The microtubule-binding fragment of microtubule-associated protein-2: location of the protease-accessible site and identification of an assembly-promoting peptide

Microtubule elongation along actin filaments induced by microtubule-associated protein 4 contributes to the formation of cellular protrusions

Actin-microtubule crosstalk is implicated in the formation of cellular protrusions, but the mechanism remains unclear. In this study, we examined the regulation of cell protrusion involving a ubiquitously expressed microtubule-associated protein (MAP) 4, and its superfamily proteins, neuronal MAP2 and tau. Fluorescence microscopy revealed that these MAPs bound to F-actin and microtubules simultaneously, and formed F-actin/microtubule hybrid bundles. The hybrid bundle-forming activity was in the order of MAP2 > MAP4 ≫ tau. Interestingly, the microtubule assembly-promoting activity of MAP4 and MAP2, but not of tau, was upregulated by their interaction with F-actin. When MAP4 was overexpressed in NG108-15 cells, the number of cell processes and maximum process length of each cell increased significantly by 28% and 30%, respectively. Super-resolution microscopy revealed that 95% of microtubules in cell processes colocalized with F-actin, and MAP4 was always found in their vicinity. These results suggest that microtubule elongation along F-actin induced by MAP4 contributes to the formation of cellular protrusions. Since MAP4, MAP2 and tau had different crosstalk activity between F-actin and microtubules, it is likely that the functional differentiation of these MAPs is a driving force for neural evolution, causing significant changes in cell morphology.

Structural insight into microtubule stabilization and kinesin inhibition by Tau family MAPs

The Tau family of microtubule-associated proteins promote microtubule stabilization or regulate microtubule-based motility. Shigematsu et al. visualized MAP4 and microtubules complexed with kinesin-1 by cryo-EM, which suggests a structural basis of microtubule stabilization and kinesin inhibition by Tau family MAPs.

The Tau family microtubule-associated proteins (MAPs) promote microtubule stabilization and regulate microtubule-based motility. They share the C-terminal microtubule-binding domain, which includes three to five tubulin-binding repeats. Different numbers of repeats formed by alternative splicing have distinct effects on the activities of these proteins, and the distribution of these variants regulates fundamental physiological phenomena in cells. In this study, using cryo-EM, we visualized the MAP4 microtubule complex with the molecular motor kinesin-1. MAP4 bound to the C-terminal domains of tubulins along the protofilaments stabilizes the longitudinal contacts of the microtubule. The strongest bond of MAP4 was found around the intertubulin–dimer interface such that MAP4 coexists on the microtubule with kinesin-1 bound to the intratubulin–dimer interface as well. MAP4, consisting of five repeats, further folds and accumulates above the intertubulin–dimer interface, interfering with kinesin-1 movement. Therefore, these cryo-EM studies reveal new insight into the structural basis of microtubule stabilization and inhibition of kinesin motility by the Tau family MAPs.

A nematode microtubule-associated protein, PTL-1, closely resembles its mammalian counterparts in overall molecular architecture

The mammalian microtubule-associated proteins (MAPs), MAP2, MAP4, and τ, are structurally similar and considered to be evolutionarily related. The primary structure of a nematode MAP, PTL-1, also reportedly resembles those of the MAPs, but only in a small portion of the molecule. In this study, we elucidated the overall domain organization of PTL-1, using a molecular dissection technique. Firstly, we isolated nematode microtubules and proved that the recombinant PTL-1 binds to nematode and porcine microtubules with similar affinities. Then, the recombinant PTL-1 was genetically dissected to generate four shorter polypeptides, and their microtubule-binding and assembly promoting activities were assessed, using porcine microtubules and tubulin. PTL-1 was found to consist of two parts, microtubule-binding and projection domains, with the former further divided into three functionally distinct subdomains. The molecular architecture of PTL-1 was proved to be quite analogous to its mammalian counterparts, MAP2, MAP4, and τ, strongly supporting their evolutionary relationships.

Actoclampin (+)-end-tracking motors: How the pursuit of profilin's role(s) in actin-based motility twice led to the discovery of how cells crawl

The path to the discovery of the actoclampins began with efforts to define profilin's role in actin-based pathogen and endosome rocketing. That research identified a set of FPPPP-containing cargo proteins and FPPPP-binding proteins that are consistently stationed within the polymerization zone during episodes of active motility. The very same biophysical clues that forced us to abandon Brownian Ratchet models guided us to the Actoclampin Hypothesis, which asserts that every propulsive filament possesses a (+)-end-tracking motor that generates the forces cells need to crawl. Each actoclampin motor is a multi-arm oligomeric complex, employing one arm to recruit/deliver Profilin•Actin•ATP to a growth-site located at the (+)-end of the lagging subfilament, while a second arm maintains an affinity-modulated binding interaction with the extreme (+)-end of the other subfilament. The alternating actions of these arms define a true molecular motor, the processivity of which explains why propelling filaments maintain full possession of their cargo. The Actoclampin Hypothesis also suggests how the energetics of tracker interactions with the (+)-end determines whether a given actoclampin is a passive (low force-producing) or active (high force-producing) motor, the latter requiring the Gibbs free energy of ATP hydrolysis. Another aim of this review is to acknowledge an earlier notional model that emerged from efforts to comprehend profilin's pivotal role(s) in actin-based cell motility.

Microtubule-associated protein (MAP) 4 interacts with microtubules in an intrinsically disordered manner

We previously used nuclear magnetic resonance (NMR) to analyze the structure of a synthetic tricosapeptide corresponding to an active site of microtubule-associated protein 4 (MAP4). To further the structural analysis, we have constructed a minimal active domain fragment of MAP4, encompassing the entire active site, and obtained its NMR spectra. The secondary structure prediction using partially assigned NMR data suggested that the fragment is largely unfolded. Two other independent techniques also demonstrated its unfolded nature, indicating that MAP4 belongs to the class of intrinsically disordered proteins (IDPs). The NMR spectra of the fragment-microtubule mixture revealed that the fragment binds to the microtubule using multiple binding sites, apparently contradicting our previous quantitative studies. Given that MAP4 is intrinsically disordered, we propose a mechanism in which any one of the binding sites is active at a time, which is one of the typical interaction mechanisms proposed for IDPs.

Functional Anchoring of the cAMP-Dependent Protein Kinase

The precision of cAMP-responsive events is controlled in part through compartmentalization of the signal transduction machinery. Recent evidence suggests that the cAMP-dependent protein kinase (PKA) is localized to specific subcellular compartments through association with A Kinase Anchoring Proteins (AKAPs). The AKAPs now represent a functionally related family of regulatory proteins that contain a conserved PKA binding domain and unique targeting sequences that direct the PKA-AKAP complex to subcellular structures. In this review, the recent evidence suggesting that AKAPs facilitate PKA anchoring close to key membrane substrates, such as glutamate receptors, calcium-activated potassium channels, and skeletal or cardiac muscle calcium channels, is surveyed.

