The spindle positioning protein Kar9p interacts with the sumoylation machinery in Saccharomyces cerevisiae

Accurate positioning of the mitotic spindle is important for the genetic material to be distributed evenly in dividing cells, but little is known about the mechanisms that regulate this process. Here we report that two microtubule-associated proteins important for spindle positioning interact with several proteins in the sumoylation pathway. By two-hybrid analysis, Kar9p and Bim1p interact with the yeast SUMO Smt3p, the E2 enzyme Ubc9p, an E3 Nfi1p, as well as Wss1p, a weak suppressor of a temperature-sensitive smt3 allele. The physical interaction between Kar9p and Ubc9p was confirmed by in vitro binding assays. A single-amino-acid substitution in Kar9p, L304P disrupted its two-hybrid interaction with proteins in the sumoylation pathway, but retained its interactions with the spindle positioning proteins Bim1p, Stu2p, Bik1p, and Myo2p. The kar9-L304P mutant showed defects in positioning the mitotic spindle, with the spindle located more distally than normal. Whereas wild-type Kar9p-3GFP normally localizes to only the bud-directed spindle pole body (SPB), Kar9p-L304P-3GFP was mislocalized to both SPBs. Using a reconstitution assay, Kar9p was sumoylated in vitro. We propose a model in which sumoylation regulates spindle positioning by restricting Kar9p to one SPB. These findings raise the possibility that sumoylation could regulate other microtubule-dependent processes.     

Bim1p/Yeb1p mediates the Kar9p-dependent cortical attachment of cytoplasmic microtubules

In Saccharomyces cerevisiae, positioning of the mitotic spindle depends on the interaction of cytoplasmic microtubules with the cell cortex. In this process, cortical Kar9p in the bud acts as a link between the actin and microtubule cytoskeletons. To identify Kar9p-interacting proteins, a two-hybrid screen was conducted with the use of full-length Kar9p as bait, and three genes were identified: BIM1, STU2, and KAR9 itself. STU2 encodes a component of the spindle pole body. Bim1p is the yeast homologue of the human microtubule-binding protein EB1, which is a binding partner to the adenomatous polyposis coli protein involved in colon cancer. Eighty-nine amino acids within the third quarter of Bim1p was sufficient to confer interaction with Kar9p. The two-hybrid interactions were confirmed with the use of coimmunoprecipitation experiments. Genetic analysis placed Bim1p in the Kar9p pathway for nuclear migration. Bim1p was not required for Kar9p's cortical or spindle pole body localization. However, deletion of BIM1 eliminated Kar9p localization along cytoplasmic microtubules. Furthermore, in the bim1 mutants, the cytoplasmic microtubules no longer intersected the cortical dot of Green Fluorescent Protein-Kar9p. These experiments demonstrate that the interaction of cytoplasmic microtubules with the Kar9p cortical attachment site requires the microtubule-binding protein Bim1p.     

The cortical localization of the microtubule orientation protein, Kar9p, is dependent upon actin and proteins required for polarization

In the yeast Saccharomyces cerevisiae, positioning of the mitotic spindle requires both the cytoplasmic microtubules and actin. Kar9p is a novel cortical protein that is required for the correct position of the mitotic spindle and the orientation of the cytoplasmic microtubules. Green fluorescent protein (GFP)- Kar9p localizes to a single spot at the tip of the growing bud and the mating projection. However, the cortical localization of Kar9p does not require microtubules (Miller, R.K., and M.D. Rose. 1998. J. Cell Biol. 140: 377), suggesting that Kar9p interacts with other proteins at the cortex. To investigate Kar9p's cortical interactions, we treated cells with the actin-depolymerizing drug, latrunculin-A. In both shmoos and mitotic cells, Kar9p's cortical localization was completely dependent on polymerized actin. Kar9p localization was also altered by mutations in four genes, spa2Delta, pea2Delta, bud6Delta, and bni1Delta, required for normal polarization and actin cytoskeleton functions and, of these, bni1Delta affected Kar9p localization most severely. Like kar9Delta, bni1Delta mutants exhibited nuclear positioning defects during mitosis and in shmoos. Furthermore, like kar9Delta, the bni1Delta mutant exhibited misoriented cytoplasmic microtubules in shmoos. Genetic analysis placed BNI1 in the KAR9 pathway for nuclear migration. However, analysis of kar9Delta bni1Delta double mutants suggested that Kar9p retained some function in bni1Delta mitotic cells. Unlike the polarization mutants, kar9Delta shmoos had a normal morphology and diploids budded in the correct bipolar pattern. Furthermore, Bni1p localized normally in kar9Delta. We conclude that Kar9p's function is specific for cytoplasmic microtubule orientation and that Kar9p's role in nuclear positioning is to coordinate the interactions between the actin and microtubule networks.     

