Gene name - Eb1
Cytological map position-42C8-42C8
Function - cytoskeletal component, signaling
Symbol - Eb1
FlyBase ID: FBgn0027066
Genetic map position - 2R: 2,636,697..2,643,544 [+]
Classification - N-terminal calponin homology domain fold, microtubule binding domain, C-terminal domain for dimerization and for interaction with EB1-binding proteins
Cellular location - cytoplasmic
|Recent literature||Moriwaki, T. and Goshima, G. (2016). Five factors can reconstitute all three phases of microtubule polymerization dynamics. J Cell Biol 215: 357-368. PubMed ID: 27799364
Cytoplasmic microtubules (MTs) undergo growth, shrinkage, and pausing. However, how MT polymerization cycles are produced and spatiotemporally regulated at a molecular level is unclear, as the entire cycle has not been recapitulated in vitro with defined components. In this study, dynamic MT plus end behavior involving all three phases was reconstituted by mixing tubulin with five Drosophila melanogaster proteins (EB1, XMAP215Msps, Sentin, kinesin-13Klp10A, and CLASPMast/Orbit). When singly mixed with tubulin, CLASPMast/Orbit strongly inhibited MT catastrophe and reduced the growth rate. However, in the presence of the other four factors, CLASPMast/Orbit acted as an inducer of pausing. The mitotic kinase Plk1Polo modulated the activity of CLASPMast/Orbit and kinesin-13Klp10A and increased the dynamic instability of MTs, reminiscent of mitotic cells. These results suggest that five conserved proteins constitute the core factors for creating dynamic MTs in cells and that Plk1-dependent phosphorylation is a crucial event for switching from the interphase to mitotic mode.
End binding 1 (EB1) is an evolutionarily conserved protein that localizes to and controls the plus ends of growing microtubules (Vaughan, 2005). EB1 targets other MT-associated proteins to the plus end and thereby regulates interactions of MTs with the cell cortex, mitotic kinetochores, and different cellular organelles. EB1 also localizes to centrosomes and is required for centrosomal MT anchoring and organization of the MT network. Using a novel preparation of the Drosophila S2 cell line that promotes cell attachment and spreading, dynamics of single microtubules were visualized in real time, and it was found that depletion of EB1 by RNA-mediated inhibition (RNAi) in interphase cells causes a dramatic increase in nondynamic microtubules (neither growing nor shrinking), but does not alter overall microtubule organization. In contrast, several defects in microtubule organization are observed in RNAi-treated mitotic cells, including a drastic reduction in astral microtubules, malformed mitotic spindles, defocused spindle poles, and mispositioning of spindles away from the cell center. Similar phenotypes were observed in mitotic spindles of Drosophila embryos that were microinjected with anti-EB1 antibodies. In addition, live cell imaging of mitosis in Drosophila embryos reveals defective spindle elongation and chromosomal segregation during anaphase after antibody injection. These results reveal crucial roles for EB1 in mitosis, which is postulated to involve its ability to promote the growth and interactions of microtubules within the central spindle and at the cell cortex (Rogers, 2002).
The microtubule cytoskeleton functions as an essential scaffold that helps to organize the cytoplasm of eukaryotic cells in interphase. Microtubules, which emanate in a radial pattern from the centrosome during interphase in most eukaryotic cells, provide tracks for microtubule motors carrying membrane, RNA, and protein complexes toward and away from the cell center. Microtubules also play an important role in establishing cell polarity, as they align in the direction of cell migration and toward the interface between immune and antigen-presenting cells. During mitosis, microtubules reorganize to create a mitotic spindle that, in conjunction with motor proteins, segregates chromosomes during cell division. Mitotic spindle orientation, which is a process important in development, tissue morphogenesis, and stem cell differentiation, also involves interactions between astral microtubules, motors, and other proteins at the cell cortex (Rogers, 2002).
