A widely distributed nuclear protein immunologically related to the microtubule-associated protein MAP1 is associated with the mitotic spindle

ReMAPping the microtubule landscape: How phosphorylation dictates the activities of microtubule-associated proteins

Classical microtubule-associated proteins (MAPs) were originally identified based on their co-purification with microtubules assembled from mammalian brain lysate. They have since been found to perform a range of functions involved in regulating the dynamics of the microtubule cytoskeleton. Most of these MAPs play integral roles in microtubule organization during neuronal development, microtubule remodeling during neuronal activity, and microtubule stabilization during neuronal maintenance. As a result, mutations in MAPs contribute to neurodevelopmental disorders, psychiatric conditions, and neurodegenerative diseases. MAPs are post-translationally regulated by phosphorylation depending on developmental time point and cellular context. Phosphorylation can affect the microtubule affinity, cellular localization, or overall function of a particular MAP and can thus have profound implications for neuronal health. Here we review MAP1, MAP2, MAP4, MAP6, MAP7, MAP9, tau, and DCX, and how each is regulated by phosphorylation in neuronal physiology and disease. Developmental Dynamics 247:138-155, 2018. © 2017 Wiley Periodicals, Inc.

NuMA: a nuclear protein involved in mitotic centrosome function

Nuclear mitotic apparatus protein, NuMA, is an abundant 240 kDa protein with microtubule (MT) binding capacity via its carboxyl terminal region. Structurally, it has been shown to be a double-strand coiled-coil that has a high potential to form filamentous polymers. During interphase, NuMA locates within the nucleus but rapidly redistributes to the separating centrosomes during early mitosis. Xenopus NuMA associates with MT minus end-directed motor cytoplasmic dynein and its motility-activating complex dynactin at mitotic centrosomal regions. This NuMA-motor complex binds the free ends of MTs, converging and tethering spindle MT ends to the poles. A similar scenario appears to be true in higher vertebrates as well. As a mitotic centrosomal component, NuMA is essential for the organization and stabilization of spindle poles from early mitosis until at least the onset of anaphase. The cell cycle-dependent distribution and function of NuMA is regulated by phosphorylation and dephosphorylation, and p34/CDC2 activity is important to the mitotic role of NuMA. This review summarizes data about the structural features and mitotic function of NuMA with particular emphasis on the newly discovered NuMA-motor complex in spindle organization. Furthermore, NuMA may represent a large group of proteins whose mitotic function is sequestered in the nucleus during interphase.

XMAP310: a Xenopus rescue-promoting factor localized to the mitotic spindle

To understand the role of microtubule-associated proteins (MAPs) in the regulation of microtubule (MT) dynamics we have characterized MAPs prepared from Xenopus laevis eggs (Andersen, S.S.L., B. Buendia, J.E. Domínguez, A. Sawyer, and E. Karsenti. 1994. J. Cell Biol. 127:1289-1299). Here we report on the purification and characterization of a 310-kD MAP (XMAP310) that localizes to the nucleus in interphase and to mitotic spindle MTs in mitosis. XMAP310 is present in eggs, oocytes, a Xenopus tissue culture cell line, testis, and brain. We have purified XMAP310 to homogeneity from egg extracts. The purified protein cross-links pure MTs. Analysis of the effect of this protein on MT dynamics by time-lapse video microscopy has shown that it increases the rescue frequency 5-10-fold and decreases the shrinkage rate twofold. It has no effect on the growth rate or the catastrophe frequency. Microsequencing data suggest that XMAP230 and XMAP310 are novel MAPs. Although the three Xenopus MAPs characterized so far, XMAP215 (Vasquez, R.J., D.L. Gard, and L. Cassimeris. 1994. J. Cell Biol. 127:985-993), XMAP230, and XMAP310 are localized to the mitotic spindle, they have distinct effects on MT dynamics. While XMAP215 promotes rapid MT growth, XMAP230 decreases the catastrophe frequency and XMAP310 increases the rescue frequency. This may have important implications for the regulation of MT dynamics during spindle morphogenesis and chromosome segregation.