The role of the proteins Kar9 and Myo2 in orienting the mitotic spindle of budding yeast

BACKGROUND: Two genetic 'pathways' contribute to the fidelity of nuclear segregation during the process of budding in the yeast Saccharomyces cerevisiae. An early pathway, involving Kar9p and other proteins, orients the mitotic spindle along the mother-bud axis. Upon the onset of anaphase, cytoplasmic dynein provides the motive force for nuclear movement into the bud. Loss of either pathway results in nuclear-migration defects; loss of both is lethal. Here, to visualize the functional steps leading to correct spindle orientation along the mother-bud axis, we imaged live yeast cells expressing Kar9p and dynein as green fluorescent protein fusions. RESULTS: Transport of Kar9p into the bud was found to require the myosin Myo2p. Kar9p interacted with microtubules through the microtubule-binding protein Bim1p and facilitated microtubule penetration into the bud. Once microtubules entered the bud, Kar9p provided a platform for microtubule capture at the bud cortex. Kar9p was also observed at sites of microtubule shortening in the bud, suggesting that Kar9p couples microtubule shortening to nuclear migration. CONCLUSIONS: Thus, Kar9p provides a key link between the actin cytoskeleton and microtubules early in the cell cycle. A cooperative mechanism between Kar9p and Myo2p facilitates the pre-anaphase orientation of the spindle. Later, Kar9p couples microtubule disassembly with nuclear migration.     

Spindle orientation in Saccharomyces cerevisiae depends on the transport of microtubule ends along polarized actin cables

Microtubules and actin filaments interact and cooperate in many processes in eukaryotic cells, but the functional implications of such interactions are not well understood. In the yeast Saccharomyces cerevisiae, both cytoplasmic microtubules and actin filaments are needed for spindle orientation. In addition, this process requires the type V myosin protein Myo2, the microtubule end-binding protein Bim1, and Kar9. Here, we show that fusing Bim1 to the tail of the Myo2 is sufficient to orient spindles in the absence of Kar9, suggesting that the role of Kar9 is to link Myo2 to Bim1. In addition, we show that Myo2 localizes to the plus ends of cytoplasmic microtubules, and that the rate of movement of these cytoplasmic microtubules to the bud neck depends on the intrinsic velocity of Myo2 along actin filaments. These results support a model for spindle orientation in which a Myo2-Kar9-Bim1 complex transports microtubule ends along polarized actin cables. We also present data suggesting that a similar process plays a role in orienting cytoplasmic microtubules in mating yeast cells.     

The microtubule-based motor Kar3 and plus end-binding protein Bim1 provide structural support for the anaphase spindle

In budding yeast, the mitotic spindle is comprised of 32 kinetochore microtubules (kMTs) and ~8 interpolar MTs (ipMTs). Upon anaphase onset, kMTs shorten to the pole, whereas ipMTs increase in length. Overlapping MTs are responsible for the maintenance of spindle integrity during anaphase. To dissect the requirements for anaphase spindle stability, we introduced a conditionally functional dicentric chromosome into yeast. When centromeres from the same sister chromatid attach to opposite poles, anaphase spindle elongation is delayed and a DNA breakage-fusion-bridge cycle ensues that is dependent on DNA repair proteins. We find that cell survival after dicentric chromosome activation requires the MT-binding proteins Kar3p, Bim1p, and Ase1p. In their absence, anaphase spindles are prone to collapse and buckle in the presence of a dicentric chromosome. Our analysis reveals the importance of Bim1p in maintaining a stable ipMT overlap zone by promoting polymerization of ipMTs during anaphase, whereas Kar3p contributes to spindle stability by cross-linking spindle MTs.     

