H3 Thr3 phosphorylation is crucial for meiotic resumption and anaphase onset in oocyte meiosis

Haspin-catalyzed histone H3 threonine 3 (Thr3) phosphorylation facilitates chromosomal passenger complex (CPC) docking at centromeres, regulating indirectly chromosome behavior during somatic mitosis. It is not fully known about the expression and function of H3 with phosphorylated Thr3 (H3T3-P) during meiosis in oocytes. In this study, we investigated the expression and sub-cellular distribution of H3T3-P, as well as its function in mouse oocytes during meiotic division. Western blot analysis revealed that H3T3-P expression was only detected after germinal vesicle breakdown (GVBD), and gradually increased to peak level at metaphase I (MI), but sharply decreased at metaphase II (MII). Immunofluorescence showed H3T3-P was only brightly labeled on chromosomes after GVBD, with relatively high concentration across the whole chromosome axis from pro-metaphase I (pro-MI) to MI. Specially, H3T3-P distribution was exclusively limited to the local space between sister centromeres at MII stage. Haspin inhibitor, 5-iodotubercidin (5-ITu), dose- and time-dependently blocked H3T3-P expression in mouse oocytes. H3T3-P inhibition delayed the resumption of meiosis (GVBD) and chromatin condensation. Moreover, the loss of H3T3-P speeded up the meiotic transition to MII of pro-MI oocytes in spite of the presence of non-aligned chromosomes, even reversed MI-arrest induced with Nocodazole. The inhibition of H3T3-P expression distinguishably damaged MAD1 recruitment on centromeres, which indicates the spindle assembly checkpoint was impaired in function, logically explaining the premature onset of anaphase I. Therefore, Haspin-catalyzed histone H3 phosphorylation is essential for chromatin condensation and the following timely transition from meiosis I to meiosis II in mouse oocytes during meiotic division.     

Polo‐like kinase‐1 triggers histone phosphorylation by Haspin in mitosis

Histone modifications coordinate the chromatin localization of key regulatory factors in mitosis. For example, mitotic phosphorylation of Histone H3 threonine‐3 (H3T3ph) by Haspin creates a binding site for the chromosomal passenger complex (CPC). However, how these histone modifications are spatiotemporally controlled during the cell cycle is unclear. Here we show that Plk1 binds to Haspin in a Cdk1‐phosphorylation‐dependent manner. Reducing Plk1 activity decreases the phosphorylation of Haspin and inhibits H3T3ph, particularly in prophase, suggesting that Plk1 is required for initial activation of Haspin in early mitosis. These studies demonstrate that Plk1 can positively regulate CPC recruitment in mitosis.     

Impaired maturation of the siderophore pyoverdine chromophore in Pseudomonas fluorescens ATCC 17400 deficient for the cytochrome c biogenesis protein CcmC

Here we show that during the meiotic maturation of Xenopus oocytes, histone H3 becomes phosphorylated on serine-10 at about the time of maturation promoting factor activation and meiosis I entry. However, overexpression of cAMP-dependent protein kinase that blocks entry into M phase, also leads to massive serine-10 phosphorylation of histone H3 in intact Xenopus oocytes but does not cause chromosome condensation. We also show that the phosphorylation of histone H3 during oocyte maturation requires the activation of the mitogen-activated protein kinase/p90Rsk pathway. Our results indicate that in G2-arrested oocytes, which are about to enter M phase, histone H3 phosphorylation is not sufficient for chromosome condensation.     

AtHaspin phosphorylates histone H3 at threonine 3 during mitosis and contributes to embryonic patterning in Arabidopsis

Post-translational histone modifications regulate many aspects of chromosome activity. Threonine 3 of histone H3 is highly conserved, but the significance of its phosphorylation is unclear, and the identity of the corresponding kinase in plants is unknown. Therefore, we characterized the candidate kinase in Arabidopsis thaliana, called AtHaspin. Recombinant AtHaspin in vitro phosphorylates histone H3 at threonine 3. Reduction of H3 threonine 3 phosphorylation level and reduced chromatin condensation in interphase nuclei by AtHaspin RNAi supports the proposition that this kinase is involved in histone H3 phosphorylation in vivo in mitotic cells. In addition, we provide a developmental function for a Haspin kinase. At the whole plant level, altered expression of the kinase induced pleiotropic phenotypes with defects in floral organs and vascular tissue. It reduced fertility and modified adventitious shoot apical meristems that then gave rise to plants with multi-rosettes and multi-shoots. Haspin mutant embryos frequently showed alteration in division plane orientation that could be traced back to the earliest divisions of embryo development, thus Haspin contributes to embryonic patterning.     

