MgcRacGAP is involved in cytokinesis through associating with mitotic spindle and midbody

Rho GTPases in hematopoietic cells

The ubiquitous Rho GTPases are instrumental in the organization of the actin cytoskeleton, but also for the control of gene expression. Here we review the role of the major members of this family, i.e., RhoA, Rac1, Rac2, and Cdc42, and their intracellular signaling in hematopoietic cells. Although these proteins have been classically implicated in chemotaxis, there are now clear indications on how differential signaling toward other, more specific functions, such as phagocytosis or the production of reactive oxygen species, is regulated by relatively small differences in primary sequence. The identification of mutations in these GTPases or their regulators has provided novel insights in their function as well as their relevance for the development of hematological diseases.

Cleavage furrow formation and ingression during animal cytokinesis: a microtubule legacy

Cytokinesis ensures the proper partitioning of the nuclear and cytoplasmic contents into independent daughter cells at the end of cell division. Although the metazoan mitotic spindle has been implicated in the placement and advancement of the cleavage furrow, the molecules responsible for these processes have remained elusive. Recent studies have provided insights into the role of different microtubule structures and associated proteins in cleavage furrow positioning and ingression together with the signalling events that regulate the dynamics of the equatorial cell cortex during cytokinesis. We try to unify these findings into a general model of cytokinesis in which both astral and central spindle microtubules have the ability to induce furrowing. We further propose that the evolutionarily conserved centralspindlin complex serves as a master controller of cell cleavage in Drosophila by promoting both furrow formation and ingression. The same mechanism might be conserved in other organisms.

MgcRacGAP controls the assembly of the contractile ring and the initiation of cytokinesis

Initiation of cytokinesis requires the establishment of the cleavage plane, the assembly of the contractile ring, and the ingression of the cleavage furrow. MgcRacGAP, a GTPase-activating protein for RhoA, is required for cytokinesis, but the mechanism of its action remains unknown. We report here that MgcRacGAP is required for the assembly of anillin and myosin into the contractile ring. In addition, MgcRacGAP is required for the localized activation of myosin through the RhoA-mediated phosphorylation of the myosin regulatory light chain. Cells with MgcRacGAP RNA interference (RNAi) failed cytokinesis without any ingression of the cleavage furrow. Paradoxically, MgcRacGAP, a GTPase-activating protein, associates during cytokinesis with ECT2, a guanine nucleotide exchange factor for RhoA, and the localization of ECT2 to both the central spindle and the contractile ring depends on MgcRacGAP. Knockdown of ECT2 phenocopies that of MgcRacGAP. We conclude that MgcRacGAP controls the initiation of cytokinesis by regulating ECT2, which in turn induces the assembly of the contractile ring and triggers the ingression of the cleavage furrow.

Inhibition of cyclin-dependent kinase 1 induces cytokinesis without chromosome segregation in an ECT2 and MgcRacGAP-dependent manner

Cleavage furrow formation marks the onset of cell division during early anaphase. The small GTPase RhoA and its regulators ECT2 and MgcRacGAP have been implicated in furrow ingression in mammalian cells, but the signaling upstream of these molecules remains unclear. We now show that the inhibition of cyclin-dependent kinase (Cdk)1 is sufficient to initiate cytokinesis. When mitotically synchronized cells were treated with the Cdk-specific inhibitor BMI-1026, the initiation of cytokinesis was induced precociously before chromosomal separation. Cytokinesis was also induced by the Cdk1-specific inhibitor purvalanol A but not by Cdk2/Cdk5- or Cdk4-specific inhibitors. Consistent with initiation of precocious cytokinesis by Cdk1 inhibition, introduction of anti-Cdk1 monoclonal antibody resulted in cells with aberrant nuclei. Depolymerization of mitotic spindles by nocodazole inhibited BMI-1026-induced precocious cytokinesis. However, in the presence of a low concentration of nocodazole, BMI-1026 induced excessive membrane blebbing, which appeared to be caused by formation of ectopic cleavage furrows. Depletion of ECT2 or MgcRacGAP by RNA interference abolished both of the phenotypes (precocious furrowing after nocodazole release and excessive blebbing in the presence of nocodazole). RNA interference of RhoA or expression of dominant-negative RhoA efficiently reduced both phenotypes. RhoA was localized at the cleavage furrow or at the necks of blebs. We propose that Cdk1 inactivation is sufficient to activate a signaling pathway leading to cytokinesis, which emanates from mitotic spindles and is regulated by ECT2, MgcRacGAP, and RhoA. Chemical induction of cytokinesis will be a valuable tool to study the initiation mechanism of cytokinesis.