The protein phosphatase PP2A/Bα binds to the microtubule-associated proteins Tau and MAP2 at a motif also recognized by the kinase Fyn: implications for tauopathies

The predominant brain microtubule-associated proteins MAP2 and tau play a critical role in microtubule cytoskeletal organization and function. We have previously reported that PP2A/Bα, a major protein phosphatase 2A (PP2A) holoenzyme, binds to and dephosphorylates tau, and regulates microtubule stability. Here, we provide evidence that MAP2 co-purifies with and is dephosphorylated by endogenous PP2A/Bα in bovine gray matter. It co-localizes with PP2A/Bα in immature and mature human neuronal cell bodies. PP2A co-immunoprecipitates with and directly interacts with MAP2. Using in vitro binding assays, we show that PP2A/Bα binds to MAP2c isoforms through a region encompassing the microtubule-binding domain and upstream proline-rich region. Tau and MAP2 compete for binding to and dephosphorylation by PP2A/Bα. Remarkably, the protein-tyrosine kinase Fyn, which binds to the proline-rich RTPPKSP motif conserved in both MAP2 and tau, inhibits the interaction of PP2A/Bα with either tau or MAP2c. The corresponding synthetic RTPPKSP peptide, but not the phosphorylated RpTPPKSP version, competes with Tau and MAP2c for binding to PP2A/Bα. Significantly, down-regulation of PP2A/Bα and deregulation of Fyn-Tau protein interactions have been linked to enhanced tau phosphorylation in Alzheimer disease. Together, our results suggest that PP2A/Bα is part of segregated MAP2 and tau signaling scaffolds that can coordinate the action of key kinases and phosphatases involved in modulating neuronal plasticity. Deregulation of these compartmentalized multifunctional protein complexes is likely to contribute to tau deregulation, microtubule disruption, and altered signaling in tauopathies.

RGS2 promotes formation of neurites by stimulating microtubule polymerization

Regulator of G-protein signaling (RGS) proteins interact with alpha subunits of heterotrimeric G-proteins via the RGS domain and attenuate their activity by accelerating GTPase activity. RGS2, a member of the RGS family, regulates synaptic development via hereto unknown mechanism. In this study, we found that RGS2 directly interacted with tubulin via a short region at the N-terminus: amino acids 41-60. RGS2 enhanced microtubule polymerization in vitro, and the tubulin binding region was necessary and sufficient for this activity. In Vero cells, polymerization of microtubule was stimulated when peptides containing the tubulin binding region were microinjected. Immunocytochemical analysis showed that endogenous RGS2 was localized at the termini of neurites in differentiated PC12 cells. Over-expression of RGS2 enhanced the nerve growth factor-induced neurite outgrowth in PC12 cells, while specific knock-down of endogenous RGS2 suppressed the neurite outgrowth. These findings demonstrate that RGS2 contributes to the neuronal cell differentiation via regulation of microtubule dynamics.

Structural insights into Alzheimer filament assembly pathways based on site-directed mutagenesis and S-glutathionylation of three-repeat neuronal Tau protein

Although Tau and MAP2 readily assemble into straight filaments (SFs), Tau's unique ability to form paired-helical filaments (PHFs) may offer clues as to why Tau's microtubule-binding region (MTBR) is the exclusive building block of the neurofibrillary tangles that accumulate during Alzheimer's disease. To learn more about the factors permitting Tau to form both SFs and PHFs, we investigated the microtubule binding, thiol oxidation, and polymerization reactions of the monomer and dimer forms of Tau and MAP2 MTBRs. This review focuses on electron microscopic evidence (1) that facilitated the identification of amino acid residues within 3-repeat Tau that promote PHF formation; and (2) provided experimental evidence for the polymerization of S-glutathionylated three-repeat Tau, a reaction that unambiguously demonstrates that disulfide-linked Tau-S-S-Tau dimer formation is not a compulsory step in filament assembly. We also consider these findings within the context of current views on the genetic and biochemical basis of Tau fibrillogenesis.

Truncation of the projection domain of MAP4 (microtubule-associated protein 4) leads to attenuation of microtubule dynamic instability

MAP4, a ubiquitous heat-stable MAP, is composed of an asymmetric structure common to the heat-stable MAPs, consisting of an N-terminal projection (PJ) domain and a C-terminal microtubule (MT)-binding (MTB) domain. Although the MTB domain has been intensively studied, the role of the PJ domain, which protrudes from MT-wall and does not bind to MTs, remains unclear. We investigated the roles of the PJ domain on the dynamic instability of MTs by dark-field microscopy using various PJ domain deletion constructs of human MAP4 (PJ1, PJ2, Na-MTB and KDM-MTB). There was no obvious difference in the dynamic instability between the wtMAP4 and any fragments at 0.1 microM, the minimum concentration required to stabilize MTs. The individual MTs stochastically altered between polymerization and depolymerization phases with similar profiles of length change as had been observed in the presence of MAP2 or tau. We also examined the effects at the increased concentrations of 0.7 microM, and found that in some cases the dynamic instability was almost entirely attenuated. The length of both the polymerization and depolymerization phases decreased and "pause-phases" were occasionally observed, especially in the case of PJ1, PJ2 or Na-MTB. No obvious change was observed in the increased concentration of wtMAP4 and KDM-MTB. Additionally, the profiles of MT length change were quite different in 0.7 microM PJ2. Relatively rapid and long depolymerization phases were sometimes observed among quite slow length changes. Perhaps, this unusual profile could be due to the uneven distribution of PJ2 along the MT lattice. These results indicate that the PJ domain of MAP4 participates in the regulation of the dynamic instability.

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.

The number of repeat sequences in microtubule-associated protein 4 affects the microtubule surface properties

The microtubule-binding domain of MAP4, a ubiquitous microtubule-associated protein, contains a Repeat region with tandemly organized repeat sequences. In this study, we focused on the variations of the Repeat region, and searched for MAP4 isoforms with diverse Repeat region organizations. We successfully isolated four types of MAP4 cDNAs, which differed from each other in both the number and the arrangement of the repeat sequences, from a single source (bovine adrenal gland). To examine the functional differences among the isoforms, we prepared the microtubule-binding domain polypeptides of three of the four isoforms, and examined their activities. The isoform fragments showed similar degrees of microtubule assembly promoting activity and microtubule binding affinity. This result suggested that the Repeat region variation is not important for the control of microtubule dynamics, which is believed to be the main function of MAPs. On the other hand, the microtubule bundle-forming activity differed among the isoform fragments. The bundle formation was augmented by increasing the number of repeat sequences in the fragments. Based on these results, we propose the hypothesis that the role of the MAP4 isoforms is to regulate the surface charge of microtubules.