Gamma-tubulin is required for proper recruitment and assembly of Kar9-Bim1 complexes in budding yeast

Microtubule plus-end-interacting proteins (+TIPs) promote the dynamic interactions between the plus ends (+ends) of astral microtubules and cortical actin that are required for preanaphase spindle positioning. Paradoxically, +TIPs such as the EB1 orthologue Bim1 and Kar9 also associate with spindle pole bodies (SPBs), the centrosome equivalent in budding yeast. Here, we show that deletion of four C-terminal residues of the budding yeast gamma-tubulin Tub4 (tub4-delta dsyl) perturbs Bim1 and Kar9 localization to SPBs and Kar9-dependent spindle positioning. Surprisingly, we find Kar9 localizes to microtubule +ends in tub4-delta dsyl cells, but these microtubules fail to position the spindle when targeted to the bud. Using cofluorescence and coaffinity purification, we show Kar9 complexes in tub4-delta dsyl cells contain reduced levels of Bim1. Astral microtubule dynamics is suppressed in tub4-delta dsyl cells, but it are restored by deletion of Kar9. Moreover, Myo2- and F-actin-dependent dwelling of Kar9 in the bud is observed in tub4-delta dsyl cells, suggesting defective Kar9 complexes tether microtubule +ends to the cortex. Overproduction of Bim1, but not Kar9, restores Kar9-dependent spindle positioning in the tub4-delta dsyl mutant, reduces cortical dwelling, and promotes Bim1-Kar9 interactions. We propose that SPBs, via the tail of Tub4, promote the assembly of functional +TIP complexes before their deployment to microtubule +ends.     

The CLIP-170 homologue Bik1p promotes the phosphorylation and asymmetric localization of Kar9p

Accurate positioning of the mitotic spindle in Saccharomyces cerevisiae is coordinated with the asymmetry of the two poles and requires the microtubule-to-actin linker Kar9p. The asymmetric localization of Kar9p to one spindle pole body (SPB) and microtubule (MT) plus ends requires Cdc28p. Here, we show that the CLIP-170 homologue Bik1p binds directly to Kar9p. In the absence of Bik1p, Kar9p localization is not restricted to the daughter-bound SPB, but it is instead found on both SPBs. Kar9p is hypophosphorylated in bik1delta mutants, and Bik1p binds to both phosphorylated and unphosphorylated isoforms of Kar9p. Furthermore, the two-hybrid interaction between full-length KAR9 and the cyclin CLB5 requires BIK1. The binding site of Clb5p on Kar9p maps to a short region within the basic domain of Kar9p that contains a conserved phosphorylation site, serine 496. Consistent with this, Kar9p is found on both SPBs in clb5delta mutants at a frequency comparable with that seen in kar9-S496A strains. Together, these data suggest that Bik1p promotes the phosphorylation of Kar9p on serine 496, which affects its asymmetric localization to one SPB and associated cytoplasmic MTs. These findings provide further insight into a mechanism for directing centrosomal inheritance.     

Yeast Cdk1 translocates to the plus end of cytoplasmic microtubules to regulate bud cortex interactions

The budding yeast spindle aligns along the mother- bud axis through interactions between cytoplasmic microtubules (CMs) and the cell cortex. Kar9, in complex with the EB1-related protein Bim1, mediates contacts of CMs with the cortex of the daughter cell, the bud. Here we established a novel series of events that target Kar9 to the bud cortex. First, Kar9 binds to spindle pole bodies (SPBs) in G(1) of the cell cycle. Secondly, in G(1)/S the yeast Cdk1, Cdc28, associates with SPBs and phosphorylates Kar9. Thirdly, Kar9 and Cdc28 then move from the SPB to the plus end of CMs directed towards the bud. This movement is dependent upon the microtubule motor protein Kip2. Cdc28 activity is required to concentrate Kar9 at the plus end of CMs and hence to establish contacts with the bud cortex. The Cdc28-regulated localization of Kar9 is therefore an integral part of the program that aligns spindles.     