Aurora kinases, aneuploidy and cancer, a coincidence or a real link?

As Aurora kinases are overexpressed in a large number of cancers, and ectopic expression of Aurora generates polyploid cells containing multiple centrosomes, it has been tempting to suggest that Aurora overexpression provokes genetic instability underlying the tumorigenesis. However, examination of the evidence suggests a more complex relationship. Overexpression of Aurora-A readily transforms rat-1 and NIH3T3 cells, but not primary cells, whereas overexpression of Aurora-B induces metastasis after implantation of tumors in nude mice. Why do polyploid cells containing abnormal centrosome numbers induced by Aurora not get eliminated at cell-cycle checkpoints? Does this phenotype determine the origin of cancer or does it only promote tumor progression? Would drugs against Aurora family members be of any help for cancer treatment? These and related questions are addressed in this review (which is part of the Chromosome Segregation and Aneuploidy series).

Anillin is a substrate of anaphase-promoting complex/cyclosome (APC/C) that controls spatial contractility of myosin during late cytokinesis

Anillin, an actin-binding protein localized at the cleavage furrow, is required for cytokinesis. Through an in vitro expression screen, we identified anillin as a substrate of the anaphase-promoting complex/cyclosome (APC/C), a ubiquitin ligase that controls mitotic progression. We found that the levels of anillin fluctuate in the cell cycle, peaking in mitosis and dropping drastically during mitotic exit. Ubiquitination of anillin required a destruction-box and was mediated by Cdh1, an activator of APC/C. Overexpression of Cdh1 reduced the levels of anillin, whereas inactivation of APC/C(Cdh1) increased the half-life of anillin. Functionally, anillin was required for the completion of cytokinesis. In anillin knockdown cells, the cleavage furrow ingressed but failed to complete the ingression. At late cytokinesis, the cytosol and DNA in knockdown cells underwent rapid myosin-based oscillatory movement across the furrow. During this movement, RhoA and active myosin were absent from the cleavage furrow, and myosin was redistributed to cortical patches, which powers the random oscillatory movement. We concluded that anillin functions to maintain the localization of active myosin, thereby ensuring the spatial control of concerted contraction during cytokinesis.

Site selection for the cleavage furrow at cytokinesis

The question of how the site for division of the cytoplasm is determined at the end of mitosis has been studied for over a century, and it remains an active, controversial and fascinating problem in cell biology. This problem draws on the use of several model cell types, with the goal of understanding and identifying how the cell cycle regulates signals between the mitotic apparatus and the cell cortex. Studies in different cell types and using a vast array of techniques reveal different answers: these might reflect differences in experimental approaches, multiple and redundant mechanisms and, importantly, diversity in biology. In this article (which is part of the Cytokinesis series), we present a summary and critique of the major models for the roles of the mitotic apparatus microtubules in stimulating furrow formation at cytokinesis.

A microtubule-dependent zone of active RhoA during cleavage plane specification

Cytokinesis in animal cells results from the assembly and constriction of a circumferential array of actin filaments and myosin-2. Microtubules of the mitotic apparatus determine the position at which the cytokinetic actomyosin array forms, but the molecular mechanisms by which they do so remain unknown. The small GTPase RhoA has previously been implicated in cytokinesis. Using four-dimensional microscopy and a probe for active RhoA, we show that active RhoA concentrates in a precisely bounded zone before cytokinesis and is independent of actin assembly. Cytokinetic RhoA activity zones are common to four echinoderm species, the vertebrate Xenopus laevis, and the highly asymmetric cytokinesis accompanying meiosis. Microtubules direct the formation and placement of the RhoA activity zone, and the zone is repositioned after physical spindle displacement. We conclude that microtubules specify the cytokinetic apparatus via a dynamic zone of local RhoA activity.