In vitro assembly of Alzheimer-like filaments. How a small cluster of charged residues in Tau and MAP2 controls filament morphology

Although the microtubule-binding regions (MTBRs) of both Tau and MAP2 can undergo self-assembly into straight filaments (SFs) in vitro, only the Tau MTBR forms paired helical filaments (PHFs). Moreover, Tau appears to be the exclusive building block of the neuropathic filaments observed in Alzheimer's disease and certain frontotemporal dementias (FTDs). Despite significant conservation in the MTBR sequences, there are two persistently different stretches of amino acids (designated here as Module-A and Module-B) between Tau and MAP2 from a number of organisms. To evaluate the role of charged residues in these modules as potential morphology-specifying elements, we used site-directed mutagenesis to replace selected residues within the MAP2 MTBR by residues at corresponding positions in Tau. We then employed electron microscopy to determine the frequency of occurrence of SF and PHF morphology in filaments assembled from these mutant microtubule-binding regions. Our experimental results indicate that a very small number of residues are especially significant determinants of filament morphology; this inference is also supported by the observation that site-directed substitutions of individual Tau residues into MAP2 Module-B likewise result in the formation of PHF-like structures. Because the Module-B in Tau contains two naturally occurring FTD mutations, residues in this region may play a critical role in neuropathic filament assembly.

The projection domain of MAP4 suppresses the microtubule-bundling activity of the microtubule-binding domain

Microtubule-associated protein 4 (MAP4), a major MAP expressed in proliferating non-neuronal cells, consists of an N-terminal projection (PJ) domain and a C-terminal microtubule-binding (MTB) domain. The PJ domain of MAP4 is divided into three regions; the N-terminal acidic region (the Na-region), the multiple KDM-repeated sequence region (the KDM-region), and the b-region followed by the MTB domain. To investigate roles of the PJ domain, we prepared three truncated forms of human MAP4 with different PJ domain lengths; PJ1, PJ2 and MTB with deletion of about one-third, two-third and all of the PJ domain, respectively, and examined their effects on bundle formation of microtubules (MTs). MTs polymerized by full length MAP4 were singly distributed as observed by both negative staining electron microscopy and dark field microscopy. MTs with PJ1 were also separated in solution but became pairs when pelleted by centrifugation. PJ2 formed planar two-dimensional bundles consisting of several MTs (the 2D-bundle). MTB induced large bundles of many MTs, tightly packed without space in between (termed the 3D-bundle). To study how the PJ domain decreases the bundle-forming activity of the MTB domain of MAP4, we made three additional deletion-mutants of MAP4, called Na-MTB, KDM-MTB and Na-PJ2. Na-MTB and KDM-MTB, in which the KDM/b-region and both of Na- and b-regions were deleted respectively, were prepared by fusing the Na-region or KDM-region to MTB. Both of Na-MTB and KDM-MTB suppressed the 3D-bundle formation as effectively as PJ2. MTs polymerized with Na-PJ2, the KDM-deletion mutant made by adding the Na-region to PJ2, were singular and did not become bundles. These results indicated that the PJ domain kept individual MTs separated by suppressing the bundle-forming ability of the MTB domain. The suppressive activity of the PJ domain was correlated with the length, but not the amino acid sequence, of the PJ.

MAP2 and tau bind longitudinally along the outer ridges of microtubule protofilaments

MAP2 and tau exhibit microtubule-stabilizing activities that are implicated in the development and maintenance of neuronal axons and dendrites. The proteins share a homologous COOH-terminal domain, composed of three or four microtubule binding repeats separated by inter-repeats (IRs). To investigate how MAP2 and tau stabilize microtubules, we calculated 3D maps of microtubules fully decorated with MAP2c or tau using cryo-EM and helical image analysis. Comparing these maps with an undecorated microtubule map revealed additional densities along protofilament ridges on the microtubule exterior, indicating that MAP2c and tau form an ordered structure when they bind microtubules. Localization of undecagold attached to the second IR of MAP2c showed that IRs also lie along the ridges, not between protofilaments. The densities attributable to the microtubule-associated proteins lie in close proximity to helices 11 and 12 and the COOH terminus of tubulin. Our data further suggest that the evolutionarily maintained differences observed in the repeat domain may be important for the specific targeting of different repeats to either alpha or beta tubulin. These results provide strong evidence suggesting that MAP2c and tau stabilize microtubules by binding along individual protofilaments, possibly by bridging the tubulin interfaces.

Regulation of microtubule-associated proteins

Microtubule-associated proteins (MAPs) function to regulate the assembly dynamics and organization of microtubule polymers. Upstream regulation of MAP activities is the major mechanism used by cells to modify and control microtubule assembly and organization. This review summarizes the functional activities of MAPs found in animal cells and discusses how these MAPs are regulated. Mechanisms controlling gene expression, isoform-specific expression, protein localization, phosphorylation, and degradation are discussed. Additional regulatory mechanisms include synergy or competition between MAPs and the activities of cofactors or binding partners. For each MAP it is likely that regulation in vivo reflects a composite of multiple regulatory mechanisms.

Tau-66: evidence for a novel tau conformation in Alzheimer's disease

We have characterized a novel monoclonal antibody, Tau-66, raised against recombinant human tau. Immunohistochemistry using Tau-66 reveals a somatic-neuronal stain in the superior temporal gyrus (STG) that is more intense in Alzheimer's disease (AD) brain than in normal brain. In hippocampus, Tau-66 yields a pattern similar to STG, except that neurofibrillary lesions are preferentially stained if present. In mild AD cases, Tau-66 stains plaques lacking obvious dystrophic neurites (termed herein 'diffuse reticulated plaques') in STG and the hippocampus. Enzyme-linked immunosorbent assay (ELISA) analysis reveals that Tau-66 is specific for tau, as there is no cross-reactivity with MAP2, tubulin, Abeta(1-40), or Abeta(1-42), although Tau-66 fails to react with tau or any other polypeptide on western blots. The epitope of Tau-66, as assessed by ELISA testing of tau deletion mutants, appears discontinuous, requiring residues 155-244 and 305-314. Tau-66 reactivity exhibits buffer and temperature sensitivity in an ELISA format and is readily abolished by SDS treatment. Taken together these lines of evidence indicate that the Tau-66 epitope is conformation-dependent, perhaps involving a close interaction of the proline-rich and the third microtubule-binding regions. This is the first indication that tau can undergo this novel folding event and that this conformation of tau is involved in AD pathology.