Nuclear components with microtubule-organizing properties in multicellular eukaryotes: functional and evolutionary considerations

The nucleus and the microtubular cytoskeleton of eukaryotic cells appear to be structurally and functionally interrelated. Together they constitute a "cell body". One of the most important components of this body is a primary microtubule-organizing center (MTOC-I) located on or near the nuclear surface and composed of material that, in addition to constitutive centrosomal material, also comprises some nuclear matrix components. The MTOC-I shares a continuity with the mitotic spindle and, in animal cells, with the centrosome also. Secondary microtubule-organizing centers (MTOC-IIs) are a special feature of walled plant cells and are found at the plasma membrane where they organize arrays of cortical MTs that are essential for ordered cell wall synthesis and hence for cellular morphogenesis. MTOC-IIs are held to be similar in origin to the MTOC-I, but their material has been translocated to the cell periphery, perhaps by MTs organized and radiating from the MTOC-I. Many intranuclear, matrix-related components have been identified to participate in MT organization during mitosis and cytokinesis; some of them also seem to be related to the condensation and decondensation of chromatin during the mitotic chromosome cycle.

A protein related to brain microtubule-associated protein MAP1B is a component of the mammalian centrosome

The centrosome is the main microtubule organizing center of mammalian cells. Structurally, it is composed of a pair of centrioles surrounded by a fibro-granular material (the pericentriolar material) from which microtubules are nucleated. However, the nature of centrosomal molecules involved in microtubules nucleation is still obscure. Since brain microtubule-associated proteins (MAPs) lower the critical tubulin concentration required for microtubule nucleation in tubulin solution in vitro, we have examined their possible association with centrosomes. By immunofluorescence, monoclonal and polyclonal antibodies raised against MAP1B stain the centrosome in cultured cells as well as purified centrosomes, whereas antibodies raised against MAP2 give a completely negative reaction. The MAP1B-related antigen is localized to the pericentriolar material as revealed by immunoelectron microscopy. In preparations of purified centrosomes analyzed on poly-acrylamide gels, a protein that migrates as brain MAP1B is present. After blotting on nitrocellulose, it is decorated by anti-MAP1B antibodies and the amino acid sequence of proteolytic fragments of this protein is similar to brain MAP1B. Moreover, brain MAP1B and its centrosomal counterpart share the same phosphorylation features and have similar peptide maps. These data strongly suggest that a protein homologue to MAP1B is present in centrosomes and it is a good candidate for being involved in the nucleating activity of the pericentriolar material.

Identification of a 102 kDa protein (cytocentrin) immunologically related to keratin 19, which is a cytoplasmically derived component of the mitotic spindle pole

The mAb RK7, previously shown to recognize keratin 19, was also found to cross-react with a biologically unrelated 102 kDa protein, which becomes associated with the poles of the mitotic apparatus. This newly identified protein, called cytocentrin, is a stable cellular component, may be at least in part phosphorylated, and displays a cell cycle-dependent cellular localization. In interphase cells, it is diffusely distributed in the cytosol and shows no affinity for cytoplasmic microtubules. It becomes localized to the centrosome in early prophase, prior to nuclear envelope breakdown, separation of replicated centrosomes, and nucleation of mitotic apparatus microtubules. During metaphase, cytocentrin is located predominately at the mitotic poles, often appearing as an aggregate of small globular sub-components; it also associates with some polar microtubules. In late anaphase/early telophase cytocentrin dissociates entirely from the mitotic apparatus and becomes temporarily localized with microtubules in the midbody, from which it disappears by late telophase. In taxol-treated cells cytocentrin was associated with the center of the miniasters but also showed affinity for some cytoplasmic microtubules. Studies employing G2-synchronized cells and nocodazole demonstrated that cytocentrin can become associated with mitotic centrosomes independently of tubulin polymerization and that microtubules regrow from antigen-containing foci. We interpret these results to suggest that cytocentrin is a cytoplasmic protein that becomes specifically activated or modified at the onset of mitosis so that it can affiliate with the mitotic poles where it may provide a link between the pericentriolar material and other components of the mitotic apparatus.

Heterogeneity of microtubule organizing center components as revealed by monoclonal antibodies to mammalian centrosomes and to nucleus-associated bodies from dictyostelium