Novel phosphorylation of histone H3 at threonine 11 that temporally correlates with condensation of mitotic and meiotic chromosomes in plant cells

A novel mitosis-specific phosphorylation site in histone H3 at threonine 11 has been described for mammalian cells. This modification is restricted to the centromeric region while phosphorylation at the classical H3 sites, Ser10 and Ser28 occurs along the entire chromosomal arms. Using phosphorylation state-specific antibodies we found that phosphorylation at threonine 11 occurs also in plant cells, during mitosis as well as meiosis. However, in contrast to animal cells, ph(Thr11)H3 was distributed along the entire length of condensed chromosomes, whereas H3 phosphorylated at Ser10 and Ser28 appeared to be restricted to centromeric/pericentromeric chromatin. Phosphorylation at Thr11 started in prophase and ended in telophase, it correlated with the condensation of mitotic and meiotic chromosomes and was independent of the distribution of late replicating heterochromatin and Giemsa-banding positive regions. Interestingly, treatment of cells with the phosphatase inhibitor cantharidin revealed a high level of Thr11 phosphorylation in interphase cells, in this case particularly in pericentromeric regions. These data show that histone modifications are highly dynamic. Moreover, animal and plant organisms may have evolved individual histone codes.     

Cohesion and centromere activity are required for phosphorylation of histone H3 in maize

Haspin‐mediated phosphorylation of histone H3 at threonine 3 (H3T3ph) promotes proper deposition of Aurora B at the inner centromere to ensure faithful chromosome segregation in metazoans. However, the function of H3T3ph remains relatively unexplored in plants. Here, we show that in maize (Zea mays L.) mitotic cells, H3T3ph is concentrated at pericentromeric and centromeric regions. Additional weak H3T3ph signals occur between cohered sister chromatids at prometaphase. Immunostaining on dicentric chromosomes reveals that an inactive centromere cannot maintain H3T3ph at metaphase, indicating that a functional centromere is required for H3T3 phosphorylation. H3T3ph locates at a newly formed centromeric region that lacks detectable CentC sequences and strongly reduced CRM and ZmBs repeat sequences at metaphase II. These results suggest that centromeric localization of H3T3ph is not dependent on centromeric sequences. In maize meiocytes, H3T3 phosphorylation occurs at the late diakinesis and extends to the entire chromosome at metaphase I, but is exclusively limited to the centromere at metaphase II. The H3T3ph signals are absent in the afd1 (absence of first division) and sgo1 (shugoshin) mutants during meiosis II when the sister chromatids exhibit random distribution. Further, we show that H3T3ph is mainly located at the pericentromere during meiotic prophase II but is restricted to the inner centromere at metaphase II. We propose that this relocation of H3T3ph depends on tension at the centromere and is required to promote bi‐orientation of sister chromatids.     

Phosphorylation of mitogen-activated protein kinase is regulated by protein kinase C, cyclic 3',5'-adenosine monophosphate, and protein phosphatase modulators during meiosis resumption in rat oocytes

Mitogen-activated protein (MAP) kinase, protein kinase C (PKC), cAMP, and okadaic acid (OA)-sensitive protein phosphatases (PPs) have been suggested to be involved in oocyte meiotic resumption. However, whether these protein kinases and phosphatases act by independent pathways or interact with each other in regulating meiosis resumption is unknown. In the present study, we aimed to determine the regulation of meiosis resumption and MAP kinase phosphorylation by PKC, cAMP, and OA-sensitive PPs in rat oocytes using an in vitro oocyte maturation system and Western blot analysis. We found that ERK1 and ERK2 isoforms of MAP kinases existed in a dephosphorylated (inactive) form in germinal vesicle breakdown (GVBD)-incompetent and GVBD-competent germinal vesicle intact (GVI) oocytes as well as GVBD oocytes at equivalent levels. These results indicate that MAP kinases are not responsible for the initiation of normal meiotic resumption in rat oocytes. However, when GVBD-incompetent and GVBD-competent oocytes were incubated in vitro for 5 h, MAP kinases were phosphorylated (activated) in GVBD-competent oocytes, but not in meiotic-incompetent oocytes, suggesting that oocytes acquire the ability to phosphorylate MAP kinase during acquisition of meiotic competence. We also found that both meiosis resumption and MAP kinase phosphorylation were inhibited by PKC activation or cAMP elevation. Moreover, these inhibitory effects were overcome by OA, which inhibited PP1/PP2A activities. These results suggest that both cAMP elevation and PKC activation inhibit meiosis resumption and MAP kinase phosphorylation at a step prior to OA-sensitive protein phosphatases. In addition, inhibitory effects of cAMP elevation on meiotic resumption and MAP kinase phosphorylation were not reversed by calphostin C-induced PKC inactivation, indicating that cAMP inhibits both meiotic resumption and MAP kinase activation in a PKC-independent manner.     