Klp67A destabilises pre-anaphase microtubules but subsequently is required to stabilise the central spindle

Klp67A is a member of the Kip3 subfamily of microtubule destabilising kinesins, the loss of which results in abnormally long and stable pre-anaphase microtubules. Here we examine its role during cytokinesis in Drosophila primary spermatocytes that require the coordinated interaction of an interior and peripheral set of central spindle microtubules. In mutants anaphase B spindles elongated with normal kinetics but bent towards the cortex. Both peripheral and interior spindle microtubules then formed diminished bundles of abnormally positioned central spindle microtubules associated with the pavarotti-KLP and KLP3A motor proteins. The minus ends of these were poorly aligned as revealed by Asp protein localisation. Furrows always initiated at the sites of central spindle bundles but could be unilateral or nonequatorially positioned. Ectopic furrows were stimulated by the interior central spindle and formed only after this structure buckled and contacted the cortex. Furrows often halted and regressed as they could not be sustained by the central spindles that became increasing unstable over time and often completely degraded. Consistent with this, actin and anillin failed to form homogenous bands. Thus, the Klp67A microtubule catastrophe factor is required for cytokinesis by regulating both the formation and stability of the central spindle.

Function and regulation of Tumbleweed (RacGAP50C) in neuroblast proliferation and neuronal morphogenesis

Drosophila RacGAP50C and its homologues act as part of a complex with a kinesin-like protein (Pavarotti/Zen-4) that is essential for the formation of the central spindle and completion of cytokinesis [Mishima, M., Kaitna, S. & Glotzer, M. (2002) Dev. Cell 2, 41-54; Somers, W. G. & Saint, R. (2003) Dev. Cell 4, 29-39; Jantsch-Plunger et al. (2000) J. Cell Biol. 149, 1391-1404]. We report here that RacGAP50C corresponds to the tumbleweed (tum) gene previously identified based on its defects in dendrite development of sensory neurons [Gao, F. B., Brenman, J. E., Jan, L. Y. & Jan, Y. N. (1999) Genes Dev. 13, 2549-2561]. Using mushroom body neurogenesis and morphogenesis as a model, we show that Tumbleweed (Tum), Pavarotti, and their association are required for neuroblast proliferation. Tum with a mutation predicted to disrupt the GTPase-activating protein (GAP) activity still largely retains its activity in regulating cell division but is impaired in its activity to limit axon growth. We also provide evidence that Tum and Pavarotti regulate the subcellular localization of each other in postmitotic neurons and that cytoplasmic accumulation of both proteins disrupts axon development in a GAP-dependent manner. Taken together with previous studies of RacGAP50C in regulating cytokinesis, we propose that Tum serves as a scaffolding protein in regulating cell division but acts as a GAP to limit axon growth in postmitotic neurons.

Ect2 and MgcRacGAP regulate the activation and function of Cdc42 in mitosis

Although Rho regulates cytokinesis, little was known about the functions in mitosis of Cdc42 and Rac. We recently suggested that Cdc42 works in metaphase by regulating bi-orient attachment of spindle microtubules to kinetochores. We now confirm the role of Cdc42 by RNA interference and identify the mechanisms for activation and down-regulation of Cdc42. Using a pull-down assay, we found that the level of GTP-Cdc42 elevates in metaphase, whereas the level of GTP-Rac does not change significantly in mitosis. Overexpression of dominant-negative mutants of Ect2 and MgcRacGAP, a Rho GTPase guanine nucleotide exchange factor and GTPase activating protein, respectively, or depletion of Ect2 by RNA interference suppresses this change of GTP-Cdc42 in mitosis. Depletion of Ect2 also impairs microtubule attachment to kinetochores and causes prometaphase delay and abnormal chromosomal segregation, as does depletion of Cdc42 or expression of the Ect2 and MgcRacGAP mutants. These results suggest that Ect2 and MgcRacGAP regulate the activation and function of Cdc42 in mitosis.