The molecular composition of two morphologically distinct microtubule-organizing centers (MTOCs) was compared by probing with monoclonal antibodies raised against (i) nucleus-associated bodies (NABs) isolated in a complex with nuclei from the cellular slime mold Dictyostelium discoideum and (ii) mammalian mitotic spindles isolated from Chinese hamster ovary (CHO) cells. The staining patterns observed by immunofluorescence microscopy in whole CHO cells and Dictyostelium amoebae showed that the distribution of thirteen MTOC antigens is heterogeneous. Not all antibodies recognized the MTOC in both interphase and mitosis. Most of the anti-MTOC antibodies cross-reacted with other cellular organelles such as nuclei, Golgi apparatus-like aggregates and cytoskeletal elements. Two antibodies, CHO3 and AX3, recognized phosphorylated epitopes present in both mammalian centrosomes and Dictyostelium NABs. On immunoblots, most of the antibodies showed multiple bands, often of high molecular weight, indicating that the antigenic determinants are shared among different molecules. One antibody inhibited the regrowth of microtubules onto centrosomes in vitro after addition of exogenous tubulin to detergent-lysed CHO cells on coverslips; this antibody binds to an antigen(s) that might be essential for the microtubule-nucleating activity of centrosomes. These observations demonstrate that molecular components in different MTOCs exhibit a variety of distinct subcellular localizations and functional properties, and that some antigenic molecules have been conserved among morphologically distinct MTOCs.

Specific association of an M-phase kinase with isolated mitotic spindles and identification of two of its substrates as MAP4 and MAP1B

Isolated mammalian (Chinese hamster ovary [CHO]) metaphase spindles were found to be enriched in a histone H1 kinase whose activity was mitotic-cycle dependent. Two substrates for the kinase were identified as MAP1B and MAP4. Partially purified spindle kinase retained activity for the spindle microtubule-associated proteins (MAPs) as well as brain and other tissue culture MAPs; on phosphorylation, spindle MAPs exhibited increased immunoreactivity with MPM-2, a monoclonal antibody specific for a subset of mitotic phosphoproteins. Immunofluorescence using an anti-thiophosphoprotein antibody localized in vitro phosphorylated spindle proteins to microtubule fibers, centrosomes, kinetochores, and midbodies. The fractionated spindle kinase was reactive with anti-human p34cdc2 antibodies and with an anti-human cyclin B but not an anti-human cyclin A antibody. We conclude that spindle MAPs undergo mitotic cycle-dependent phosphorylations in vivo and associate with a kinase that remains active on spindle isolation and may be related to p34cdc2.

Chromosomal passengers: toward an integrated view of mitosis

The major events of mitosis have traditionally been considered to represent two distinct pathways and have been studied by two separate groups of workers. The chromosomal events (chromosome condensation and sister chromatid disjunction) have been the principal focus for one group, while the cytoskeletal events (nuclear envelope breakdown, chromosomal movements, cytokinesis) have been the focus for the other. This historical division is epitomized by the view of many cell biologists, which was aptly caught by Mazia's comparison of the role of the chromosome arms in mitosis to that of "the corpse at the funeral" which "provide a reason for the proceedings but do not take an active part in them" (Mazia 1961). More recent studies have demonstrated that the role of the chromosomes in mitotic movements is somewhat more active than this. That the kinetochore may play an important role in chromosome movements has long been suspected (see early references in Mazia 1961) but was only proven rather recently (Brinkley et al. 1988; Gorbsky et al. 1987; Nicklas 1989). This has led to a burst of recent interest in all aspects of kinetochore structure and function. Our studies have led us to ask whether chromosomes may play an even more extensive role in the events of mitosis. We suggest here that in addition to their active role in movements, the chromosome may make important structural contributions to the anaphase spindle and cleavage furrow, which are normally thought of as "cytoskeletal" functions. These structural contributions may be made by members of a new class of "chromosomal passenger" proteins that use the chromosomes as a means of conveyance so that they are correctly positioned at the metaphase plate to carry out their nonchromosomal functions during anaphase and the subsequent mitotic events.

Centrophilin: a novel mitotic spindle protein involved in microtubule nucleation

A novel protein has been identified which may serve a key function in nucleating spindle microtubule growth in mitosis. This protein, called centrophilin, is sequentially relocated from the centromeres to the centrosomes to the midbody in a manner dependent on the mitotic phase. Centrophilin was initially detected by immunofluorescence with a monoclonal, primate-specific antibody (2D3) raised against kinetochore-enriched chromosome extract from HeLa cells (Valdivia, M. M., and B. R. Brinkley. 1985. J. Cell Biol. 101:1124-1134). Centrophilin forms prominent crescents at the poles of the metaphase spindle, gradually diminishes during anaphase, and bands the equatorial ends of midbody microtubules in telophase. The formation and breakdown of the spindle and midbody correlates in time and space with the aggregation and disaggregation of centrophilin foci. Immunogold EM reveals that centrophilin is a major component of pericentriolar material in metaphase. During recovery from microtubule inhibition, centrophilin foci act as nucleation sites for the assembly of spindle tubules. The 2D3 probe recognizes two high molecular mass polypeptides, 180 and 210 kD, on immunoblots of whole HeLa cell extract. Taken together, these data and the available literature on microtubule dynamics point inevitably to a singular model for control of spindle tubule turnover.