Aurora kinase B modulates chromosome alignment in mouse oocytes

The elevated incidence of aneuploidy in human oocytes warrants study of the molecular mechanisms regulating proper chromosome segregation. The Aurora kinases are a well‐conserved family of serine/threonine kinases that are involved in proper chromosome segregation during mitosis and meiosis. Here we report the expression and localization of all three Aurora kinase homologs, AURKA, AURKB, and AURKC, during meiotic maturation of mouse oocytes. AURKA, the most abundantly expressed homolog, localizes to the spindle poles during meiosis I (MI) and meiosis II (MII), whereas AURKB is concentrated at kinetochores, specifically at metaphase of MI (Met I). The germ cell‐specific homolog, AURKC, is found along the entire length of chromosomes during both meiotic divisions. Maturing oocytes in the presence of the small molecule pan‐Aurora kinase inhibitor, ZM447439 results in defects in meiotic progression and chromosome alignment at both Met I and Met II. Over‐expression of AURKB, but not AURKA or AURKC, rescues the chromosome alignment defect suggesting that AURKB is the primary Aurora kinase responsible for regulating chromosome dynamics during meiosis in mouse oocytes. Mol. Reprod. Dev. 76: 1094–1105, 2009. © 2009 Wiley‐Liss, Inc.     

Meiotic regulation of TPX2 protein levels governs cell cycle progression in mouse oocytes

Formation of female gametes requires acentriolar spindle assembly during meiosis. Mitotic spindles organize from centrosomes and via local activation of the RanGTPase on chromosomes. Vertebrate oocytes present a RanGTP gradient centred on chromatin at all stages of meiotic maturation. However, this gradient is dispensable for assembly of the first meiotic spindle. To understand this meiosis I peculiarity, we studied TPX2, a Ran target, in mouse oocytes. Strikingly, TPX2 activity is controlled at the protein level through its accumulation from meiosis I to II. By RNAi depletion and live imaging, we show that TPX2 is required for spindle assembly via two distinct functions. It controls microtubule assembly and spindle pole integrity via the phosphorylation of TACC3, a regulator of MTOCs activity. We show that meiotic spindle formation in vivo depends on the regulation of at least a target of Ran, TPX2, rather than on the regulation of the RanGTP gradient itself.     

