using double-stranded DNA as a template. The major activity eluted had an apparent molecular mass of about 300 kDa. Product analyses showed that de novo DNA synthesis occurs with formation of replication bubbles, thereby suggesting that this fraction catalyzes the initiation of DNA replication. Activities of DNA polymerase alpha-primase and DNA helicase overlapped with the DNA synthetic activity, but the elution profiles of the enzymes differed from that of the DNA synthetic activity. Therefore, this 300-kDa fraction may contain a component which differs from the above enzymes yet is essential for initiation of DNA replication.

Positive regulation of mu-calpain action by polyphosphoinositides

Whether calcium is the only major intracellular activator of calpain has not yet been established. Here we demonstrate that polyphosphoinositides may play critical roles in the activation process of mu-calpain. Experiments with purified enzyme, substrate (fodrin), and phospholipids show that only polyphosphoinositides but not other lipids significantly promote calpain action in the physiological intracellular calcium range of 10(-7) to 10(-6) M. The effect of polyphosphoinositide is exerted through both a reduction in the calcium concentration required for calpain autolysis and an increase in the Vmax of the proteolytic reaction. Neomycin, a polyphosphoinositide-binding antibiotic, inhibits both polyphosphoinositide-assisted proteolysis in test tubes and calcium-induced calpain activation coupled with substrate proteolysis in intact cells. This implies that the presence of polyphosphoinositides may actually be a prerequisite for calpain activation inside cells.

Inhibition of eukaryotic DNA polymerase alpha with a novel lysophosphatidic acid (PHYLPA) isolated from myxoamoebae of Physarum polycephalum

A specific inhibitor of DNA polymerase alpha was isolated from the lipid fraction prepared from myxoamoebae of a true slime mold, Physarum polycephalum. The purified substance was subjected to structural studies by fast atom bombardment mass spectroscopy, infrared spectroscopy, and two-dimensional nuclear magnetic resonance spectroscopy. The structure of this substance was thereby suggested to be a novel lysophosphatidic acid (LPA) composed of cyclic phosphate and cyclopropane-containing hexadecanoic acid. Then we named this substance PHYLPA (Physarum LPA). PHYLPA inhibited more than 80% of the affinity-purified calf thymus DNA polymerase alpha activity at a concentration of 10 micrograms/ml (approximately 20 microM). Inhibition was observed for DNA polymerase alpha but not for DNA polymerase beta or gamma from various eukaryotic species, nor did it inhibit DNA polymerase I from E. coli. From kinetic analyses, the inhibition was considered to be caused by the interaction of PHYLPA with the template DNA.

Expression of a 66-kD heat shock protein associated with the process of cyst formation of a true slime mold, Physarum polycephalum

Under unfavorable conditions for growth, haploid myxoamoebae of Physarum polycephalum retracted their pseudopodia and changed their cell shape into disk-like form, after which they constructed the cell walls to form microcysts. These morphological changes of haploid cells were associated with changes in intracellular distribution of actin filaments. Staining with phalloidin showed that actin filaments were almost uniformly distributed throughout the cytoplasm of the myxoamoebae. When these cells were transferred to a cyst-inducing medium, the actin structures changed into short rods or dots, after which the rods/dots disappeared in the microcysts. An incubation of the myxoamoebae in the cyst-inducing medium caused the synthesis of several proteins, among which a 66-kD protein was most prominently induced. The morphological changes and the induction of the 66-kD protein was pronounced at elevated temperatures, e.g. 40 degrees C. The 66-kD protein was not induced, however, when plasmodia of the same species were incubated at 40 degrees C. We found that the 66-kD protein was co-precipitated with polymerized actin and bound to ATP-agarose. A double staining of the disk-shaped cells with anti-66-kD protein antibody and phalloidin revealed superimposable localization of the 66-kD protein and actin filaments in the short rods or dots. Although the induction of the 66-kD protein was enhanced at high temperatures, the protein was immunologically unrelated to the common heat shock proteins, HSP70 and HSP90, those are highly conserved during evolution. These results indicate that the 66-kD protein is a novel heat shock protein which is specifically expressed during cyst formation.