Ablation of PRC1 by small interfering RNA demonstrates that cytokinetic abscission requires a central spindle bundle in mammalian cells, whereas completion of furrowing does not

The temporal and spatial regulation of cytokinesis requires an interaction between the anaphase mitotic spindle and the cell cortex. However, the relative roles of the spindle asters or the central spindle bundle are not clear in mammalian cells. The central spindle normally serves as a platform to localize key regulators of cell cleavage, including passenger proteins. Using time-lapse and immunofluorescence analysis, we have addressed the consequences of eliminating the central spindle by ablation of PRC1, a microtubule bundling protein that is critical to the formation of the central spindle. Without a central spindle, the asters guide the equatorial cortical accumulation of anillin and actin, and of the passenger proteins, which organize into a subcortical ring in anaphase. Furrowing goes to completion, but abscission to create two daughter cells fails. We conclude the central spindle bundle is required for abscission but not for furrowing in mammalian cells.

Filling the GAPs in cell dynamics control: BPGAP1 promotes cortactin translocation to the cell periphery for enhanced cell migration

Cells undergo dynamic changes in morphology or motility during cellular division and proliferation, differentiation, neuronal pathfinding, wound healing, apoptosis, host defense and organ development. These processes are controlled by signalling events relayed through cascades of protein interactions leading to the establishment and maintenance of cytoskeletal networks of microtubules and actin. Various regulators, including the Rho small GTPases (guanine nucleotide triphosphatases), serve as master switches to fine-tune the amplitude, duration as well as the integration of such circuitry responses. Rho GTPases are activated by guanine nucleotide-exchange factors and inactivated by GAPs (GTPase-activating proteins). Although normally down-regulating signalling pathways by catalysing their GTPase activity, many GAPs exist with various protein modules, the functions of which still largely remain unknown. BPGAP1 is a novel RhoGAP that co-ordinately regulates pseudopodia and cell migration through the interplay of its BNIP-2 and Cdc42GAP homology domains serving as a homophilic/heterophilic interaction device, an enzymic RhoGAP domain that inactivates RhoA and a proline-rich region that binds the Src homology-3 domain of cortactin. Both proteins co-localize to cell periphery and enhance cell migration. As a molecular scaffold in cortical actin assembly and organization, cortactin and its interaction with small GTPases, GAPs and tyrosine kinases seems set to provide further insights to the multiplicity and complexity of cell dynamics control. Elucidating how these processes might be individually or co-ordinately regulated through cortactin remains an exciting future challenge.

Cell Type-specific Regulation of RhoA Activity during Cytokinesis

Rho family GTPases play pivotal roles in cytokinesis. By using probes based on the principle of fluorescence resonance energy transfer (FRET), we have shown that in HeLa cells RhoA activity increases with the progression of cytokinesis. Here we show that in Rat1A cells RhoA activity remained suppressed during most of the cytokinesis. Consistent with this observation, the expression of C3 toxin inhibited cytokinesis in HeLa cells but not in Rat1A cells. Furthermore, the expression of a dominant negative mutant of Ect2, a Rho GEF, or Y-27632, an inhibitor of the Rho-dependent kinase ROCK, inhibited cytokinesis in HeLa cells but not in Rat1A cells. In contrast to the activity of RhoA, the activity of Rac1 was suppressed during cytokinesis and started increasing at the plasma membrane of polar sides before the abscission of the daughter cells in both HeLa and Rat1A cells. This type of Rac1 suppression was shown to be essential for cytokinesis because a constitutively active mutant of Rac1 induced a multinucleated phenotype in both HeLa and Rat1A cells. Moreover, the involvement of MgcRacGAP/CYK-4 in this suppression of Rac1 during cytokinesis was shown by the use of a dominant negative mutant. Because ML-7, an inhibitor of myosin light chain kinase, delayed the cytokinesis of Rat1A cells and because Pak, a Rac1 effector, is known to suppress myosin light chain kinase, the suppression of the Rac1-Pak pathway by MgcRacGAP may play a pivotal role in the cytokinesis of Rat1A cells.