Identification of cAMP-dependent protein kinase holoenzymes in preantral- and preovulatory-follicle-enriched ovaries, and their association with A-kinase-anchoring proteins

Undifferentiated cells from preantral (PA) follicles respond to high levels of cAMP in a different manner than do differentiated cells from preovulatory (PO) follicles. We hypothesized that this differential response of PA and PO cells to cAMP could be due, in part, to either a difference in the profile of isoforms that comprise the cAMP-dependent protein kinase (PKA) holoenzymes and/or a difference in the interaction of PKA with A-kinase-anchoring proteins (AKAPs). To test these hypotheses, PKA activity, PKA holoenzymes, PKA subunits and AKAPs from PA and PO ovaries were compared. Soluble PKA holoenzymes and regulatory (R) subunits were separated by DEAE-cellulose chromatography and sucrose-density-gradient centrifugation. PKA R subunits were distinguished by photoaffinity labelling, autophosphorylation, size, isoelectric point and immunoreactivity. AKAPs were identified by RII subunit overlay assays and immunoreactivity. The results showed that extracts from PA and PO ovaries exhibited equivalent PKA holoenzyme profiles and activities, characterized by low levels of PKA type I (PKAI) holoenzyme and two distinct PKAII holoenzyme peaks, one containing only RIIbeta subunits (PKAIIbeta) and one containing both PKAIIbeta and PKAIIalpha holoenzymes. Both PA and PO ovarian extracts also contained PKA catalytic (C)-subunit-free RIalpha, while only PO ovaries exhibited C-subunit-free RIIbeta. Consistent with the elevated levels of C-subunit-free RIIbeta in PO cells, PKA activation in PO cells required higher concentrations of forskolin than that in PA cells. While extracts of PA and PO ovaries exhibited a number of similar AKAPs, including four prominent ones reactive with anti-AKAP-KL antisera (where AKAP-KL is an AKAP especially abundant in kidney and liver), cAMP-agarose affinity chromatography revealed two major differences in AKAP binding to purified R subunits. PO ovaries contained increased levels of AKAP80 (AKAP of 80 kDa) bound selectively to R subunits in DEAE-cellulose peak 2 (comprising PKAIIbeta and RIalpha), but not to R subunits in DEAE-cellulose peak 3 (comprising PKAIIalpha, PKAIIbeta and RIIbeta). PO ovaries also showed increased binding of R subunits to AKAPs reactive with anti-AKAP-KL antisera at 210, 175, 150 and 115 kDa. Thus in PO ovaries, unlike in PA ovaries, the majority of AKAPs are bound to R subunits. These results suggest that altered PKA-AKAP interactions may contribute to the distinct responses of PA and PO follicles to high levels of cAMP, and that higher cAMP levels are required to activate PKA in PO ovaries.

Targeting of PKA, PKC and protein phosphatases to cellular microdomains

The intracellular responses to many distinct extracellular signals involve the direction of broad-based protein kinases and protein phosphatases to catalyse quite specific protein phosphorylation/dephosphorylation events. It is now clear that such specificity is often achieved through subcellular targeting of distinct pools of kinase or phosphatase towards particular substrates at specific subcellular locations. Given the dynamic nature of protein phosphorylation reactions, coordinated control of both kinase and phosphatases is often required and complexes formed by common scaffold or targeting proteins exist to direct both kinase and phosphatase to the same subcellular location. In many cases more than one kinase or phosphatase is required and binding proteins which target more than one kinase or phosphatase have now been identified. This review summarizes recent findings relating to the concept of targeting PKA, PKC and the major serine/threonine phosphatases, PP1, PP2A and PP2B, through the formation of multi-enzyme signalling complexes.

The kinetochore of higher eucaryotes: a molecular view

This review summarizes results concerning the molecular nature of the higher eucaryotic kinetochore. The first major section of this review includes kinetochore proteins whose general functions remain to be determined, precluding their entry into a discrete functional category. Many of the proteins in this section, however, are likely to be involved in kinetochore formation or structure. The second major section is concerned with how microtubule motor proteins function to cause chromosome movement. The microtubule motors dynein, CENP-E, and MCAK have all been observed at the kinetochore. While their precise functions are not well understood, all three are implicated in chromosome movement during mitosis. Finally, the last section deals with kinetochore components that play a role in the spindle checkpoint; a checkpoint that delays mitosis until all kinetochores have attached to the mitotic spindle. Brief reviews of kinetochore morphology and of an important technical breakthrough that enabled the molecular dissection of the kinetochore are also included.

Disulfide-cross-linked tau and MAP2 homodimers readily promote microtubule assembly

The neuronal proteins Tau and MAP2 use homologous C-terminal MT-binding regions (MTBRs) to interact with microtubules, F-actin, and intermediate filaments. Although Tau-MTBR is the principal component of pronase-treated Alzheimer paired helical filaments, both Tau and MAP2 form filaments in vitro from disulfide-linked homodimers. That the critical thiol lies within a domain needed for MT binding raised the question: Does disulfide formation block Tau-Tau or MAP2-MAP2 dimer binding to microtubules, thereby acting to divert dimers toward filament formation? We now report that cross-linked Tau and MAP2 homodimers readily promote tubulin polymerization and that monomer and dimer affinity for MTs is surprisingly similar. Therefore, disulfide cross-bridging into homodimers is unlikely to be a drive force for filament formation in Alzheimer's disease.

Actin-based motility of the intracellular pathogen Listeria monocytogenes: assessing the inhibitory specificity of ABM-1 peptide analogues

Actin-Based Motility motifs [ABM-1 sequence = (D/E)FPPPPX(D/E), where X = P or T, and ABM-2 sequence = XPPPPP, where X denotes G, A, L, P, and S] facilitate assembly of an activated motility complex. Potent inhibition of intracellular motility of pathogens by ABM-1 and ABM-2 peptide analogues has served as a criterion for investigating actin-based motility. To assess the specificity of ABM-1 peptide inhibitors, we microinjected proline-rich peptides into Listeria-infected PtK2 host cells. Use of a combinatorial ABM-1 peptide library (empirical formula = D1E2F2P4T1) demonstrated that high-potency inhibition requires a precise sequence, and not merely a particular amino acid composition. Calculated concentrations of specific sequences in this library indicate that the entire (D/E)FPPPPX(D/E) motif is needed to achieve high-affinity inhibition in living cells. The failure of the well known proline-rich SH3 binding antagonists VSL-12 or APP-12 to inhibit Listeria motility also indicates that SH3 interactions are unlikely to control actin-based motility directly.

A new model for microtubule-associated protein (MAP)-induced microtubule assembly. The Pro-rich region of MAP4 promotes nucleation of microtubule assembly in vitro

The microtubule-binding domains of microtubule-associated protein (MAP) 2, tau, and MAP4 are divided into three distinctive regions: the Pro-rich region, the AP sequence region and the tail region (Aizawa, H., Emori, Y., Murofushi, H., Kawasaki, H., Sakai., H., and Suzuki, K. (1990) J. Biol. Chem. 265, 13849-13855). Electron microscopic observation showed that the taxol-stabilized microtubules alone and those mixed with the A4T fragment (containing the AP sequence region and the tail region) had a long, wavy appearance, while those mixed with the PA4T fragment (containing the Pro-rich region, the AP sequence region, and the tail region) or the PA4 fragment (containing the Pro-rich region and the AP sequence region) were shorter and straighter. Stoichiometries of the binding between the fragments and the tubulin dimers were approximately between 1 and 2, suggesting that not all of the AP sequences in the AP sequence region bound to tubulin. Binding affinity of the PA4T fragment is only four times higher than that of the A4T fragment, while the microtubule nucleating activity of the PA4T fragment is far greater. Based on these results, we propose that the nucleation of microtubule assembly is promoted by the bridging activity of the Pro-rich region in the MAPs.