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.

Autolytic transition of mu-calpain upon activation as resolved by antibodies distinguishing between the pre- and post-autolysis forms

A novel method to observe the autolytic activation of a mammalian cytoplasmic calcium protease, mu-calpain, was developed using a set of antipeptidic antibodies capable of distinguishing between the pre- and post-autolysis forms of the enzyme. Antibodies raised against synthetic peptides designed to match the N-terminal sequences of the pre- and post-autolysis forms of the mu-calpain large subunit reacted specifically with the corresponding form of calpain and not with the other. The antibodies were specific and sensitive enough to detect the antigens in crude cell lysates. The relevance of the immunochemical detection of calpain activation was confirmed by the observation that proteolysis of a substrate protein by purified mu-calpain paralleled autolysis at various pCa as probed by these antibodies and that autolysis preceded substrate proteolysis. We also observed calcium-dependent autolysis of calpain accompanying subsequent proteolysis of substrate in intact cells using the antibodies. The method will provide a novel approach to assess the physiological targets of the enzyme by determining the local intracellular sites of calpain activation.

Stimulation of DNA polymerase alpha activity by microtubule-associated proteins

Microtubule-associated protein 2 (MAP2) isolated from porcine brains stimulated the activity of DNA polymerase alpha immunopurified from calf thymus or human lymphoma cells, in a dose-dependent manner. This stimulation was pronounced when activated DNA or poly(dA).(dT)10 was used as the template-primer. DNA polymerase alpha bound to a MAP2-immobilized column, whereas preincubation of the enzyme with unbound MAP2 prevented binding to the column. These events suggested that a physical binding occurred between the polymerase and MAP2. Kinetic analyses revealed that MAP2 decreased the Km value of the polymerase for deoxyribonucleotides, irrespective of the species of template-primer. A concomitant increase in Vmax was observed; however, the extent of the increase depended on the species of template-primer. MAP2 also decreased the Km value of the polymerase for template-primers when activated DNA of poly(dA).(dT)10 was used as the template-primer. Product analyses showed that MAP2 did not significantly alter the processivity of the polymerase and the increment of Vmax is considered to be due to an increase in the frequency of initiation of DNA synthesis. The stimulation by MAP2 occurred specifically in the activity of DNA polymerase alpha, but not DNA polymerases beta, gamma, and I from Escherichia coli. Other MAPs, tau and 190-kDa MAP, could substitute for MAP2. Thus, the specific stimulation of DNA polymerase alpha by MAPs supports the notion of a possible involvement of MAPs or MAP-like proteins in DNA replication, in vivo.

Microtubule destabilization by cdc2/H1 histone kinase: phosphorylation of a "pro-rich region" in the microtubule-binding domain of MAP-4

Microtubule-associated protein-4 (MAP-4), a major MAP in proliferating cells, consists of a microtubule-binding domain and a projection domain protruding from the microtubule wall. The former contains a Pro-rich region and an assembly-promoting (AP) sequence region which is common to the neuron-specific MAPs, MAP-2 and tau1. In this paper, we describe the phosphorylation of the Pro-rich region of MAP-4 and the suppression of its assembly-promoting activity by cdc2/H1 histone kinase. This inactivation of MAP-4 may cause disassembly of the interphase microtubular network at the end of the G2 phase of the cell cycle.

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.

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.

