A protein factor essential for microtubule assembly.

A heat stable protein essentail for microtubule assembly has been isolated. This protein, which we designate tau (tau), is present in association with tubulin purified from porcine brain by repeated cycles of polymerization. Tau is separated from tubulin by ion exchange chromatography on phosphocellulose. In the absence of tau, tubulin exists entirely as a 6S dimer of two polypeptide chains (alpha and beta tubulin) with a molecular weight of 120,000, which will not assemble into microtubules in vitro. Addition of tau completely restores tubule-forming capacity. Under nonpolymerizing conditions, tau converts 6S dimers to 36S rings-structures which have been implicated as intermediates in tubule formation. Hence, tau appears to act on the 6S tubulin dimer, activating it for polymerization. The unique ability of tau to restore the normal features of in vitro microtubule assembly makes it likely that tau is a major regulator of microtubule formation in cells.     

Complementary dimerization of microtubule-associated tau protein: Implications for microtubule bundling and tau-mediated pathogenesis

Tau is an intrinsically unstructured microtubule (MT)-associated protein capable of binding to and organizing MTs into evenly spaced parallel assemblies known as "MT bundles." How tau achieves MT bundling is enigmatic because each tau molecule possesses only one MT-binding region. To dissect this complex behavior, we have used a surface forces apparatus to measure the interaction forces of the six CNS tau isoforms when bound to mica substrates in vitro. Two types of measurements were performed for each isoform: symmetric configuration experiments measured the interactions between two tau-coated mica surfaces, whereas "asymmetric" experiments examined tau-coated surfaces interacting with a smooth bare mica surface. Depending on the configuration (of which there were 12), the forces were weakly adhesive, strongly adhesive, or purely repulsive. The equilibrium spacing was determined mainly by the length of the tau projection domain, in contrast to the adhesion force/energy, which was determined by the number of repeats in the MT-binding region. Taken together, the data are incompatible with tau acting as a monomer; rather, they indicate that two tau molecules associate in an antiparallel configuration held together by an electrostatic "zipper" of complementary salt bridges composed of the N-terminal and central regions of each tau monomer, with the C-terminal MT-binding regions extending outward from each end of the dimeric backbone. This tau dimer determines the length and strength of the linker holding two MTs together and could be the fundamental structural unit of tau, underlying both its normal and pathological action.     

The fragile X protein controls microtubule-associated protein 1B translation and microtubule stability in brain neuron development

The fragile X mental retardation protein (FMRP) is a selective RNA-binding protein implicated in regulating translation of its mRNA ligands. The absence of FMRP results in fragile X syndrome, one of the leading causes of inherited mental retardation. Delayed dendritic spine maturation was found in fragile X mental retardation patients as well as in Fmr1 knockout (KO) mice, indicating the functional requirement of FMRP in synaptic development. However, the biochemical link between FMRP deficiency and the neuronal impairment during brain development has not been defined. How FMRP governs normal synapse development in the brain remains elusive. We report here that the developmentally programmed FMRP expression represses the translation of microtubule associated protein 1B (MAP1B) and is required for the accelerated decline of MAP1B during active synaptogenesis in neonatal brain development. The lack of FMRP results in misregulated MAP1B translation and delayed MAP1B decline in the Fmr1 KO brain. Furthermore, the aberrantly elevated MAP1B protein expression leads to abnormally increased microtubule stability in Fmr1 KO neurons. Together, these results indicate that FMRP plays critical roles in controlling cytoskeleton organization during neuronal development, and the abnormal microtubule dynamics is a conceivable underlying factor for the pathogenesis of fragile X mental retardation.     

Angiotensin II stimulates two myelin basic protein/microtubule-associated protein 2 kinases in cultured vascular smooth muscle cells.