Microtubule associated protein (MAP1B) is present in cultured oligodendrocytes and co-localizes with tubulin

Differentiation of oligodendrocytes is accompanied by the extension of processes and the assembly of the myelin membrane. It is likely that the cytoskeleton plays an important role in this process in terms of changes in cell shape, transport of myelin components, and organization of the myelin membrane. Oligodendrocytes contain microtubules (MT) which associate with other components of the cytoskeleton, and microtubule associated proteins (MAPs) may mediate some of these interactions. In this study we have shown the presence of MAP1B in oligodendrocytes grown in primary glial cultures by double-label immunofluorescence using antibodies to galactocerebroside (GC) and MAP1B. The staining of the cultures showed that GC-positive oligodendrocytes were also stained with MAP1B antibodies. However, MAP1B stain was limited to cell bodies and processes, whereas GC stain was also seen in flattened membrane sheets and punctate staining in processes. MAP1B staining was also compared with that of myelin proteolipid (PLP), myelin basic protein (MBP) and beta-tubulin in secondary glial cultures that were enriched for oligodendrocytes. The results showed a typical staining of cell bodies and membranous profiles using PLP antibodies, and the staining of cell bodies and flattened regions of membranous sheets by MBP antibodies. In contrast, both polyclonal and monoclonal antibodies to MAP1B showed a uniform diffuse staining of cell bodies, major processes, and fine interconnected processes. Double-labeling of the cells showed that MAP1B was co-localized with tubulin, but was not present in glial fibrillary acidic protein (GFAP)-positive astrocytes. Western and Northern blot analyses of primary glial cultures showed that MAP1B had a molecular mass of 320 kDa and a mRNA of 10 kb. These values are identical to those previously reported for brain MAP1B (Safaei and Fischer, 1989) and demonstrate the presence of MAP1B in oligodendrocytes.

Changes in microtubule-associated protein MAP1B phosphorylation during rat brain development

Microtubule-associated protein MAP1B from neonatal rat brain was separated on sodium dodecyl sulfate-containing polyacrylamide gels into two isoforms (high and low MAP1B), both of which were recognized by a panel of monoclonal and polyclonal antibodies against MAP1B. In addition, SMI31, a monoclonal antibody directed against phosphorylated epitopes of the neurofilament proteins, showed phosphatase-sensitive reactivity against the high isoform of MAP1B. The antigenic relationship between the phosphorylated isoform of MAP1B and neurofilaments was confirmed by the reactivity of SMI31 with the immunoprecipitated MAP1B protein. After dephosphorylation of MAP1B with alkaline phosphatase, the higher-molecular-weight isoform of MAP1B was no longer detectable with phosphate-insensitive anti-MAP1B antibodies, whereas there was a significant increase in the immunoreactivity of the lower-molecular-weight MAP1B isoform. These data suggest that the structural microheterogeneity of MAP1B is due to differences in phosphorylation. The two isoforms were present in all brain regions of the young rat. During brain development, the general decrease in MAP1B levels was accompanied by changes in the relative amount of the two isoforms. In particular, the phosphorylated isoform of MAP1B decreased dramatically to almost undetectable levels in adult brain. This conclusion was further supported by immunoblotting analysis that showed the disappearance of phosphorylated epitopes of MAP1B early during brain development. In addition, dephosphorylation experiments demonstrated the phosphatase sensitivity of the phosphorylated isoform throughout development.