The MEN mediates the effects of the spindle assembly checkpoint on Kar9-dependent spindle pole body inheritance in budding yeast

Many asymmetrically dividing cells segregate the poles of the mitotic spindle non-randomly between their two daughters. In budding yeast, the protein Kar9 localizes almost exclusively to the astral microtubules emanating from the old spindle pole body (SPB) and promotes its movement toward the bud. Thereby, Kar9 orients the spindle relative to the division axis. Here, we show that beyond perturbing Kar9 distribution, activation of the spindle assembly checkpoint (SAC) randomizes SPB inheritance. Inactivation of the B-type cyclin Clb5 led to a SAC-dependent defect in Kar9 orientation and SPB segregation. Furthermore, unlike the Clb4-dependent pathway, the Clb5- and SAC-dependent pathways functioned genetically upstream of the mitotic exit network (MEN) in SPB specification and Kar9-dependent SPB inheritance. Together, our study indicates that Clb5 functions in spindle assembly and that the SAC controls the specification and inheritance of yeast SPBs through inhibition of the MEN.     

The MEN mediates the effects of the spindle assembly checkpoint on Kar9-dependent spindle pole body inheritance in budding yeast

Many asymmetrically dividing cells segregate the poles of the mitotic spindle non-randomly between their two daughters. In budding yeast, the protein Kar9 localizes almost exclusively to the astral microtubules emanating from the old spindle pole body (SPB) and promotes its movement toward the bud. Thereby, Kar9 orients the spindle relative to the division axis. Here, we show that beyond perturbing Kar9 distribution, activation of the spindle assembly checkpoint (SAC) randomizes SPB inheritance. Inactivation of the B-type cyclin Clb5 led to a SAC-dependent defect in Kar9 orientation and SPB segregation. Furthermore, unlike the Clb4-dependent pathway, the Clb5- and SAC-dependent pathways functioned genetically upstream of the mitotic exit network (MEN) in SPB specification and Kar9-dependent SPB inheritance. Together, our study indicates that Clb5 functions in spindle assembly and that the SAC controls the specification and inheritance of yeast SPBs through inhibition of the MEN.     

Differential contribution of Bud6p and Kar9p to microtubule capture and spindle orientation in S. cerevisiae

In Saccharomyces cerevisiae, spindle orientation is controlled by a temporal and spatial program of microtubule (MT)-cortex interactions. This program requires Bud6p/Aip3p to direct the old pole to the bud and confine the new pole to the mother cell. Bud6p function has been linked to Kar9p, a protein guiding MTs along actin cables. Here, we show that Kar9p does not mediate Bud6p functions in spindle orientation. Based on live microscopy analysis, kar9Delta cells maintained Bud6p-dependent MT capture. Conversely, bud6Delta cells supported Kar9p-associated MT delivery to the bud. Moreover, additive phenotypes in bud6Delta kar9Delta or bud6Delta dyn1Delta mutants underscored the separate contributions of Bud6p, Kar9p, and dynein to spindle positioning. Finally, tub2C354S, a mutation decreasing MT dynamics, suppressed a kar9Delta mutation in a BUD6-dependent manner. Thus, Kar9p-independent capture at Bud6p sites can effect spindle orientation provided MT turnover is reduced. Together, these results demonstrate Bud6p function in MT capture at the cell cortex, independent of Kar9p-mediated MT delivery along actin cables.     

Slk19p is a centromere protein that functions to stabilize mitotic spindles.

We have identified a novel centromere-associated gene product from Saccharomyces cerevisiae that plays a role in spindle assembly and stability. Strains with a deletion of SLK19 (synthetic lethal Kar3p gene) exhibit abnormally short mitotic spindles, increased numbers of astral microtubules, and require the presence of the kinesin motor Kar3p for viability. When cells are deprived of both Slk19p and Kar3p, rapid spindle breakdown and mitotic arrest is observed. A functional fusion of Slk19p to green fluorescent protein (GFP) localizes to kinetochores and, during anaphase, to the spindle midzone, whereas Kar3p-GFP was found at the nuclear side of the spindle pole body. Thus, these proteins seem to play overlapping roles in stabilizing spindle structure while acting from opposite ends of the microtubules.     