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 newly identified membrane protein localized exclusively in intracellular organelles of neurons

We report here cloning of the cDNA of a novel membrane protein, termed p24, which, of the eight mouse tissues tested, was found only in brain where it is localized exclusively in neurons. The cDNA encodes 196 amino acids with a molecular weight of approximately 24000. P24 contains two putative membrane spanning domains and a sequence in the hydrophilic tail homologous to the microtubule-binding domain of microtubule-associated proteins, such as TAU and MAP-2. We prepared antibodies to p24 and demonstrated that the protein is rich in nerve fibers of the cerebral cortex, anterior cerebral nuclei and hypothalamus. When neuroblastoma Neuro 2a cells were treated with retinoic acid to induce differentiation, p24 mRNA increased but the p24 protein was not detected. The protein expressed from the p24 cDNA in non-neuronal Cos-7 cells was 24 kDa in size and were localized only in lysosomes. These findings indicate that p24 is a neuron-specific membrane protein localized in intracellular organelles of highly differentiated neural cells and suggest that it may play a role in the neural organelle transport system.

Identification of a 15-kDa cAMP-dependent protein kinase-anchoring protein associated with skeletal muscle L-type calcium channels

Voltage-dependent potentiation of skeletal muscle L-type calcium channels requires phosphorylation by cAMP-dependent protein kinase (PKA) that is localized by binding to a cAMP-dependent protein kinase-anchoring protein (AKAP). L-type calcium channels purified from rabbit skeletal muscle contain an endogenous co-purifying protein kinase activity that phosphorylates the alpha1 and beta subunits of the channel. The co-purifying kinase also phosphorylates a known PKA peptide substrate, is stimulated by cAMP, and is inhibited by PKA inhibitor peptide-(5-24), indicating that it is PKA. PKA activity co-immunoprecipitates with the calcium channel, suggesting that the channel and the kinase are physically associated. Using biotinylated type II regulatory subunit of PKA (RII) as a probe, we have identified a 15-kDa RII-binding protein in purified calcium channel preparations, which we have designated AKAP-15. Anti-peptide antibodies directed against the alpha1 subunit of the calcium channel co-immunoprecipitate AKAP-15. Together, these findings demonstrate a physical link between PKA and the calcium channel and suggest that AKAP-15 may mediate their interaction.

Functional interactions between the proline-rich and repeat regions of tau enhance microtubule binding and assembly

Tau is a neuronal microtubule-associated protein that promotes microtubule assembly, stability, and bundling in axons. Two distinct regions of tau are important for the tau-microtubule interaction, a relatively well-characterized "repeat region" in the carboxyl terminus (containing either three or four imperfect 18-amino acid repeats separated by 13- or 14-amino acid long inter-repeats) and a more centrally located, relatively poorly characterized proline-rich region. By using amino-terminal truncation analyses of tau, we have localized the microtubule binding activity of the proline-rich region to Lys215-Asn246 and identified a small sequence within this region, 215KKVAVVR221, that exerts a strong influence on microtubule binding and assembly in both three- and four-repeat tau isoforms. Site-directed mutagenesis experiments indicate that these capabilities are derived largely from Lys215/Lys216 and Arg221. In marked contrast to synthetic peptides corresponding to the repeat region, peptides corresponding to Lys215-Asn246 and Lys215-Thr222 alone possess little or no ability to promote microtubule assembly, and the peptide Lys215-Thr222 does not effectively suppress in vitro microtubule dynamics. However, combining the proline-rich region sequences (Lys215-Asn246) with their adjacent repeat region sequences within a single peptide (Lys215-Lys272) enhances microtubule assembly by 10-fold, suggesting intramolecular interactions between the proline-rich and repeat regions. Structural complexity in this region of tau also is suggested by sequential amino-terminal deletions through the proline-rich and repeat regions, which reveal an unusual pattern of loss and gain of function. Thus, these data lead to a model in which efficient microtubule binding and assembly activities by tau require intramolecular interactions between its repeat and proline-rich regions. This model, invoking structural complexity for the microtubule-bound conformation of tau, is fundamentally different from previous models of tau structure and function, which viewed tau as a simple linear array of independently acting tubulin-binding sites.

In vitro polymerization of embryonic MAP-2c and fragments of the MAP-2 microtubule binding region into structures resembling paired helical filaments

The microtubule-associated protein Tau is widely regarded as the principal component of paired helical filaments comprising Alzheimer neurofibrillary tangles. Tau fragments containing the non-identical repeat region formed structures resembling paired helical filaments (Schweers, O., Mandelkow, M., Biernat, J., and Mandelkow, E. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 8463-8467). MAP-2, the other structurally related neuronal microtubule-associated protein, has not been implicated in paired helical filament formation. We now describe the assembly of paired helical filament-like structures from MAP-2 polypeptides containing only 100 residues. A dimeric species, stabilized by an interchain disulfide, appears to be involved in the assembly reaction. We also investigated the polymerization of embryonic MAP-2c, which, except for its microtubule binding region, is structurally distinct from Tau. Full-length MAP-2c formed paired helical filament-like polymers. Polymerized MAP-2c and the microtubule binding region fragment readily bound thioflavin-S, a dye that stains paired helical filaments in the histochemical diagnosis of Alzheimer's disease. Our unprecedented finding that a small MAP-2 microtubule binding region fragment and MAP-2c can form structures resembling straight filaments or Pronase-treated paired helical filaments raises fundamental questions concerning the role of MAP-2 in the pathobiology of Alzheimer disease.