Cloning of a cDNA encoding MAP1B in rat brain: regulation of mRNA levels during development

This article describes the isolation of a microtubule-associated protein 1B (MAP1B) cDNA clone from a rat brain lambda gt11 library and the study of MAP1B mRNA expression during brain development. On Northern blots, the cDNA hybridized with an mRNA of greater than 10 kilobases which was present only in the brain. The identity of the cDNA was confirmed by the characterization of the antiserum against the fusion protein, and also by comparing both the original antibody and the anti-fusion protein antiserum with a panel of well-studied monoclonal antibodies against different forms of MAP1 and MAP2. The regulation of MAP1B mRNA during development was studied in whole brain, cerebral cortex, hypothalamus, brainstem, and olfactory bulbs. The steady-state levels of MAP1B mRNA in all tissues examined were relatively low in the adult compared to developing brains. This decrease varied in different brain regions, and its time course appeared to coincide with the pattern of postnatal developmental and morphological events. The developmental patterns of the MAP1B mRNA and protein in the brain were similar, suggesting that expression of this protein is under transcriptional control. The RNA blots were also probed with beta-actin and beta-tubulin to compare the levels of MAP1B mRNA with other cytoskeletal elements and as controls for the quality of the RNA.

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.

225-Kilodalton phosphoprotein associated with mitotic centrosomes in sea urchin eggs

Protein phosphorylation during development of sea urchin eggs from fertilization to first cleavage was examined by labeling cells with specific antiphosphoprotein antibodies. Indirect immunofluorescence staining with monoclonal antithiophosphoprotein antibody (Gerhart et al.: Cytobios 43:335-347, 1985) has revealed that nuclei as well as centrosomes, kinetochores, and midbodies were specifically thiophosphorylated in developing eggs incubated with adenosine 5'-O (3-thiotriphosphate) (ATP-gamma-S). The phosphorylation reaction required Mg2+ but was not dependent on cAMP or calmodulin in detergent-extracted models. Centrosomes were purified by fractionation of isolated mitotic spindles with 0.5 M KCl extraction. The thiophosphoproteins were retained in the purified centrosomes and the antibody recognized a major 225-Kd polypeptide on immunoblots. In an independent preparation, a monoclonal antiphosphoprotein antibody (CHO3) was found also to react with mitotic poles and stained a 225-Kd polypeptide, confirming the centrosome specificity of this protein. Immunoelectron microscopy showed that the 225-Kd thiophosphoprotein was found at mitotic poles associated with granules to which mitotic microtubules were directly attached. Unlike centrosomes in permeabilized eggs, those in isolated spindles could not be thiophosphorylated, possibly due to inactivation or loss of either phosphorylation enzymes or cofactors, or both, during isolation. The immunofluorescence labeling of thiophosphate could be inhibited by ATP and AMP.PNP in a concentration-dependent manner. Exogenous ATP could abolish thiophosphate-staining more effectively when added with phosphatase inhibitors, suggesting a dynamic state in which centrosomal proteins are being phosphorylated and dephosphorylated in rapid succession by the action of protein kinase(s) and phosphatase(s).

Altered levels of microtubule proteins in brains of Alzheimer's disease patients

The amount of microtubule protein present in the total soluble protein from brains of Alzheimer's disease patients and from brains of non-Alzheimer age-matched controls, were determined by radioimmunoassay. No differences were found in the amount of tubulin or microtubule-associated protein MAP2 present in either group. However, the amount of tau protein or MAP1 from the brains of Alzheimer's disease patients was about half of that present in their control counterparts.

A casein kinase II-related activity is involved in phosphorylation of microtubule-associated protein MAP-1B during neuroblastoma cell differentiation

A neuroblastoma protein related to the brain microtubule-associated protein, MAP-1B, as determined by immunoprecipitation and coassembly with brain microtubules, becomes phosphorylated when N2A mouse neuroblastoma cells are induced to generate microtubule-containing neurites. To characterize the protein kinases that may be involved in this in vivo phosphorylation of MAP-1B, we have studied its in vitro phosphorylation. In brain microtubule protein, MAP-1B appears to be phosphorylated in vitro by an endogenous casein kinase II-like activity which also phosphorylates the related protein MAP-1A but scarcely phosphorylates MAP-2. A similar kinase activity has been detected in cell-free extracts of differentiating N2A cells. Using brain MAP preparations devoid of endogenous kinase activities and different purified protein kinases, we have found that MAP-1B is barely phosphorylated by cAMP-dependent protein kinase, Ca/calmodulin-dependent protein kinase, or Ca/phospholipid-dependent protein kinase whereas MAP-1B is one of the preferred substrates, together with MAP-1A, for casein kinase II. Brain MAP-1B phosphorylated in vitro by casein kinase II efficiently coassembles with microtubule proteins in the same way as in vivo phosphorylated MAP-1B from neuroblastoma cells. Furthermore, the phosphopeptide patterns of brain MAP-1B phosphorylated in vitro by either purified casein kinase II or an extract obtained from differentiating neuroblastoma cells are identical to each other and similar to that of in vivo phosphorylated neuroblastoma MAP-1B. Thus, we suggest that the observed phosphorylation of a protein identified as MAP-1B during neurite outgrowth is mainly due to the activation of a casein kinase II-related activity in differentiating neuroblastoma cells. This kinase activity, previously implicated in beta-tubulin phosphorylation (Serrano, L., J. Díaz-Nido, F. Wandosell, and J. Avila, 1987. J. Cell Biol. 105: 1731-1739), may consequently have an important role in posttranslational modifications of microtubule proteins required for neuronal differentiation.