Phorbol ester-induced translocation of PKC epsilon to the nucleus in fibroblasts: identification of nuclear PKC epsilon-associating proteins

We show that phorbol ester treatment of NIH 3T3 fibroblasts induces rapid translocation of PKC from a perinuclear site to the nucleus, extending findings in PC12 and NG108-15 cells and in myocytes. We have immunoprecipitated the PKC from nuclei isolated from phorbol ester-treated fibroblasts and identified six proteins which associate with nuclear PKC. These have been characterised as matrin 3, transferrin, Rac GTPase activating protein 1, vimentin, beta-actin and annexin II by MALDI-TOF-MS. We have confirmed that these proteins associate with PKC by gel overlay and/or dot blotting assays. The role of these PKC-associating proteins in the nucleus and their interaction with PKC are considered.

Cytokinesis: progress on all fronts

Cell multiplication requires sequestration of the duplicated and segregated genome into two daughter cells. The mitotic spindle is critical for orchestrating sister chromatid separation and division plane positioning. During anaphase, spindle microtubules become bundled to form the central spindle, which is essential for completion of cytokinesis. Central spindle assembly is mediated by a microtubule-associated protein and a kinesin-RhoGAP complex, both of which are regulated by phosphorylation/dephosphorylation. The central spindle also plays a role in cleavage furrow positioning, which appears to involve activation of RhoA. New results have provided some initial clues as to how furrow positioning is achieved. Particularly notable is the discovery that a protein activated by RhoA, formin, has actin nucleation activity.

A GTPase-activating protein binds STAT3 and is required for IL-6-induced STAT3 activation and for differentiation of a leukemic cell line

We previously identified a guanosine triphosphatase (GTPase)-activating protein (GAP) male germ cell Rac GAP (MgcRacGAP) that enhanced interleukin-6 (IL-6)-induced macrophage differentiation of murine M1 leukemia cells. Later, MgcRacGAP was found to play crucial roles in cell division. However, how MgcRacGAP enhanced IL-6-induced differentiation remained elusive. Here we show that MgcRacGAP enhances IL-6-induced differentiation through enhancement of signal transducer and activator of transcription-3 (STAT3) activation. MgcRacGAP, Rac, and STAT3 formed a complex in IL-6-stimulated M1 cells, where MgcRacGAP interacted with Rac1 and STAT3 through its cysteine-rich domain and GAP domain. In reporter assays, the wild-type MgcRacGAP enhanced transcriptional activation of STAT3 while a GAP-domain deletion mutant (DeltaGAP) did not significantly enhance it, suggesting that the GAP domain was required for enhancement of STAT3-dependent transcription. Intriguingly, M1 cells expressing DeltaGAP had no effect on the differentiation signal of IL-6, while forced expression of MgcRacGAP rendered M1 cells hyperresponsive to the IL-6-induced differentiation. Moreover, knockdown of MgcRacGAP by RNA interference profoundly suppressed STAT3 activation, implicating MgcRacGAP in the STAT3-dependent transcription. All together, our data not only reveal an important role for MgcRacGAP in STAT3 activation, but also demonstrate that MgcRacGAP regulates IL-6-induced cellular differentiation in which STAT3 plays a pivotal role.

Animal cell division: a fellowship of the double ring?

Despite a century of research into the nature of animal cell division, a molecular explanation for the positioning of the actomyosin contractile ring has remained elusive. The discovery of a novel interaction between regulators of Rho family small GTPases has revealed a link between the mitotic microtubules and the contractile ring during the later stages of mitosis. The properties of the interacting Rho regulators suggest a molecular model for the positioning and initiation of contractile ring furrowing in animal cells. In this 'double ring' model, centralspindlin complexes, localized by the action of their kinesin-like protein component, position and activate a cortical equatorial ring of Rho GTPase exchange factors. The resulting ring of activated Rho would then trigger a cascade of events leading to formation and constriction of the contractile ring.