The APC-associated protein EB1 associates with components of the dynactin complex and cytoplasmic dynein intermediate chain

Human EB1 is a highly conserved protein that binds to the carboxyl terminus of the human adenomatous polyposis coli (APC) tumor suppressor protein [1], a domain of APC that is commonly deleted in colorectal neoplasia [2]. EB1 belongs to a family of microtubule-associated proteins that includes Schizosaccharomyces pombe Mal3 [3] and Saccharomyces cerevisiae Bim1p [4]. Bim1p appears to regulate the timing of cytokinesis as demonstrated by a genetic interaction with Act5, a component of the yeast dynactin complex [5]. Whereas the predominant function of the dynactin complex in yeast appears to be in positioning the mitotic spindle [6], in animal cells, dynactin has been shown to function in diverse processes, including organelle transport, formation of the mitotic spindle, and perhaps cytokinesis [7] [8] [9] [10]. Here, we demonstrate that human EB1 can be coprecipitated with p150(Glued), a member of the dynactin protein complex. EB1 was also found associated with the intermediate chain of cytoplasmic dynein (CDIC) and with dynamitin (p50), another component of the dynactin complex, but not with dynein heavy chain, in a complex that sedimented at approximately 5S in a sucrose density gradient. The association of EB1 with members of the dynactin complex was independent of APC and was preserved in the absence of an intact microtubule cytoskeleton. The molecular interaction of EB1 with members of the dynactin complex and with CDIC may be important for microtubule-based processes.     

Asymmetric loading of Kar9 onto spindle poles and microtubules ensures proper spindle alignment

Spindle alignment is the process in which the two spindle poles are directed toward preselected and opposite cell ends. In budding yeast, the APC-related molecule Kar9 is required for proper alignment of the spindle with the mother-bud axis. We find that Kar9 localizes to the prospective daughter cell spindle pole. Kar9 is transferred from the pole to cytoplasmic microtubules, which are then guided in a myosin-dependent manner to the bud. Clb4/Cdc28 kinase phosphorylates Kar9 and accumulates on the pole destined to the mother cell. Mutations that block phosphorylation at Cdc28 consensus sites result in localization of Kar9 to both poles and target them both to the bud. Thus, Clb4/Cdc28 prevents Kar9 loading on the mother bound pole. In turn, asymmetric distribution of Kar9 ensures that only one pole orients toward the bud. Our results indicate that Cdk1-dependent spindle asymmetry ensures proper alignment of the mitotic spindle with the cell division axis.     

The Cdc31p-binding protein Kar1p is a component of the half bridge of the yeast spindle pole body

KAR1 has been identified as an essential gene which is involved in karyogamy of mating yeast cells and in spindle pole body duplication of mitotic cells (Rose, M. D., and G. R. Fink. 1987. Cell. 48:1047-1060). We investigated the cell cycle-dependent localization of the Kar1 protein (Kar1p) and its interaction with other SPB components. Kar1p is associated with the spindle pole body during the entire cell cycle of yeast. Immunoelectron microscopic studies with anti-Kar1p antibodies or with the monoclonal antibody 12CA5 using an epitope-tagged, functional Kar1p revealed that Kar1p is associated with the half bridge or the bridge of the spindle pole body. Cdc31p, a Ca(2+)-binding protein, was previously identified as the first component of the half bridge of the spindle pole body (Spang, A., I. Courtney, U. Fackler, M. Matzner, and E. Schiebel. 1993. J. Cell Biol. 123:405-416). Using an in vitro assay we demonstrate that Cdc31p specifically interacts with a short sequence within the carboxyl terminal half of Kar1p. The potential Cdc31p- binding sequence of Kar1p contains three acidic amino acids which are not found in calmodulin-binding peptides, explaining the different substrate specificities of Cdc31p and calmodulin. Cdc31p was also able to bind to the carboxy terminus of Nuflp/Spc110p, another component of the SPB (Kilmartin, J. V., S. L. Dyos, D. Kershaw, and J. T. Finch. 1993. J. Cell Biol. 123:1175-1184). The association of Kar1p with the spindle pole body was independent of Cdc31p. Cdc31p, on the other hand, was not associated with SPBs of kar1 cells.     