High molecular weight microtubule-associated proteins contain O-linked-N-acetylglucosamine

We have examined the post-translational modification of high molecular weight microtubule-associated proteins (MAPs) have shown that MAP1, MAP2, and MAP4 are glycosylated. The presence of carbohydrate residues on these proteins was indicated by labeling with biotin hydrazide following periodate oxidation, a specific and well established method for detecting saccharide moieties on proteins. Both MAP2 and MAP4 were also labeled in vitro by UDP-[3H]galactose in the presence of galactosyltransferase. Labeling by galactosyltransferase indicated that MAP2 and MAP4 contained terminal nonreducing GlcNAc residues, and they appeared to be O-linked to the proteins as shown by their sensitivity to beta-elimination. Chromatographic analysis showed that the GlcNAc residues were directly linked to the proteins as monosaccharides. Thus, we have added MAP2 and MAP4 to the list of intracellular O-GlcNAc-modified proteins, which includes other cytoskeletal proteins such as cytokeratins 8, 13, and 18 and neurofilament proteins NF-L and NF-M. We further characterized the O-GlcNAc modification of MAP2, and stoichiometric analysis indicated that nearly 10% of the MAP2 isolated from rat brain is modified by O-GlcNAc. However, this estimate is thought to reflect the minimal level of O-GlcNAc modification present on MAP2. We have also shown that both the O-GlcNAc and biotin hydrazide-reactive carbohydrate moieties are located on the projection domain of MAP2. Three O-GlcNAc-containing peaks were observed following fast protein liquid chromatography of a tryptic digest of MAP2, suggesting that multiple modification sites exist. The specific modification sites and functional significance of the O-GlcNAc glycosylation on the high Mr MAPs remain to be determined.

Functional analysis of the MAP2 repeat domain

The neuronal microtubule-associated protein MAP2 binds to microtubules via a domain near its C terminus containing a set of 3 or 4 imperfect repeats of a 31 amino acid motif. Using naturally occurring and mutated forms of the molecule containing between 1 and 4 repeats we have examined the contribution that these repeats make to MAP2 function and explored the significance of their repetition. The experiments utilised the short 3- and 4-repeat splice variants MAP2c and MAP2d that are expressed in developing neurons and in glia respectively, and mutant 1- and 2-repeat versions that were produced by using in vitro mutagenesis to remove further 31 amino acid units while leaving the rest of the molecule unaltered. The properties of these MAP2 variants were compared both with respect to their influence on microtubules in transfected non-neuronal cells and their ability to promote microtubule assembly in vitro. We found that each of the known effects of MAP2, including the bundling of microtubules and induction of process formation in living cells, are expressed by the 1-repeat form MAP2c3, which contains only the third repeat (R3). A second 1-repeat form, MAP2c4, which contains only R4, interacts more weakly with tubulin in vitro and does not bind to microtubules in transfected cells. The microtubule-related properties of MAP2 thus arise mainly from a single predominant repeat unit, R3. In vitro assembly experiments showed that the primary effect of all the repeats is to lower the critical concentration of tubulin required for microtubule assembly but that they differ greatly in potency. The results did not reveal a separate function related to the repetition of the repeat motifs, but instead suggest that its purpose is to tailor the efficiency of MAP2 to the cellular environment in which it has to function.

Domains of tau protein, differential phosphorylation, and dynamic instability of microtubules

The dynamic instability of microtubules is thought to be regulated by MAPs and phosphorylation. Here we describe the effect of the neuronal microtubule-associated protein tau by observing the dynamics of single microtubules by video microscopy. We used recombinant tau isoforms and tau mutants, and we phosphorylated tau by the neuronal kinases MARK (affecting the KXGS motifs within tau's repeat domain) and cdk5 (phosphorylating Ser-Pro motifs in the regions flanking the repeats). The variants of tau can be broadly classified into three categories, depending on their potency to affect microtubule dynamics. "Strong" tau variants have four repeats and both flanking regions. "Medium" variants have one to three repeats and both flanking regions. "Weak" variants lack one or both of the flanking regions, or have no repeats; with such constructs, microtubule dynamics is not significantly different from that of pure tubulin. N- or C-terminal tails of tau have no influence on dynamic instability. The two ends of microtubules (plus and minus) showed different activities but analogous behavior. These results are consistent with the "jaws" model of tau where the flanking regions are considered as targeting domains whereas the addition of repeats makes them catalytically active in terms of microtubule stabilization. The dominant changes in the parameters of dynamic instability induced by tau are those in the dissociation rate and in the catastrophe rate (up to 30-fold). Other rates change only moderately or not at all (association rate increased up to twofold, rates of rescue or rapid shrinkage decreased up to approximately twofold). The order of repeats has little influence on microtubule dynamics (i.e., repeats can be re-arranged or interchanged), arguing in favor of the "distributed weak binding" model proposed by Butner and Kirschner (1991); however, we confirmed the presence of a "hotspot" of binding potential involving Lys274 and Lys281 observed by Goode and Feinstein, 1994. Phosphorylation of Ser-Pro motifs by cdk5 (mainly Ser 202, 235, and 404) in the flanking regions had a moderate effect on microtubule dynamics while phosphorylation at the "Alzheimer"-site Ser262 MARK eliminated tau's interactions with microtubules. In both cases the predominant effects of phosphorylation are on the rates of tubulin dissociation and catastrophe whereas the effects on the rates of association or rescue are comparatively small.

Subpopulations of tau interact with microtubules and actin filaments in various cell types

It has been demonstrated that microtubule-associated proteins (MAPs) interact with tubulin in vitro and in vivo. However, there is no clear evidence on the possible roles of the interactions of MAPs in vivo with other cytoskeletal components in maintaining the integrity of the cell architecture. To address this question we extracted the neuronal cytoskeleton from brain cells and studied the selective dissociation of specific molecular isospecies of tau protein under various experimental conditions. Tau, and in some cases MPA-2, were analysed by the use of anti-idiotypic antibodies that recognize epitopes on their tubulin binding sites. Fractions of microtubule-bound tau isoforms were extracted with 0.35 M NaCl or after the addition of nocodazole to allow microtubule depolymerization. Protein eluted with this inhibitor contained most of the assembled tubulin dimer pool and part of the remaining tau and MAP-2. When the remaining cytoskeletal pellet was treated with cytochalasin D to allow depolymerization of actin filaments, only tau isoforms were extracted. Immunoprecipitation studies along with immunolocalization experiments in cell lines containing tau-like components supported the findings on the roles of tau isospecies as linkers between tubulin in the microtubular structure with actin filaments. Interestingly, in certain types of cells, antibody-reactive tau isospecies were detected by immunofluorescence with a discrete distribution pattern along actin filaments, which was affected by cytochalasin disruption of the actin filament network. These results suggest the possible in vivo roles of subsets of tau protein in modulating the interactions between microtubules and actin filaments.