Microtubules are intrinsically dynamic, which allows the microtubule cytoskeleton to rapidly rearrange in response to internal or external cues. Within a population of microtubules at steady-state, individual microtubules undergo transitions between phases of prolonged polymerization and depolymerization. This behavior, known as 'dynamic instability,' is enabled by the hydrolysis of GTP after monomeric tubulin becomes incorporated into the microtubule. Dynamic instability is modulated by various microtubule-associated proteins (MAPs) and motor proteins, some of which act to promote microtubule assembly and stability, whereas others induce their depolymerization. Although many MAPs bind along the length of microtubules, two classes of MAPs localize selectively to the plus ends of growing microtubules: the Cap-Gly proteins (e.g., CLIP-170, p150glued subunit of dynactin) and the EB1 protein family (Schuyler, 2001). The mechanism by which these proteins interact selectively with microtubule plus ends and their biological roles are poorly understood. Current work, however, suggests that microtubule plus end-binding proteins mediate interactions between microtubule ends and the cell cortex, kinetochores, endosomes, and dynein motor complexes (Tirnauer, 2000; Schuyler, 2001; Rogers, 2002 and references therein).
EB1 was first discovered in a yeast two-hybrid screen for proteins that interact with the human adenomatous polyposis coli (APC) tumor suppressor protein (Su, 1995). Homologous proteins have been identified subsequently in many organisms including budding and fission yeast, Drosophila, and Caenorhabditis elegans (Tirnauer, 2000). The budding yeast EB1 homologue, BIM1, has received the most attention to date. In yeast, Bim1p is a nonessential gene product that performs at least three related functions: (1) it localizes to the plus ends of cytoplasmic microtubules, where it increases dynamic instability (Tirnauer, 1999); (2) Bim1p links microtubule ends to the cell cortex to facilitate orientation of the spindle toward the bud site by binding to a multiprotein complex containing Kar9 and myosin (Myo2p) (Korinek, 2000; Lee, 2000; Miller, 2000; Yin, 2000), and (3) through its participation in spindle orientation, Bim1p indirectly participates in a checkpoint that delays cytokinesis pending mitotic exit (Muhua, 1998). A mitotic function also has been assigned to the EB1 homologue Mal3 in (Beinhauer, 1997) Schizosaccharomyces pombe (Rogers, 2002).
In higher eukaryotes, the functions of EB1 proteins remain poorly understood. In epithelial cells of the early Drosophila embryo, EB1 is required to direct the axis of cell division (Lu, 2001), although the mechanism by which it performs this function was not resolved. In vertebrate cells, the only activity attributed to EB1 is its ability to bind the COOH terminus of the APC tumor suppressor protein (see Drosophila APC) and target it to the tips of growing microtubules (Mimori-Kiyosue, 2000a; Mimori-Kiyosue, 2000b). The functional significance of these interactions has not been ascertained, although truncations of the COOH-terminal EB1 binding domain of APC are frequently associated with sporadic and familial colorectal cancers (Rogers, 2002).
Given the high degree of evolutionary conservation, EB1 proteins very likely perform important functions in higher eukaryotes. However, given that budding yeast and higher eukaryotes exhibit considerable differences both in their interphase microtubule organization and in their mechanisms of mitosis, extrapolating results from yeast BIM1 to metazoan cells becomes precarious. This study investigated the role of EB1 in Drosophila cells in culture by decreasing EB1 protein levels using RNA-mediated inhibition (RNAi) technology and in Drosophila embryos by injecting antibodies against EB1. These complementary techniques and preparations have demonstrated that EB1 influences microtubule dynamics and plays a particularly critical role in the assembly, dynamics, and positioning of the mitotic spindle. Interference of EB1 function in these metazoan cells shows similar yet distinct phenotypes from those described in lower eukaryotes (Rogers, 2002).
Thus interfering with EB1 produces effects on spindle structure, the most common being an overall decrease in the length of the spindle, a lack of astral microtubules, a defocusing of the spindle poles, and dissociation of centrosomes from the spindle. Mitotic spindles in EB1-depleted cells lose the ability to self-center and are generally mispositioned within the cell. In living Drosophila embryos, where spindles proceed into anaphase without a checkpoint arrest, it was found that EB1 plays an important role late in mitosis during anaphase spindle elongation and chromosome segregation (Rogers, 2002).
During interphase, loss of EB1 does not alter microtubule length or distribution and produces no obvious effect on cell morphology. However, by imaging GFP-tubulin, it was found that loss of EB1 causes the majority (60%) of microtubules to enter a 'paused' state in which they are neither growing nor shrinking. Microtubules assembled from purified tubulin rarely exhibit such static behavior. Therefore, pausing most likely reflects the action of a cellular factor that suppresses microtubule dynamics, possibly by capping the microtubule end. EB1 appears to promote dynamic behavior, at least in part, by antagonizing the actions of this yet unknown factor, either by directly competing for tubulin sites or by inducing a conformation at the microtubule end that prevents capping. Interestingly, these findings are similar to those obtained for Bim1p in S. cerevisiae, which show that microtubules in bim1-null cells are less dynamic in G1 of the cell cycle, spending >60% of their lifetimes in a paused state (Tirnauer, 1999). Although these effects of EB1 on interphase microtubule dynamics are not crucial to the formation of the microtubule network in S2 cells, it is speculated that they may be important for dynamic rearrangements of the microtubule cytoskeletal network that occur during cell migration and other polarized cell shape changes (Rogers, 2002).