In cultured vascular smooth muscle cells, angiotensin II (Ang II) stimulated a cytosolic protein kinase activity toward myelin basic protein (MBP) in a time- and dose-dependent manner. Phorbol 12-myristate 13-acetate (PMA) and phorbol 12,13-dibutyrate also increased the MBP kinase activity. Downregulation of protein kinase C by prolonged treatment of the cells with phorbol 12,13-dibutyrate markedly attenuated the Ang II- and PMA-induced MBP kinase activation. The Ang II- and PMA-stimulated MBP kinase activities were resolved almost equally into two distinct fractions on Mono-Q HR5/5 column chromatography (kinase 1 and kinase 2). The kinase assay in polyacrylamide gel revealed that apparent molecular masses of kinase 1 and kinase 2 were 40 and 45 kd, respectively. Microtubule-associated protein 2 also served as a substrate for both the kinases. Immunoblot analysis with an antiphosphotyrosine antibody suggested that both the kinases were tyrosine-phosphorylated during the action of Ang II. Phosphoamino acid analysis revealed that Ang II and PMA induced phosphorylation of both the kinases on serine/threonine as well as tyrosine residues. Phosphopeptide mapping patterns of kinase 1 and kinase 2 isolated from Ang II-stimulated cells were almost identical with those from PMA-stimulated cells. These results indicate that in vascular smooth muscle cells Ang II activates two species of MBP/microtubule-associated protein 2 kinases mainly through the protein kinase C-signaling pathway and suggest that tyrosine and serine/threonine phosphorylation may be involved in this process.     

ASAP, a human microtubule-associated protein required for bipolar spindle assembly and cytokinesis

We have identified a unique human microtubule-associated protein (MAP) named ASAP for ASter-Associated Protein. ASAP localizes to microtubules in interphase, associates with the mitotic spindle during mitosis, localizes to the central body during cytokinesis and directly binds to purified microtubules by its COOH-terminal domain. Overexpression of ASAP induces profound bundling of cytoplasmic microtubules in interphase cells and aberrant monopolar spindles in mitosis. Depletion of ASAP by RNA interference results in severe mitotic defects: it provokes aberrant mitotic spindle, delays mitotic progression, and leads to defective cytokinesis or cell death. These results suggest a crucial role for ASAP in the organization of the bipolar mitotic spindle, mitosis progression, and cytokinesis and define ASAP as a key factor for proper spindle assembly.     

The heat shock protein Gp96 binds to human neutrophils and monocytes and stimulates effector functions

The endoplasmic reticulum (ER)-resident heat shock protein Gp96 is involved in protein folding and is released into the extracellular space after necrotic cell death. In this context, Gp96 has immunostimulatory properties: it activates dendritic cells or macrophages and delivers associated peptides into the antigen presentation pathway, resulting in the induction of specific T-cell responses. The inflammatory response after necrotic tissue damage leads to the recruitment of polymorphonuclear neutrophils (PMNs) and monocytes, allowing them to make their first encounter with Gp96. We therefore investigated whether PMNs and monocytes interact with Gp96. We were able to show that PMNs and monocytes specifically bind fluorescein isothiocyanate (FITC)-conjugated Gp96. The binding of Gp96-FITC was competed by lipopolysaccharide (LPS) or fucoidan, a known inhibitor of scavenger receptors. Interestingly, the binding of LPS-FITC was also competed not only by fucoidan, but by Gp96, suggesting that LPS and Gp96 share a common receptor on PMNs. One important effector function of PMNs is the clearance of an inflammatory site by phagocytosis. We therefore assessed the influence of Gp96 on phagocytic activity using fluorochrome-labeled polystyrene beads. We found a marked enhancement of phagocytosis in the presence of Gp96 and concluded that PMNs not only bind Gp96, but are also activated by it. Additionally, Gp96-stimulated PMNs and especially monocytes release large amounts of interleukin-8, a potent neutrophil-attracting chemokine. In conclusion, we demonstrate that Gp96 specifically binds to and activates PMNs and monocytes, extending the function of Gp96 as a danger signal to additional members of the innate immune system.     

Structure and phosphorylation of microtubule-associated protein 2 (MAP 2).

Chymotryptic fragments of microtubule-associated protein 2 (MAP 2) containing the portion of the molecule responsible for promoting microtubule assembly were identified. These assembly-promoting fragments displaced intact MAP 2, but not MAP 1, from assembled microtubules. This indicates that the association of MAP 2 with the microtubule surface is reversible. Both the assembly-promoting fragments and fragments representing the portion of the MAP 2 molecule observed as a projection on the microtubule surface were found to contain sites for endogenous cyclic AMP-dependent phosphorylation. The projection fragments were capable of endogenous phosphorylation even after their physical separation from microtubules. This suggests an intimate association of a kinase activity with the projections. Detailed analysis of the properties of the chymotryptic fragments of MAP 2 has led to a map of the molecule showing the major sites of proteolytic attack and the sites of phosphorylation.     