Analysis of MAP 4 function in living cells using green fluorescent protein (GFP) chimeras

MAP 4 is a ubiquitous microtubule-associated protein thought to play a role in the polymerization and stability of microtubules in interphase and mitotic cells. We have analyzed the behavior of protein domains of MAP 4 in vivo using chimeras constructed from these polypeptides and the green fluorescent protein (GFP). GFP-MAP 4 localizes to microtubules; this is confirmed by colocalization of GFP-MAP 4 with microtubules that have incorporated microinjected rhodamine-tubulin, and by loss of localized fluorescence after treatment of cells with anti-microtubule agents. Different subdomains of MAP 4 have distinct effects on microtubule organization and dynamics. The entire basic domain of MAP 4 reorganizes microtubules into bundles and stabilizes these arrays against depolymerization with nocodazole. Within the basic domain, the PGGG repeats, which are conserved with MAP 2 and tau, have a weak affinity for microtubules and are dispensable for microtubule binding, whereas the MAP 4-unique PSP region can function independently in binding. The projection domain shows no microtubule localization, but does modulate the association of various binding subdomains with microtubules. The acidic carboxy terminus of MAP 4 strongly affects the microtubule binding characteristics of the other domains, despite constituting less than 6% of the protein. These data show that MAP 4 association with microtubules is modulated by sequences both within and outside the basic domain. Further, our work demonstrates that GFP chimeras will allow an in vivo analysis of the effects of MAPs and their variants on microtubule dynamics in real time.

Tau domains, phosphorylation, and interactions with microtubules

We consider the interactions of tau protein with microtubules from two points of view, phosphorylation and domain structure. Tau can be phosphorylated at many sites and by several kinases, notably by proline-directed kinases (MAPK, GSK-3, cdk5) which generate Alzheimer-like antibody epitopes. Other kinases phosphorylate Ser 262, a site that has a particularly pronounced influence on the affinity of tau for microtubules. All of these sites can be cleared by phosphatases PP-2a and calcineurin. The site Ser262 lies within the repeat domain of tau. However, when probing the domains of tau for their effects on microtubule binding, nucleation, assembly, or bundling, the repeat domain has only a weak influence. Whereas the repeat domain of tau binds to microtubules with low affinity, repeat-less tau binds strongly yet unproductively in terms of microtubule assembly. Productive binding of tau to microtubules depends on the combination of (some) repeats with the flanking regions, as if the flanking regions acted as "jaws" for the proper positioning of tau on the microtubule surface.

A 205 kDa protein from non-neuronal cells in culture contains tubulin binding epitopes

Microtubule-associated proteins (MAPs) interact with tubulin in vitro and in vivo. Despite that there is a large amount of information on the roles of these proteins in neurons, the data on non-neuronal MAPs or MAPs-related proteins is scarce. There is an increasing number of microtubule-interacting proteins that have been identified in different cultured cell lines, and some of them share common functional epitopes with the most well-known MAPs, MAP-2 and tau. In a search for tubulin-interacting proteins in non-neuronal cells we identified a 205 kDa protein in the monkey kidney Vero cells in culture, on the basis of immunological studies and affinity chromatography. This protein interacts with the C-terminal moiety of beta-tubulin and cosediments with taxol assembled microtubules, but it was not recovered after successive cycles of assembly and disassembly. The presence of neuronal MAPs such as MAP-1, MAP-2 and tau was not detected in these cells. Interestingly, the studies showed that the 205 kDa protein contained a tubulin binding motif which was recognized by site-directed antibodies that also tag tubulin binding epitopes on MAP-2 and tau. This characteristic led us to designate this protein as MBD-205, a component that shares binding domains with these MAPs, rather than as a marker of the MAPs family. On the other hand, immunofluorescence experiments using site-specific antibodies, i.e. MAP-reacting monoclonal anti-idiotypic reagent MTB6.22 and a polyclonal antibody to the second tau repeat, revealed a MBD-205 co-localization with membrane structures and microtubule-organizing centers in Vero cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Non-cooperative binding of the MAP-2 microtubule-binding region to microtubules

Microtubule-associated protein (MAP)-2 is a multi-domain cytoskeletal protein that copurifies with brain microtubules (MTs) through repeated cycles of warm polymerization and cold disassembly. Recent equilibrium binding studies of high molecular weight MAP-2ab to taxol-stabilized MTs suggest that the interactions are highly cooperative, as indicated by sigmoidal binding curves, non-linear Scatchard plots, and an apparent all-or-none response in MAP binding in titration experiments (Wallis, K. T., Azhar, S., Rho, M. B., Lewis, S. A., Cowan, N. J., and Murphy, D. B. (1993) J. Biol. Chem. 268, 15158-15167). To learn more about the mechanism of MAP-2 binding to MTs, we investigated the binding properties of bacterially expressed MT-binding region (MTBR) of bovine brain MAP-2. Scatchard plots of the binding data showed no evidence of cooperativity, as reflected by the linear plots of v/[MTBR]free versus v. The stoichiometry was 1-1.1 mol of MTBR/mol of tubulin dimer, and the dissociation constant for the MTBR was 1.1 microM. Bovine brain tau protein competitively inhibited MAP-2 binding, as evidenced by an increased Kd value for MTBR binding to MTs. Although the second repeat peptide m2 (VTSK-CGSLKNIRHRPGGG) is thought to play a dominant role in MAP-2 binding to MTs, a MTBR mutant (with m2 replaced by the third octadecapeptide repeat m3) displays an Kd of 2.8 +/- 0.1 microM and stoichiometry of 0.9 +/- 0.05 mol of MTBR/mol of tubulin dimer. Another mutant with additional copies of the second repeat, designated by us as MTBR[m12m2m32], displayed noncooperative binding with a Kd of 0.53 +/- 0.05 microM and a stoichiometry of 2.2 +/- 0.2 mol of mutant MTBR/tubulin dimer. Equilibrium sedimentation experiments demonstrated that the wild-type MTBR is monomeric, whereas MTBR[m12m2m32] self-associates to a stable dimer over the concentration range used in our MT binding studies. This finding indicates that only one of the two MT-binding sites on the dimer is probably linked to a microtubule at any given time.

Sequence analysis of MAP2 function in living cells

Microtubule-associated protein 2 (MAP2) is an abundant neuron-specific protein that binds to microtubules through a domain near its carboxyl terminus that contains either three or four similar repeats of a 31 amino acid motif. When expressed in non-neuronal cells by transfection MAP2 stabilises microtubules and induces their rearrangement into long bundles that are capable of supporting process outgrowth. To investigate which elements in the MAP2 sequence are involved in these functions we have constructed a series of deletion mutants of the short embryonic form of MAP2, MAP2c, and transfected them into non-neuronal cells. This showed that the strength of binding to microtubules increased with the number of repeats present in the construct. However, the repeat domain itself was insufficient for microtubule binding, which required in addition contiguous sequences either amino-terminal or carboxyl-terminal to the repeats themselves. Particularly on the amino-terminal side of the repeats, where there is a proline-rich domain, step-wise increases in the length of neighbouring sequence produced a gradual increase in microtubule binding. The apparent strength of binding to microtubules produced by mutant MAP2 forms was further correlated with the degree of bundling they induced as well as with the ability of the resulting microtubules to support process outgrowth. These results indicate that the interaction of MAP2 with microtubules is mediated by the combined action of several weak binding sites, including each of the repeat motifs and elements in the sequences on either side of them, whose additive effect produces the strong binding of the native MAP2 molecule. The results further indicate that both the bundling and stiffening of microtubules by MAP2 are correlated with the strength of its binding to them and suggest that these properties are a direct result of microtubule stabilisation.