Spindle pole bodies exploit the mitotic exit network in metaphase to drive their age-dependent segregation

Like many asymmetrically dividing cells, budding yeast segregates mitotic spindle poles nonrandomly between mother and daughter cells. During metaphase, the spindle positioning protein Kar9 accumulates asymmetrically, localizing specifically to astral microtubules emanating from the old spindle pole body (SPB) and driving its segregation to the bud. Here, we show that the SPB component Nud1/centriolin acts through the mitotic exit network (MEN) to specify asymmetric SPB inheritance. In the absence of MEN signaling, Kar9 asymmetry is unstable and its preference for the old SPB is disrupted. Consistent with this, phosphorylation of Kar9 by the MEN kinases Dbf2 and Dbf20 is not required to break Kar9 symmetry but is instead required to maintain stable association of Kar9 with the old SPB throughout metaphase. We propose that MEN signaling links Kar9 regulation to SPB identity through biasing and stabilizing the age-insensitive, cyclin-B-dependent mechanism of symmetry breaking.     

Mutation of Ser172 in yeast β tubulin induces defects in microtubule dynamics and cell division

Ser172 of β tubulin is an important residue that is mutated in a human brain disease and phosphorylated by the cyclin-dependent kinase Cdk1 in mammalian cells. To examine the role of this residue, we used the yeast S. cerevisiae as a model and produced two different mutations (S172A and S172E) of the conserved Ser172 in the yeast β tubulin Tub2p. The two mutants showed impaired cell growth on benomyl-containing medium and at cold temperatures, altered microtubule (MT) dynamics, and altered nucleus positioning and segregation. When cytoplasmic MT effectors Dyn1p or Kar9p were deleted in S172A and S172E mutants, cells were viable but presented increased ploidy. Furthermore, the two β tubulin mutations exhibited synthetic lethal interactions with Bik1p, Bim1p or Kar3p, which are effectors of cytoplasmic and spindle MTs. In the absence of Mad2p-dependent spindle checkpoint, both mutations are deleterious. These findings show the importance of Ser172 for the correct function of both cytoplasmic and spindle MTs and for normal cell division.     

Nud1p links astral microtubule organization and the control of exit from mitosis

The budding yeast spindle pole body (SPB) not only organizes the astral and nuclear microtubules but is also associated with a number of cell-cycle regulators that control mitotic exit. Here, we describe that the core SPB component Nud1p is a key protein that functions in both processes. The astral microtubule organizing function of Nud1p is mediated by its interaction with the gamma-tubulin complex binding protein Spc72p. This function of Nud1p is distinct from its role in cell-cycle control: Nud1p binds the spindle checkpoint control proteins Bfa1p and Bub2p to the SPB, and is part of the mitotic exit network (MEN) in which it functions upstream of CDC15 but downstream of LTE1. In conditional lethal nud1-2 cells, the MEN component Tem1p, a GTPase, is mislocalized, whereas the kinase Cdc15p is still associated with the SPB. Thus, in nud1-2 cells the failure of Tem1p to interact with Cdc15p at the SPB probably prevents mitotic exit.     

The Saccharomyces cerevisiae homolog of p24 is essential for maintaining the association of p150Glued with the dynactin complex

Stu1 is the Saccharomyces cerevisiae member of the CLASP family of microtubule plus-end tracking proteins and is essential for spindle formation. A genomewide screen for gene deletions that are lethal in combination with the temperature-sensitive stu1-5 allele identified ldb18Delta. ldb18Delta cells exhibit defects in spindle orientation similar to those caused by a block in the dynein pathway. Consistent with this observation, ldb18Delta is synthetic lethal with mutations affecting the Kar9 spindle orientation pathway, but not with those affecting the dynein pathway. We show that Ldb18 is a component of dynactin, a complex required for dynein activity in yeast and mammalian cells. Ldb18 shares modest sequence and structural homology with the mammalian dynactin component p24. It interacts with dynactin proteins in two-hybrid and co-immunoprecipitation assays, and comigrates with them as a 20 S complex during sucrose gradient sedimentation. In ldb18Delta cells, the interaction between Nip100 (p150(Glued)) and Jnm1 (dynamitin) is disrupted, while the interaction between Jnm1 and Arp1 is not affected. These results indicate that p24 is required for attachment of the p150(Glued) arm to dynamitin and the remainder of the dynactin complex. The genetic interaction of ldb18Delta with stu1-5 also supports the notion that dynein/dynactin helps to generate a spindle pole separating force.