Antibodies to microtubule-associated protein 2 in patients with neuropsychiatric systemic lupus erythematosus

OBJECTIVE: Microtubule-associated protein 2 (MAP-2), a cellular protein restricted to neurons, is important in the control of cytoskeletal integrity and other neuronal functions. We undertook this study to examine the presence of autoantibodies to MAP-2 in neuropsychiatric systemic lupus erythematosus (NPSLE). METHODS: Sera from 100 patients with SLE, 74 patients with other neurologic disorders and injuries (including cerebrovascular accidents, brain trauma, brain tumors, and demyelinating disorders), and 60 normal controls were examined both by enzyme immunoassays and by Western immunoblotting for autoantibodies to MAP-2. Sera designated positive for antibodies to MAP-2 were required to be positive in both assays. RESULTS: Seventeen percent of SLE patients had autoantibodies to MAP-2, in contrast to 4% of neurologic injury/disease control patients (P = 0.028) and 1.7% of normal controls. In SLE, anti-MAP-2 positivity in both assays was associated with neuropsychiatric symptoms in 76.5% of patients, whereas the absence of anti-MAP-2 was associated with neuropsychiatric symptoms in 19.7% of patients (P = 0.0002). The neuropsychiatric symptoms in the former group included psychosis, seizure, neuropathy, and cerebritis. CONCLUSION: Autoantibodies to MAP-2, a neuron-restricted cytoskeletal protein, appear to be another immune marker for NPSLE.     

Expression of microtubule-associated protein 2 by reactive astrocytes.

After an injury to the central nervous system, a dramatic change in the astrocytes bordering the wound occurs. The most characteristic feature of this process, termed reactive gliosis, is the upregulation of the intermediate filament protein, glial fibrillary acidic protein. In the present study, we show that reactive astrocytes express high levels of microtubule-associated protein 2 (MAP-2), a protein normally found in the somatodendritic compartment of neurons. When sections of injured brain are double-stained with antibodies directed against MAP-2 and glial fibrillary protein, all of the reactive astrocytes are found to contain MAP-2. The high levels of this protein appear to represent a permanent change in reactive astrocytes. In parallel quantitative studies, an elevated level of MAP-2 in the injured brain is confirmed by an immunoblot analysis of injured and normal white matter. This report demonstrates the direct involvement of a microtubule protein in the process of reactive gliosis.     

Nucleation and the kinetics of microtubule assembly.

A kinetic model was developed for the purpose of interpreting scanning calorimetric data of microtubule assembly. The model consists of two steps. The first step is a highly exothermic nucleation with a strong dependence upon temperature and concentration. The second step is the elongation of the nuclei or microtubules by the addition of tubulin dimer to growing ends. Computer fits to the data provided the values of the parameters of the model. The model successfully simulated various experiments.     

Heterogeneity of microtubule-associated protein 2 during rat brain development.

The electrophoretic pattern of the large microtubule-associated protein, MAP2, changes during rat brain development. Immunoblots of NaDodSO4 extracts obtained from the cerebral cortex, cerebellum, and thalamus at 10-15 days after birth reveal only a single electrophoretic species when probed with any of three MAP2 monoclonal antibodies. By contrast, adult MAP2 contains two immunoreactive species, MAP2a and MAP2b. The single band of MAP2 from immature brain electrophoretically comigrates with adult MAP2b. Between postnatal days 17 and 18, immature MAP2 simultaneously resolves into two species in both the cerebellum and cerebral cortex. Immunoblots of NaDodSO4 extracts from spinal cord demonstrate the adult complement of MAP2 by day 10, indicating that MAP2 does not change coordinately throughout the entire central nervous system. In vitro cAMP-dependent phosphorylation of immature MAP2 causes a band split reminiscent of that seen during brain development in vivo. The possibility that the developmentally regulated changes observed in MAP2 during brain maturation are due to timed phosphorylation events is discussed.     