Modulation of the Chromatin Phosphoproteome by the Haspin Protein Kinase

Recent discoveries have highlighted the importance of Haspin kinase activity for the correct positioning of the kinase Aurora B at the centromere. Haspin phosphorylates Thr³ of the histone H3 (H3), which provides a signal for Aurora B to localize to the centromere of mitotic chromosomes. To date, histone H3 is the only confirmed Haspin substrate. We used a combination of biochemical, pharmacological, and mass spectrometric approaches to study the consequences of Haspin inhibition in mitotic cells. We quantified 3964 phosphorylation sites on chromatin-associated proteins and identified a Haspin protein-protein interaction network. We determined the Haspin consensus motif and the co-crystal structure of the kinase with the histone H3 tail. The structure revealed a unique bent substrate binding mode positioning the histone H3 residues Arg² and Lys⁴ adjacent to the Haspin phosphorylated threonine into acidic binding pockets. This unique conformation of the kinase-substrate complex explains the reported modulation of Haspin activity by methylation of Lys⁴ of the histone H3. In addition, the identification of the structural basis of substrate recognition and the amino acid sequence preferences of Haspin aided the identification of novel candidate Haspin substrates. In particular, we validated the phosphorylation of Ser¹³⁷ of the histone variant macroH2A as a target of Haspin kinase activity. MacroH2A Ser¹³⁷ resides in a basic stretch of about 40 amino acids that is required to stabilize extranucleosomal DNA, suggesting that phosphorylation of Ser¹³⁷ might regulate the interactions of macroH2A and DNA. Overall, our data suggest that Haspin activity affects the phosphorylation state of proteins involved in gene expression regulation and splicing.Eukaryotic protein kinases (ePK)¹ constitute a large family of enzymes that coordinate virtually any cellular processes by the phosphorylation of their target proteins at specific sites (1, 2). Active kinases often modulate the activity of other enzymes, including other kinases, thus amplifying and extending an initial signal that affect sometimes thousands of proteins (3). This creates a highly complex network of feedback and forward loops where multiple kinases can mutually influence each other''s activity. Kinases adopt three molecular strategies to select and specifically phosphorylate their substrates in the crowded environment of a cell (2). First, tight control of cellular kinase localization assures that only proteins present in the close proximity of the kinase can be phosphorylated; second, the kinase specific activity can be regulated via post-translational modifications or the recruitment of cofactor molecules; and third, the recognition of specific consensus motifs on substrates ensures that phosphorylation only occurs at the intended site or sites (2).The Haspin kinase is a member of the ePK family that structurally diverges from most ePKs (1, 4). The Haspin kinase domain displays structural features that have never been observed in other ePK family members (5, 6). Specifically, the possibility of activation loop phosphorylation, a frequent regulatory mechanisms to control kinase activity, is absent in Haspin (5). Haspin is characterized by an active conformation that is stabilized by a hydrophobic lock of the helix αC inducing a stable S conformation of the structurally unique activation segment. These specific structural features also create a structurally diverse substrate binding site comprising a highly electronegative cleft for the histone H3 basic tails (5). Interestingly, the recognition of H3 has been shown to be modulated by methylation at H3 residue Lys⁴, thus coupling Haspin activity with epigenetic mechanisms of chromatin regulation (5). Histone H3 that is phosphorylated at Thr³ is so far the only well-characterized Haspin substrate (7). H3Thr³ phosphorylation (H3Thr^3ph) is required for the localization of Aurora B at the centromere (8–10). Inactivation of Haspin catalytic activity by ATP mimetic inhibitors induces Aurora B centromeric delocalization, leading to a loss of phosphorylation in chromatin associated Aurora B substrates (11, 12). To date, apart from this well-characterized centromeric function of Haspin activity, the broader cellular functions of the kinase and the phosphorylation events that control these remain essentially unknown.In this study, we used an integrated biochemical, proteomic, pharmacologic, and structural biology approach to study the Haspin kinase, its substrates and the cellular consequences of its activity. Specifically, we determined a new mode of kinase substrate binding and identified a Haspin kinase substrate recognition motif. We identified 3964 phosphorylation sites in chromatin-associated proteins, quantified their response to Haspin inhibition, and verified the mitotic phosphorylation of MacroH2A Ser¹³⁷ (13) as directly dependent by Haspin activity. Altogether, our data suggest that Haspin regulates the phosphorylation of proteins involved in mechanisms that control gene expression, including the modifications of histones, and provide evidence for novel molecular effects of Haspin activity on mitotic chromatin.     

Involvement of mitogen-activated protein kinase (MAPK) pathway in LH- and meiosis-activating sterol (MAS)-induced maturation in rat and mouse oocytes

Gonadotropic stimulation of meiotic resumption in mice is dependent upon mitogen-activated protein kinase (MAPK) activation in the somatic compartment of the follicle. By contrast, spontaneous resumption of meiosis is independent of MAPK activation. In view of the suggested role of meiosis-activating sterol (MAS) in oocyte maturation we have (i) compared MAPK activation in rat preovulatory follicles stimulated by LH or by accumulation of endogenous MAS by using an inhibitor of MAS conversion, AY9944; (ii) examined whether stimulation of meiosis by MAS is dependent upon MAPK activation using denuded oocytes (DO) of Mos- null mice (hereafter Mos(-/-)) with oocytes unable to activate MAPK. Rat preovulatory follicles responded to LH or AY9944 stimulation by MAPK activation. Inhibition of MAPK phosphorylation blocked both LH- and AY9944 triggered resumption of meiosis. In mouse cumulus-enclosed oocytes (CEOs) and DOs AY9944 stimulated GVB in wild-type and Mos(-/-) mouse CEOs cultured with hypoxanthine (Hx). Addition of MAS or AY9944 to mouse DOs cultured with Hx induced resumption of meiosis only in wild-type and Mos(+/-) oocytes, but they were ineffective in Mos(-/-) oocytes. The observed sluggish activation of MAPK induced by AY9944 in rat follicle-enclosed oocytes (FEO) may cause the delay in meiotic resumption in response to MAS and AY9944 stimulation. Further, it is incompatible with the suggested role of MAS as an obligatory mediator of LH in the induction of meiotic maturation. MAPK/MOS activation, whether in the somatic compartment or in denuded oocytes, is required for MAS- like LH-, FSH-, or EGF-induced resumption of meiosis.     