Although EB1 loss does not dramatically change the number of microtubules during interphase, it does decrease microtubule lengths and numbers in mitosis. In a study of microtubule dynamics at the G2/M transition in vertebrate cells, is has been observed that microtubule polymer levels dramatically decrease upon entry into prophase, but polymer levels increase as mitosis progresses and chromosomes become attached to microtubules. In EB1-depleted S2 cells, the extent of microtubule disassembly in prophase is more severe than in control cells, and may reflect an inability of the cell to reestablish microtubule polymer levels later in mitosis. This decrease in microtubule polymer was not observed with RNAi of another plus end-binding protein, CLIP-190. The basis for the mitotic-specific effect of EB1 on microtubule stability may be due either to a change in how EB1 interacts with microtubules or in the activities of other microtubule-associated proteins. The latter possibility is favored because it has been shown that assembly-promoting factors, such as XMAP215/TOG, are downregulated in mitosis, which allows depolymerization factors, such as the KIN-I kinesins or stathmin/OP18, to predominate. Thus, EB1 may play a particularly important role in mitosis in counteracting microtubule depolymerization factors. The most direct way to test these ideas would be to observe microtubule behavior in EB1-depleted mitotic cells in real time. However, due to the loss of astral microtubules and the bundling of microtubules in the interpolar regions, it was not possible to resolve the behavior of individual microtubules in GFP-tubulin-transfected, EB1-depleted cells (Rogers, 2002).
This finding for EB1 differs from that obtained for Bim1p in S. cerevisiae, since bim1Delta cells do not exhibit significant defects in microtubule behavior in preanaphase or anaphase, even though there are subtle changes in microtubule dynamics and spindle positioning (Tirnauer, 1999). The role of EB1 also differs from its orthologue Mal3 in fission yeast, as null mutants exhibit abnormally short cytoplasmic microtubules but no defects in their spindle morphology. These differences may not be due to different molecular mechanisms of EB1, but rather due to the distinct processes for creating the spindle and executing chromosome movements (Rogers, 2002).
The most frequent phenotype observed in EB1-depleted mitotic spindles is the failure to form astral microtubules, which may underlie many of the aberrant spindle phenotypes produced by EB1 RNAi- and antibody-injected embryos, although other unknown roles of EB1 (e.g., interactions with other proteins) may play a role as well. In the absence of EB1, the central spindle still contains kinetochore fibers, often they are partially or fully detached from the centrosomes, which gives rise to defocused or 'splayed apart' microtubules at the poles. In wild-type spindles, it is speculated that astral microtubules nucleated from the spindle poles intercalate with microtubule bundles in the central spindle to focus them to the poles via microtubule cross-linking proteins or through motor proteins such as Ncd or cytoplasmic dynein (Rogers, 2002).
Loss of astral microtubules is also likely to underlie the spindle positioning defects that were observe in EB1-depleted cells. Spindle positioning has been speculated to involve a balancing of forces generated either by growing astral microtubules pushing against the cell cortex or by cortically bound motor complexes containing dynein and Lis1 pulling on astral microtubules. Similarly, yeast Bim1p has been shown to be important in orienting the mitotic spindle into the bud neck by linking microtubules to the cortically bound Kar9p complex and the actin cytoskeleton (for review see Bloom, 2000). Mammalian EB1 also has been shown to interact with dynein intermediate chain and with subunits of the dynactin complex (Berrueta, 1999), and so it may mediate motor microtubule linkages at the plasma membrane of higher eukaryotes as well (Rogers, 2002).