Psb27, a transiently associated protein, binds to the chlorophyll binding protein CP43 in photosystem II assembly intermediates

Photosystem II (PSII), a large multisubunit pigment–protein complex localized in the thylakoid membrane of cyanobacteria and chloroplasts, mediates light-driven evolution of oxygen from water. Recently, a high-resolution X-ray structure of the mature PSII complex has become available. Two PSII polypeptides, D1 and CP43, provide many of the ligands to an inorganic Mn₄Ca center that is essential for water oxidation. Because of its unusual redox chemistry, PSII often undergoes degradation followed by stepwise assembly. Psb27, a small luminal polypeptide, functions as an important accessory factor in this elaborate assembly pathway. However, the structural location of Psb27 within PSII assembly intermediates has remained elusive. Here we report that Psb27 binds to CP43 in such assembly intermediates. We treated purified genetically tagged PSII assembly intermediate complexes from the cyanobacterium Synechocystis 6803 with chemical cross-linkers to examine intermolecular interactions between Psb27 and various PSII proteins. First, the water-soluble 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) was used to cross-link proteins with complementary charged groups in close association to one another. In the His27△ctpAPSII preparation, a 58-kDa cross-linked species containing Psb27 and CP43 was identified. This species was not formed in the HT3△ctpA△psb27PSII complex in which Psb27 was absent. Second, the homobifunctional thiol-cleavable cross-linker 3,3′-dithiobis(sulfosuccinimidylpropionate) (DTSSP) was used to reversibly cross-link Psb27 to CP43 in His27△ctpAPSII preparations, which allowed the use of liquid chromatography/tandem MS to map the cross-linking sites as Psb27K⁶³↔CP43D³²¹ (trypsin) and CP43K²¹⁵↔Psb27D⁵⁸AGGLK⁶³↔CP43D³²¹ (chymotrypsin), respectively. Our data suggest that Psb27 acts as an important regulatory protein during PSII assembly through specific interactions with the luminal domain of CP43.     

Vascular endothelial growth factor binds to fibrinogen and fibrin and stimulates endothelial cell proliferation

Vascular development and response to injury are regulated by several cytokines and growth factors including the members of the fibroblast growth factor and vascular endothelial cell growth factor (VEGF) families. Fibrinogen and fibrin are also important in these processes and affect many endothelial cell properties. Possible specific interactions between VEGF and fibrinogen that could play a role in coordinating vascular responses to injury are investigated. Binding studies using the 165 amino acid form of VEGF immobilized on Sepharose beads and soluble iodine 125 ((125)I)-labeled fibrinogen demonstrated saturable and specific binding. Scatchard analysis indicated 2 classes of binding sites with dissociation constants (K(d)s) of 5.9 and 462 nmol/L. The maximum molar binding ratio of VEGF:fibrinogen was 3.8:1. Further studies characterized binding to fibrin using (125)I-labeled VEGF- and Sepharose-immobilized fibrin monomer. These also demonstrated specific and saturable binding with 2 classes of sites having K(d)s of 0.13 and 97 nmol/L and a molar binding ratio of 3.6:1. Binding to polymerized fibrin demonstrated one binding site with a K(d) of 9.3 nmol/L. Binding of VEGF to fibrin(ogen) was independent of FGF-2, indicating that there are distinct binding sites for each angiogenic peptide. VEGF bound to soluble fibrinogen in medium and to surface immobilized fibrinogen or fibrin retained its capacity to support endothelial cell proliferation. VEGF binds specifically and saturably to fibrinogen and fibrin with high affinity, and this may affect the localization and activity of VEGF at sites of tissue injury. (Blood. 2000;96:3772-3778)     

Alpha 2-macroglobulin binds and inhibits activated protein C.