Distribution of a matrix component of the midbody during the cell cycle in Chinese hamster ovary cells

Monoclonal antibodies were raised against isolated spindles of CHO (Chinese hamster ovary) cells to probe for molecular components specific to the mitotic apparatus. One of the antibodies, CHO1, recognized an antigen localized to the midbody during mitosis. Immunofluorescence staining of metaphase cells showed that although the total spindle area was labeled faintly, the antigen corresponding to CHO1 was preferentially localized in the equatorial region of the spindle. With the progression of mitosis, the antigen was further organized into discrete short lines along the spindle axis, and eventually condensed into a bright fluorescent dot at the midzone of the intercellular bridge between two daughter cells. Parallel immunostaining of tubulin showed that the CHO1-stained area corresponded to the dark region where microtubules are entrapped by the amorphous dense matrix components and possibly blocked from binding to tubulin antibody. Immunoblot analysis indicated that CHO1 recognized two polypeptides of mol wt 95,000 and 105,000. The immunoreaction was always stronger in preparations of isolated midbodies than in mitotic spindle fractions. The protein doublet was retained in the particulate matrix fraction after Sarkosyl extraction (Mullins, J. M., and J. R. McIntosh. 1982. J. Cell Biol. 94:654-661), suggesting that CHO1 antigen is indeed a component of the dense matrix. In addition to the equatorial region of spindles and midbodies, CHO1 also stained interphase centrosomes, and nuclei in a speckled pattern that was cell cycle-dependent. Thus, the midbody appears to share either common molecular component(s) or a similar epitope with interphase centrosomes and nuclei.

Structural and chemical characterization of isolated centrosomes

A procedure adapted from that described by Mitchison and Kirschner [Nature 312:232-237, 1984] was used to isolate centrosomes from human lymphoid cells. High yields of homogeneous centrosomes (60% of the theoretical total, assuming one centrosome per cell) were obtained. Centrosomes were isolated as pairs of centrioles, plus their associated pericentriolar material. Ultrastructural investigation revealed: 1) a link between both centrioles in a centrosome formed by the gathering in of a unique bundle of thin filaments surrounding each centriole; 2) a stereotypic organization of the pericentriolar material, including a rim of constant width at the proximal end of each centriole and a disc of nine satellite arms organized according to a ninefold symmetry at the distal end and; 3) an axial hub in the lumen of each centriole at the distal end surrounded by some ill-defined material. The total protein content was 2 to 3 X 10(-2) pg per isolated centrosome, a figure that suggests that the preparations were close to homogeneity. The protein composition was complex but specific, showing proteins ranging from 180 to 300 kD, one prominent band at 130 kD, and a group of proteins between 50 and 65 kD. Actin was also present in centrosome preparations. Functional studies demonstrated that the isolated centrosomes were competent to nucleate microtubules in vitro from purified tubulin in conditions in which spontaneous assembly could not occur. They were also very effective at inducing cleavage when microinjected into unfertilized Xenopus eggs.

Intranuclear appearance of the phosphorylated form of cytoskeleton-associated 350-kDa proteins in U1-ribonucleoprotein regions after growth stimulation of fibroblasts

Cytoskeleton-associated 350-kDa and 80-kDa polypeptides, which were immunoprecipitated with polyclonal antibody against microtubule-associated protein 1 (MAP-1), were rapidly phosphorylated on mitogenic stimulation of quiescent fibroblasts with serum or growth factors. The enhanced phosphorylation was evident within 5 min and reached a maximum 2 hr after the stimulation. Phosphorylated MAP-1 analogues were first detected in the cytoplasm around the microtubule-organizing center and then in the nucleus by immunofluorescent staining with a monoclonal antibody that recognized the phosphorylated form of MAP-1. The monoclonal antibody reacted with the 350-kDa protein in immunoblot analysis and immunostained intranuclear speckles; both immunoreactions were abolished by treatment with alkaline or acid phosphatase. The nuclear speckles stained by the monoclonal antibody were also stained by anti-U1 small nuclear ribonucleoprotein antibodies on double immunofluorescence, suggesting that the stained regions are sites of maturation of messenger RNA. These results support the idea that part of the cytoskeleton-associated 350-kDa protein is phosphorylated and transferred to the nuclear region of mRNA modification as a common early process after growth stimulation.