Purification and characterization of a novel intracellular acid proteinase from the plasmodia of a true slime mold, Physarum polycephalum

An acid proteinase was purified to apparent homogeneity from the plasmodia of a slime mold, Physarum polycephalum, by a combination of detergent extraction, acid precipitation, and column chromatographies on DEAE-Sephadex, hydroxylapatite, CM-Sephadex, and Sephadex G-100. The enzyme was shown to be composed of two polypeptide chains (a 31-kDa heavy chain and a 23-kDa light chain) cross-linked by disulfide bond(s). The NH2-terminal amino acid sequence of the heavy chain was determined to be Ala-Gly-Val- Asp-Gly-Tyr-Ile-Val-Pro-Tyr-Val-Ile-Phe-Asp-Leu-Tyr-Gly-Ile-Pro-Tyr and that of the light chain to be Ala-Glu-Pro-Pro-Ile. The heavy chain contained carbohydrate moiety composed of mannose, glucosamine, fucose, and glucose. The enzyme was optimally active at pH 1.7 toward hemoglobin as a substrate. Among the proteinase inhibitors tested only diazoacetyl-D,L-norleucine methyl ester, a typical aspartic proteinase inhibitor, inhibited the acid proteinase in the presence of cupric ions. It was insensitive to the other typical aspartic proteinase inhibitors, pepstatin A and 1,2-epoxy-3-(p-nitrophenoxy)propane. The enzyme hydrolyzed Lys-Pro-Ile-Glu-Phe(4-NO2)-Arg-Leu at the Phe-Phe(4-NO2) bond, but could not hydrolyze another synthetic pepsin-substrate, N-acetyl-L-phenylalanyl-3,5-diiodo-L-tyrosine. The enzyme showed a unique substrate specificity toward oxidized insulin B chain. The major cleavage sites were the bonds Gly8-Ser9, Leu11-Val12, Cya19-Gly20, and Phe24-Phe25, and the Gly8-Ser9 bond was most susceptible. These results indicate that the enzyme is a novel type of intracellular acid proteinase with a unique substrate specificity.

Interaction mechanism between microtubule-associated proteins and microtubules. A proton nuclear magnetic resonance analysis on the binding of synthetic peptide to tubulin

An amino acid sequence essential for microtubule-associated proteins (MAPs) to bind to microtubules is presented [Aizawa et al. (1989) J. Biol. Chem. 264, 5885-5890]. A synthetic peptide of 23 amino acid residues which corresponded to the sequence [tubulin binding peptide (TBP)] was active in binding to tubulin and inducing its assembly. The TBP-tubulin interaction mechanism was analyzed by proton nuclear magnetic resonance spectroscopy as a simplified model for MAP-microtubule interactions. Intraresidue transferred nuclear Overhauser effects (TRNOEs) of TBP in TBP-tubulin mixtures were analyzed, and strong binding of two Val and two Lys residues of TBP to tubulin was detected. Among the sharply peaked signals from tubulin aromatic residues, those due to Tyr ring protons broadened upon mixing with TBP, suggesting the involvement of Tyr residue(s) in the binding with TBP. Irradiation of the tubulin Tyr protons resulted in an intermolecular TRNOE at TBP methyl proton resonances. Evidently, hydrophobic interactions between Val and Tyr residues are important for the binding of TBP to tubulin. Hydrophobic interactions have not been taken into account previously in the widely accepted electrostatic model for the binding of MAPs to microtubules.

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.

The mitotic apparatus-associated 51-kDa protein from sea urchin eggs is a GTP-binding protein and is immunologically related to yeast polypeptide elongation factor 1 alpha

We investigated the biochemical characteristics of the 51-kDa protein that is a major mitotic apparatus-associated basic protein of sea urchin eggs (Toriyama, M., Ohta, K., Endo, S., and Sakai, H. (1988) Cell Motil. Cytoskeleton 9, 117-128). The amino acid composition of the 51-kDa protein was apparently different from those of tubulin, actin, histones, and myelin basic protein; yet it was similar to those of polypeptide elongation factors 1 alpha (EF-1 alpha). In addition, antibody to EF-1 alpha from yeast cross-reacted with the 51-kDa protein. [3H] GTP binding activity was detected in the phosphocellulose-purified fraction (PC fraction) which predominantly contained the 51-kDa protein and was shown to be specific to GTP, GDP, guanylyl imidodiphosphate, and ITP. Photo-affinity labeling using [alpha-32P]8-azidoguanosine triphosphate (8-azido-GTP) demonstrated that a 51-kDa polypeptide in the PC fraction specifically bound 8-azido-GTP. This GTP-binding polypeptide was bound to a GTP affinity column, could be eluted by the addition of GTP, and was immunoreactive with anti-51-kDa protein antibodies. When the PC fraction was applied to a gel filtration chromatography column, GTP binding activity was completely coeluted with the 51-kDa protein. Furthermore, the PC fraction and the gel filtration-purified fraction had EF-1 alpha activity: [14C]Phe-tRNA transferring activity to ribosomes in the presence of poly(U) and ribosome-dependent GTPase activity. The results indicate that the mitotic apparatus-associated 51-kDa protein is a GTP-binding protein and suggest that it is structurally and functionally related to yeast EF-1 alpha.