In previous studies using a nonhuman primate model of Protein C (PC) activation in vivo, immunoblotting showed substantial amounts of activated PC (APC) in a high molecular weight complex with what was presumed to be a previously unrecognized APC binding protein. This APC complex can also be formed in citrated plasma in vitro. It is of low electrophoretic mobility, sodium dodecyl sulfate (SDS) stable, with an apparent Mr of 320 Kd. Its purification from human plasma was accomplished using barium citrate adsorption, sequential polyethylene glycol (PEG) precipitations, diethylaminoethyl sepharose chromatography, AcA-34 gel filtration, and zinc-chelate affinity chromatography. This was monitored by subjecting the fractions to nondenaturing polyacrylamide gel electrophoresis (PAGE), transfer to polyvinylidene-difluoride membranes, and probing with 125I-labeled human APC. The purified APC-binding protein was homogeneous by SDS-PAGE with an Mr of 275 Kd. Its identity as alpha 2-macroglobulin (alpha 2M) was demonstrated immunochemically. Complex formation between alpha 2M and APC was found to be almost completely inhibited by EDTA, but to a lesser extent by citrate. Complex formation could also be prevented by active site inhibition with D-Phenylalanyl-L-Prolyl-L-Arginine-Chloromethyl Ketone (PPACK) or pretreatment of alpha 2M with methylamine. Incubation of APC (33 nmol/L) with alpha 2M (1 mumol/L) resulted in time-dependent inhibition of APC anticoagulant activity when measured using an activated partial thromboplastin time based APC assay. These data show that alpha 2M binds and inhibits APC in vitro and the interaction is both metal-ion and active-site dependent, requiring functionally intact alpha 2M. As the complexes formed in vitro comigrate electrophoretically with those observed in vivo after PC activation, it is suggested that alpha 2M is a physiologically relevant inhibitor involved in the processing of APC in vivo.     

Immunocytochemical localization of actin and microtubule-associated protein MAP2 in dendritic spines.

To determine whether dendritic spines contain actin, we evaluated the immunocytochemical localization of actin in the hippocampal formation and cerebral cortex of the rat. Monoclonal hybridoma antibodies were prepared against adult quail breast muscle actin. The culture supernatant of two cell lines (QAB1 and QAB2) was examined. Both antibodies bound only actin in crude brain homogenates, and neither exhibited species specificity. Electron microscopic analyses of sections reacted with QAB1 revealed staining of postsynaptic densities and dendritic microtubules but little staining of the cytoplasmic compartment of spines. However, sections reacted with QAB2 exhibited staining at the cytoplasmic compartment of spines as well as the sites stained by QAB1. We also evaluated the immunocytochemical distribution of beta-tubulin and high molecular weight microtubule-associated protein (MAP2) utilizing monoclonal antibodies. MAP2 was found in the dendritic spine as well as in the parent dendrite. However, beta-tubulin was found only in the postsynaptic density and in the microtubules of the parent dendrite. The combined results indicate that actin is present in the spine along with MAP2 and that there is a difference in the actin (or the state of actin) in the spine in comparison with other neuronal compartments.     

Role of GTP hydrolysis in microtubule treadmilling and assembly.

GTP hydrolysis accompanies addition of tubulin to microtubules. I find that hydrolysis is a requirement for the opposite-end assembly/disassembly of microtubules and consequent subunit treadmilling from one end to the other of the polymer. Neither GDP nor guanosine 5'-[beta, gamma-imido]triphosphate allows or participates in the treadmilling reaction. Therefore, there is a requirement for hydrolysis in the addition of subunits to the favored assembly end of the microtubule. Podophyllotoxin, an assembly inhibitory drug, "caps" the microtubule assembly end, preventing subunit loss from that site to equilibrium. Continued hydrolysis of GTP is required to maintain the podophyllotoxin cap. A corollary of this finding is that GTP hydrolysis is required for cap formation. Microtubules assembled in GTP enter a metastable state when all remaining GTP is hydrolyzed. This state is characterized by its ability to maintain indefinitely a subunit/polymer distribution ratio that is arbitrary and that can be altered at will by brief chilling or by addition of small amounts of GTP. This metastable state is labile to podophyllotoxin. Use of podophyllotoxin allows measurement of the microtubule treadmilling rate; use of podophyllotoxin in the absence of GTP allows measurement of the overall rate of dimer dissociation from the microtubule. Measurement of these rates has permitted determination of the efficiency with which adding dimers incorporate into the microtubule treadmill and are not lost to assembly end equilibrium. The efficiency varies with GTP concentration for unknown reasons, being high at 0.1 mM GTP and low at higher GTP concentrations.