SLX2 interacting with BLOS2 is differentially expressed during mouse oocyte meiotic maturation

Gametogenesis is a complex biological process of producing cells for sexual reproduction. Xlr super family members containing a conserved COR1 domain play essential roles in gametogenesis. In the present study, we identified that Slx2, a novel member of Xlr super family, is specifically expressed in the meiotic oocytes, which is demonstrated by western blotting and immunohistochemistry studies. In the first meiotic prophase, SLX2 is unevenly distributed in the nuclei of oocytes, during which phase SLX2 is partly co-localized with SYCP3 in synaptonemal complex and γH2AX in the nucleus of oocytes. Interestingly, the localization of SLX2 was found to be switched into the cytoplasm of oocytes after prometaphase I during oocyte maturation. Furthermore, yeast two-hybrid and coimmunoprecipitation studies demonstrated that SLX2 interacts with BLOS2, which is a novel centrosome-associated protein, and co-localized with γ-Tubulin, which is a protein marker of chromosome segregation in meiosis. These results indicated that SLX2 might get involved in chromosomes segregation during meiosis by interaction with BLOS2. In conclusion, SLX2 might be a novel gametogenesis-related protein that could play multiple roles in regulation of meiotic processes including synaptonemal complex assembly and chromosome segregation.     

Neither Aurora B activity nor histone H3 phosphorylation is essential for chromosome condensation during meiotic maturation of porcine oocytes

Aurora kinase B (AURKB) is a chromosomal passenger protein that is essential for a number of processes during mitosis. Its activity is regulated by association with two other passenger proteins, INCENP and Survivin, and by phosphorylation on Thr 232. In this study, we examine expression and phosphorylation on Thr-232 of AURKB during meiotic maturation of pig oocytes in correlation with histone H3 phosphorylation and chromosome condensation. We show that histone H3 phosphorylation on Ser-10, but not on Ser-28, correlates with progressive chromosome condensation during oocyte maturation; Ser-10 phosphorylation starts around the time of the breakdown of the nuclear envelope, with the maximal activity in metaphase I, whereas Ser-28 phosphorylation does not significantly change in maturing oocytes. Treatment of oocytes with 50 microM butyrolactone I (BL-I), an inhibitor of cyclin-dependent kinases, or cycloheximide (10 microg/ml), inhibitor of proteosynthesis, results in a block of oocytes in the germinal vesicle stage, when nuclear membrane remains intact; however, condensed chromosome fibers or highly condensed chromosome bivalents can be seen in the nucleoplasm of BL-I- or cycloheximide-treated oocytes, respectively. In these treated oocytes, no or only very weak AURKB activity and phosphorylation of histone H3 on Ser-10 can be detected after 27 h of treatment, whereas phosphorylation on Ser-28 is not influenced. These results suggest that AURKB activity and Ser-10 phosphorylation of histone H3 are not required for chromosome condensation in pig oocytes, but might be required for further processing of chromosomes during meiosis.     