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.

Microtubule-associated protein-2 stimulates DNA synthesis catalyzed by the nuclear matrix

Microtubule-associated protein-2 (MAP-2) isolated from porcine brains stimulated DNA synthesis catalyzed by the nuclear matrix isolated from Physarum polycephalum in the presence of activated DNA as exogenous templates. The degree of the stimulation depended on the amount of the nuclear matrix, but not on that of the template. MAP-2 also stimulated DNA polymerase alpha activity solubilized from nuclei, but not DNA polymerase beta activity. These results suggest that MAP-2 stimulates DNA synthesis by interacting with the putative DNA replication machinery including DNA polymerase alpha bound to the matrix. Similar stimulation occurred in the nuclear matrix isolated from HeLa and rat ascites hepatoma cells, which strongly suggests that MAP-2 is involved in the control of DNA replication in eukaryotic cells.

Immunohistochemical localization of microtubule-associated proteins in the nervous system of the small intestine of guinea pig

Layers containing Auerbach's and Meissner's plexuses were dissected from the small intestine of guinea pig and immunostained with affinity-purified antibodies against brain-specific microtubule-associated proteins (MAPs): MAP1, MAP2 and tau and a MAP with a molecular weight of 190,000 dalton purified from bovine adrenal cortex (190-kDa MAP). MAP1 antibody stained the network of nerve fibers and the cell bodies of enteric neurons in both Auerbach's and Meissner's plexuses. Staining with anti-tau antibody gave the same results. Antibody against MAP2 stained neuronal cell bodies and short thin processes extending from them. Interganglionic strands composed mainly of long processes were unstained. Anti-190-kDa MAP antibody stained both the neuronal cell bodies and bundles of nerve fibers. However, the staining was less intense than that with anti-MAP1 and tau antibodies. Differentiation in the structure of the cytoskeleton probably exists in the neuronal processes of the enteric neurons as is shown in the dendrites and axons in some neurons of the central nervous system. Thus, enteric neurons possess axon-like processes containing MAP1, tau and probably lower amounts of 190-kDa MAP. Cell bodies and dendrite-like structures of these neurons contain MAP2 in addition to MAP1, tau and 190-kDa MAP.

The molecular structure of adrenal medulla kinesin

The molecular structure of bovine adrenal kinesin was studied by electron microscopy using the low-angle rotary shadowing technique. Adrenal kinesin exhibited either a folded or an extended configuration; the ratio of the two is dependent on the salt concentration. Almost all adrenal kinesin molecules were folded in a low-ionic solution, and the ratio of extended molecules increased to 40-50% in a solution containing 1 M ammonium acetate. Kinesin in the extended configuration displayed a rod-shaped structure with a mean length of about 80 nm. The morphologies of the ends were different; one end was composed of two globular particles, similar to the two-headed structure of myosin, while the other end had a more ill-defined structure, appearing either as a globular particle, an aggregate of two to four small granules, or a frayed, fan-like structure. The folded kinesin molecule possessed a hinge region in the middle of the rod, at about 32 nm from the neck of the two heads. In our preparations, the majority of adrenal kinesin molecules were folded at physiological salt concentrations. Adrenal kinesin bound to microtubules in the presence of adenylyl imidodiphosphate (AMP-PNP) also displayed a folded morphology.