Identification of centrosomal proteins in a human lymphoblastic cell line

Highly enriched preparations of centrosomes from human T-lymphoblasts KE 37 were analyzed for their protein content. The specific pattern of polypeptides was characterized by an abundant subset of high mol. wt proteins and a major group of proteins with mol. wt ranging from 50 to 65 kd. Several immunoreactive proteins were identified, using a rabbit serum spontaneously reacting with human centrosomes. They include a family of high mol. wt ranging from 180 to 250 kd, a 130-kd protein and a 60-65 kd doublet. These antigens have the following properties: they are localized within the pericentriolar material; their abundance, as judged by centrosome labelling, changes significantly during the cell cycle, the maximum being observed at the pole of the metaphasic spindle; in Taxol-treated cells where the centrosome is no longer acting as a nucleating center, they redistribute at one end of the microtubule arrays in both mitotic and interphasic cells, as expected for nucleating, or capping, proteins. All these properties are compatible with their involvement in microtubule nucleation.

Study of the transit of an integral membrane protein from secretory granules through the plasma membrane of secreting rat basophilic leukemia cells using a specific monoclonal antibody

The monoclonal antibody 5G10 reacted specifically with an 80-kD integral membrane protein in rat basophilic leukemia (RBL) cells. Immunofluorescence microscopy studies of RBL cells, fixed and permeabilized, revealed that the 80-kD protein was located in the membrane of cytoplasmic vesicles. The vesicles were identified as secretory granules by their content in immunoreactive serotonin. Expression of the 5G10 antigen on the surface of unstimulated RBL cells was low. However, RBL cells stimulated to secrete with anti-dinitrophenyl IgE followed by dinitrophenyl-bovine serum albumin or with the Ca2+ ionophore A-23187 displayed an increased expression of the antigen on their surface. Surface exposure of the 5G10 antigen was maximal at 5 min after stimulation of secretion. Removal of dinitrophenyl-bovine serum albumin from the incubation medium resulted in internalization of 50% of the antigen within 10 min.

Nuclear protein matrix as a target for estramustine-induced cell death

The effect of estramustine [estradiol 3-N-bis(2-chloroethyl)carbamate] on the human prostatic tumor cell line 1013L was investigated. Cell proliferation experiments revealed that estramustine cytotoxicity varied during the different phases of cell growth. Maximum cell killing was found in early log phase, but cell death also occurred in the stationary phase. Mitotic arrest was found at cytotoxic concentrations throughout the log phase. Subcellular distribution studies showed that the cellular uptake of estramustine increased throughout the log phase and remained steady during the stationary phase. Nuclear uptake in contrast was similar in all phases, whereas a preferential binding to the nuclear protein matrix was found to increase throughout the log phase and even during the stationary phase of growth. This implicates the nuclear protein matrix as a target for estramustine cytotoxicity.

Microtubule-associated protein 1B: identification of a major component of the neuronal cytoskeleton

The major nontubulin proteins in purified brain microtubules are high molecular weight species traditionally classified into two groups known as microtubule-associated proteins 1 and 2 (MAP 1 and MAP 2). In an earlier study, we found that MAP 1 consisted of a complex of polypeptides and we characterized the highest molecular weight species--MAP 1A--with the use of a monoclonal antibody. In the current report, we describe four monoclonal antibodies raised against electrophoretically purified MAP 1B. All of the antibodies reacted exclusively with this protein. Together with peptide mapping, these results indicated that MAP 1B was structurally distinct from the other MAPs. Another distinctive property of MAP 1B was that most of it remained soluble during microtubule polymerization, resulting in an extreme underestimate of its abundance in the brain. Immunofluorescence microscopy of rat brain sections and cultured rat brain cells indicated that compared to MAP 1A and MAP 2, MAP 1B was particularly prominent in axonal as well as dendritic processes. Together, these data indicate that MAP 1B is a major, previously undescribed component of the neuronal cytoskeleton.