Meiotically incompetent and competent goat oocytes: timing of nuclear events and protein phosphorylation

The ability of mammalian oocytes to resume meiosis and to complete the first meiotic division is acquired sequentially during their growth phase. The acquisition of meiotic competence in goat oocytes has been previously correlated with follicular size (9). Since protein phosphorylation/dephosphorylation play a key role in oocyte maturation, it could be that in meiotically incompetent oocytes, such post-translational modifications are inadequate. The aim of this study was to analyze whether changes in oocyte proteins phosphorylation occurred during the acquisition of meiotic competence. For this propose, goat oocytes were divided into 4 classes according to follicular size and meiotic competence: Class A oocytes from follicles < 0.5 mm in diameter: Class B oocytes from follicles 0.5-0.8 mm; Class C oocytes from follicles 1-1.8 mm and class D oocytes from follicles > 3 mm. The protein phosphorylation patterns of these classes of oocytes were studied at different times of in vitro maturation. After 4h of culture, when all oocytes were in the germinal vesicle stage, only the oocytes from Class D displayed the phosphoproteins at 110 kD, 31 kD and around 63 kD. In contrast to Class D oocytes Classes B and C oocytes were partially competent to mature, they underwent germinal vesicle breakdown later than fully competent Class D oocytes and remained in early prometaphase I or in metaphase I, respectively. They exhibited the phosphoprotein changes that are associated with commitment to resume meiosis; but the changes occurred later than in Class D oocytes, which were fully competent to reach metaphase II. After 27 h of culture, the phosphorylation patterns of Class B, C and D oocytes were identical, whereas the meiotic stages reached were quite different. The phosphoprotein changes associated with oocyte maturation did not occur in meiotically incompetent Class A oocytes, which were blocked at the germinal vesicle stage. From these results it can be concluded that, at the GV stage, meiotically incompetent and competent goat oocytes display different patterns of protein phosphorylation. Once oocytes are able to resume meiosis they undergo specific phosphorylation changes, but whether these changes are markers or regulators of maturation events remains to be determined.     

Involvement of histone H3 (Ser10) phosphorylation in chromosome condensation without Cdc2 kinase and mitogen-activated protein kinase activation in pig oocytes

When oocytes resume meiosis, chromosomes start to condense and Cdc2 kinase becomes activated. However, recent findings show that the chromosome condensation does not always correlate with the Cdc2 kinase activity in pig oocytes. The objectives of this study were to examine 1) the correlation between chromosome condensation and histone H3 phosphorylation at serine 10 (Ser10) during the meiotic maturation of pig oocytes and 2) the effects of protein phosphatase 1/2A (PP1/ PP2A) inhibitors on the chromosome condensation and the involvement of Cdc2 kinase, MAP kinase, and histone H3 kinase in this process. The phosphorylation of histone H3 (Ser10) was first detected in the clump of condensed chromosomes at the diakinesis stage and was maintained until metaphase II. The kinase assay showed that histone H3 kinase activity was low in oocytes at the germinal vesicle stage (GV) and increased at the diakinesis stage and that high activity was maintained until metaphase II. Treatment of GV-oocytes with okadaic acid (OA) or calyculin-A (CL-A), the PP1/PP2A inhibitors, induced rapid chromosome condensation with histone H3 (Ser10) phosphorylation after 2 h. Both histone H3 kinase and MAP kinase were activated in the treated oocytes, although Cdc2 kinase was not activated. In the oocytes treated with CL-A and the MEK inhibitor U0126, neither Cdc2 kinase nor MAP kinase were activated and no oocytes underwent germinal vesicle breakdown (GVBD), although histone H3 kinase was still activated and the chromosomes condensed with histone H3 (Ser10) phosphorylation. These results suggest that the phosphorylation of histone H3 (Ser10) occurs in condensed chromosomes during maturation in pig oocytes. Furthermore, the chromosome condensation is correlated with histone H3 kinase activity but not with Cdc2 kinase and MAP kinase activities.     

The Drosophila meiotic mutant mei-352 is an allele of klp3A and reveals a role for a kinesin-like protein in crossover distribution

The semisterile meiotic mutant mei-352 alters the distribution of meiotic exchanges without greatly affecting their total frequency. We show that the mei-352 mutation is an allele of the klp3A gene, which encodes a kinesin-like protein of the Kinesin-4 family. The semisterility observed in mei-352 females results from a known defect of klp3A oocytes in mediating pronuclear fusion. Interestingly, other klp3A alleles also exhibit defects in meiotic recombination similar to those of mei-352. Finally, we show that the Klp3A protein localizes within the oocyte nucleus during meiotic prophase, the time at which exchange distribution is established, and extensively colocalizes with DNA. The parallel of the klp3A phenotype with a meiotic defect observed for kar3 mutants in yeast suggests a role for kinesins in early meiosis and might reflect a previously suggested role for this class of kinesins in chromosome condensation.