The difference in the binding of phosphatidylinositol distinguishes MAP2 from MAP2C and Tau

The interactions of bovine brain MAP2 and tau, recombinant murine MAP2C, and recombinant human tau with phosphatidylinositol vesicles yield apparent Kd values of 51 +/- 6 nM, 2.4 +/- 0.6 microM, 1.4 +/- 0.1 microM, and 1.6 +/- 0.2 microM, respectively. Examinations of the binding of MAP2 and/or MAP2C to phosphatidylcholine vesicles doped with phosphatidylinositol or to phosphatidylserine vesicles and of thrombin-digested MAP2C to phosphatidylinositol vesicles demonstrates that the observed high affinity of the MAP2: phosphatidylinositol binding is due to the contributions of two separate interactions. A low-affinity site (Kd = 1.5-2.5 microM) is located within the C-terminal domain and affects the nonspecific interaction of MAP2, MAP2C, and tau with anionic phospholipids. The second site, with an apparent Kd of 221 +/- 25 nM, is located within the MAP2-specific peptide, which is eliminated from MAP2C by differential gene splicing. It is proposed that the high affinity of MAP2 for phosphatidylinositol contributes to the spatial and temporal regulation of the dendritic cytoskeleton.

Arrest of Listeria movement in host cells by a bacterial ActA analogue: implications for actin-based motility

Upon entering the host cell's cytoplasm, the pathogen Listeria monocytogenes can subvert the normal contractile system of the host cell; subsequent assembly of polar actin-filament structures is likely to provide the force for rapid intracellular bacterial movement and its cell-to-cell spread. We have now investigated the functional consequences of microinjecting Listeria-infected PtK2 cells with a synthetic peptide, CFEFPPPPTDE. This peptide represents one of four related oligoproline stretches in ActA, a bacterial surface protein necessary for Listeria-induced actin assembly. Over an estimated intracellular concentration range of 80 nM to 0.8 microM, this analogue rapidly blocks the formation of the actin-filament tails and arrests intracellular bacterial motility. Over the same time scale and concentration range, introduction of the ActA analogue also causes host cell membrane retraction. Bodipyphallacidin staining reveals that microinjection of the ActA analogue results in massive retraction of the actin cytoskeleton. Microinjection of 1-20 microM poly(L-proline) (intracellular concentration) fails to block Listeria intracellular movement or polar actin-filament assembly. As observed with ActA, however, poly(L-proline) does cause membrane retraction. Our findings demonstrate the efficacy of low molecular weight peptides in efforts to distinguish mechanistic features in Listeria motility and PtK2 host cell membrane reorganization. These observations also suggest that a cytoskeletal component sensitive to specific oligoproline peptides may participate in protein-protein interactions essential for both of these actin-associated processes.

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

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

Carboxyl terminal sequences of beta-tubulin involved in the interaction of HMW-MAPs. Studies using site-specific antibodies

After the finding of the involvement of the C-terminal moieties of tubulin subunits in the interaction of MAPs, different studies have focused on the substructure of the binding domains for the different MAPs. Current biochemical evidence point to the role of a low-homology sequence between alpha and beta-subunits within the conserved region of the C-terminal domain of tubulin, in the binding of MAP-2 and tau. Another line of studies indicates that a site for interaction of the high molecular weight MAPs is located in the variable region defined by the glutamic-rich C-terminus of beta-tubulin. Here, we report the usefulness of idiotypic site-directed antibodies, produced by immunization with peptides from different beta-tubulin isoforms, to study both MAP-1 and MAP-2 binding sites on tubulin. On the basis of these results with site-specific antibodies along with previous structural information (Cross et al., 1991, Biochemistry 30: 4362-4366), we propose the role of consensus sequences, from the invariant beta-tubulin C-terminal domain in the binding of MAP-2 and from the variable domain in the interactions of MAP-1 and MAP-2.

Cytochemical polarity in lateral geniculate interneurons

To determine the cytochemical composition of presynaptic dendrites, we have examined the distribution of synapsin 1, calcium and calmodulin-dependent protein kinase II (CaM-II), microtubule-associated protein 2 (MAP-2) and spectrin in cat lateral geniculate (LGN) class III cells by immune-EM. Special attention was paid to the dendrites of these interneurons because they are both pre- and postsynaptic. The dendritic proteins MAP-2 and RBC spectrin were not observed in interneuron dendrites but these proteins were localized in relay cell dendrites. The synaptic vesicle-associated protein synapsin 1 was present in all synaptic vesicle containing profiles, including dendritic terminals. CaM-II, the major postsynaptic density protein, was found in all dendrites. Thus, the LGN interneuron dendritic compartment displays both axonal and dendritic cytochemical properties. The results suggest the possibility of unique molecular interactions in interneuron dendritic terminals.

Stiff microtubules and neuronal morphology

Neuronal processes contain high concentrations of two related microtubule-associated proteins, MAP2 and tau. When MAP2 is expressed in non-neuronal cells the microtubules appear to be stiffer than those in control cells that do not express MAP2. A stiffening effect of MAP2 is further suggested by recent experiments with microtubules reassembled in vitro and by the fact that, under appropriate circumstances, MAP2-expressing cells can be induced to form processes that are long and cylindrical. Both MAP2 and tau contain homologous microtubule-binding domains, consisting of three or four repeats of an 18 amino acid sequence, which we believe are responsible for the stiffening effect. Our hypothesis is that each repeat binds to a neighbouring tubulin subunit in the wall of the microtubule, tethering them together and reducing their freedom of movement relative to one another. Based on these considerations, we suggest that MAP2 and tau may contribute to the support of neuronal processes by making the microtubules they contain longer, more stable and stiffer than those in non-neuronal cells.

Probing modifications of the neuronal cytoskeleton

The prominent death of central neurons in Alzheimer's and Parkinson's is reflected by changes in cell shape and by the formation of characteristic cytoskeletal inclusions (neurofibrillary tangles, Lewy bodies). This review focuses on the biology of neurofilaments and microtubule-associated proteins and identifies changes that can occur to these elements from basic and clinical research perspectives. Attention is directed at certain advances in neurobiology that have been especially integral to the identification of epitope domains, protein isoforms, and posttranslational (phosphorylation) events related to the composition, development, and structure of the common cytoskeletal modifications. Recently, a number of experimental strategies have emerged to simulate the aberrant changes in neurodegenerative disorders and gain insight into possible molecular events that contribute to alterations of the cytoskeleton. Descriptions of specific systems used to induce modifications are presented. In particular, unique neural transplantation methods in animals have been used to probe possible molecular and cellular conditions concerned with abnormal cytoskeletal changes in neurons.