These results also shed light upon the recent observations of Lu (2001) who demonstrated that EB1 is required for spindle orientation in epidermoblasts of the Drosophila embryo. In this cell type, cell divisions are normally oriented within the plane of the tissue in response to lateral polarity cues established by adherens junctions formed between neighboring cells. When EB1 was reduced by RNAi, epidermal cells instead divide randomly with respect to the plane of the tissue. It was generally assumed that this effect was due to impaired interactions of microtubules with adherens junction components that served as polarity cues. Although this may be true, the current results also reveal a drastic reduction in the number and length of astral microtubules that also may underlie the defect observed in these asymmetric cell divisions (Rogers, 2002).
Inhibition of EB1 in synctitial Drosophila embryos by injection of anti-EB1 antibodies also revealed important roles for this protein during the later stages of mitosis. In these cells, the most severe mitotic defects were observed closest to the injection site, and these included dramatically reduced rates of spindle elongation throughout mitosis and defective chromosome segregation. Spindles distal to the injection site exhibited less severe structural defects, but also exhibited lagging chromosomes during anaphase. These phenotypes were not directly observed in S2 cells depleted of EB1, and it is postulated that this is due to activation of the spindle checkpoint as the result of damage to the spindle (Rogers, 2002).
Why do mitotic spindles fail to elongate during anaphase? The forces that drive spindle elongation during anaphase B are derived, at least in part, from the activities of cortical cytoplasmic dynein pulling on astral microtubules and from bipolar kinesins that push spindle poles apart by sliding antiparallel interpolar microtubule bundles. Inhibition of EB1 suppresses the formation of both astral microtubules and interpolar microtubules and eliminates the formation of midbodies during late telophase. A role for EB1 in the formation or stabilization of these subpopulations of spindle microtubules is supported by immunolocalization data showing the protein enriched on astral microtubules and in interpolar bundles and midbodies in S2 cells and embryos. The inhibition of anaphase after EB1 depletion may be a consequence of the failure to produce spindles that form the specialized microtubule structures required for elongation in anaphase. Another possible mechanism is suggested by the observation that anaphase B is accompanied by microtubule polymerization in the central spindle that may contribute to the forces that drive spindle poles apart. Since EB1 appears necessary to promote microtubule growth during mitosis, it may be that in the absence of this protein, anaphase microtubule polymerization is inhibited and spindle elongation fails. These two potential mechanisms are not mutually exclusive (Rogers, 2002).
The question of why chromosome segregation fails when EB1 is inhibited is also an important one. The simplest explanation is that, in the absence of EB1, spindle elongation during anaphase is crippled to such an extent that chromosome-to-pole movement is insufficient to drive their segregation, leading to an increased number of 4N nuclei. Alternatively, it is possible that EB1 mediates interactions between kinetochores and microtubules and in the absence of this interaction, anaphase A is affected. This is an interesting possibility in light of recent work identifying APC as a kinetochore component, although no evidence exists for a direct interaction between Drosophila APC/APC2 and EB1 (Rogers, 2002).
In conclusion, these studies reveal that EB1 is not essential for creating the microtubule network in interphase but is essential for microtubule organization in mitosis. Such cell cycle specificity, which is not common among MAPs, raises the possibility that EB1 might constitute an attractive target for small molecule inhibition of cell division in cancer chemotherapy. At least three different genes for EB1 family proteins exist in the human genome; one of which is ubiquitously expressed, and the other two are tissue specific. Selective inhibition of these mammalian genes will be required to evaluate the utility of EB1 inhibition as means of interfering with cancer growth (Rogers, 2002).
To begin analysis of EB1 function in Drosophila, the fly genome was examined for genes that exhibited homology to human EB1 (MAPRE1) (Su, 2001). The Drosophila genome contains four predicted gene products that encode proteins with a high degree of sequence similarity (>40%) to human EB1: genes CG3265, CG18190, CG15306, and CG2955. One gene (CG3265, termed here Drosophila EB1) exhibits a higher degree of sequence identity throughout its length to both human EB1 (52%) and Saccharomyces cerevisiae Bim1p (33%), making it the most likely orthologue. This gene encodes a predicted protein of 294 residues (32.5 kD) with a similar domain organization to human EB1 and Bim1p. Residues 1-134, which have the highest degree of sequence conservation among EB1 family members, constitutes the domain of the protein implicated in microtubule binding (Juwana, 1999). Residues 129-212 are enriched in serines and prolines and hence may be unstructured, and residues 213-273 are predicted to form a coiled coil. In Bim1p, the COOH-terminal coiled-coil domain binds to Kar9 (Miller, 2000), and it may mediate protein-protein interactions in other species as well (Rogers, 2002).
date revised: 15 August 2007
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