A screen for modifiers of RacGAP(84C) gain-of-function in theDrosophilaeye revealed the LIM kinase Cdi/TESK1 as a downstream effector of Rac1 during spermatogenesis

In Drosophila, RotundRacGAP/RacGAP(84C) is critical to retinal organisation and spermatogenesis. We show that eye-directed expression of RacGAP(84C) or its GTPase activating protein (GAP) domain induces a dominant rough eye phenotype which we used as a starting point in a gain-of-function screen to identify new partners of RacGAP(84C). Proteins known to function in Ras, Rho and Rac signalling were identified confirming the essential role of RacGAP(84C) in crosstalk between GTPases. Other potential RacGAP(84C) partners identified by the screen are implicated in signal transduction, DNA remodelling, cytoskeletal organisation, membrane trafficking and spermatogenesis. This latter class includes the serine/threonine kinase Center divider (Cdi), which is homologous to the human LIM kinase, Testis specific kinase 1 (TESK1), involved in cytoskeleton control through Cofilin phosphorylation. Eye-directed expression of cdi strongly suppressed the phenotypes induced by either RacGAP(84C) gain-of-function or by the dominant negative form of Rac1, Rac1N17. These results are consistent with Cdi being a specific downstream target of Rac1. We showed that Rac1 and cdi are both expressed in Drosophila testis and that homozygous Rac1 mutants exhibit poor fertility that is further reduced by introducing a cdi loss-of-function mutation in trans. Thus, results from a misexpression screen in the eye led us to a putative novel Rac1-Cdi-Cofilin pathway, regulated by RacGAP(84C), coordinating Drosophila spermatogenesis.

Cdc42 and mDia3 regulate microtubule attachment to kinetochores

During mitosis, the mitotic spindle, a bipolar structure composed of microtubules (MTs) and associated motor proteins, segregates sister chromatids to daughter cells. Initially some MTs emanating from one centrosome attach to the kinetochore at the centromere of one of the duplicated chromosomes. This attachment allows rapid poleward movement of the bound chromosome. Subsequent attachment of the sister kinetochore to MTs growing from the other centrosome results in the bi-orientation of the chromosome, in which interactions between kinetochores and the plus ends of MTs are formed and stabilized. These processes ensure alignment of chromosomes during metaphase and their correct segregation during anaphase. Although many proteins constituting the kinetochore have been identified and extensively studied, the signalling responsible for MT capture and stabilization is unclear. Small GTPases of the Rho family regulate cell morphogenesis by organizing the actin cytoskeleton and regulating MT alignment and stabilization. We now show that one member of this family, Cdc42, and its effector, mDia3, regulate MT attachment to kinetochores.

Role of the midbody matrix in cytokinesis: RNAi and genetic rescue analysis of the mammalian motor protein CHO1

CHO1 is a kinesin-like motor protein essential for cytokinesis in mammalian cells. To analyze how CHO1 functions, we established RNAi and genetic rescue assays. CHO1-depleted cells reached a late stage of cytokinesis but fused back to form binucleate cells because of the absence of the midbody matrix in the middle of the intercellular bridge. Expression of exogenous CHO1 restored the formation of the midbody matrix and rescued cytokinesis in siRNA-treated cells. By analyzing phenotypes rescued with different constructs, it was shown that both motor and stalk domains function in midbody formation, whereas the tail is essential for completion of cytokinesis after the midbody matrix has formed. During the terminal stage of cytokinesis, different subregions of the tail play distinctive roles in stabilizing the midbody matrix and maintaining an association between the midbody and cell cortex. These results demonstrate that CHO1 consists of functionally differentiated subregions that act in concert to ensure complete cell separation.

Human Mitotic Spindle-associated Protein PRC1 Inhibits MgcRacGAP Activity toward Cdc42 during the Metaphase

Although many proteins have been shown to participate in mitotic events, including cytokinesis, their specific roles and interactions remain unclear. A novel interaction of proteins is demonstrated in this report. Yeast two-hybrid screening using PRC1 (protein-regulating cytokinesis 1) cDNA, a human mitotic spindle-associated cyclin-dependent kinase (CDK) substrate, which is involved in cytokinesis, as bait was performed. Data show that the PRC1 bait bound to MgcRacGAP, which is a GTPase-activating protein (GAP) for the Rho family GTPases also involved in cytokinesis. In addition, the two proteins showed similar localization during the M phase. PRC1 was shown to bind to the COOH-terminal GAP-conserved domain of MgcRacGAP and to inhibit its GAP activity toward Cdc42. This binding and/or inhibition of MgcRacGAP GAP activity was found to depend on further binding of PRC1 to the basic region (125-285 amino acids) of MgcRacGAP. Furthermore, the basic region was phosphorylated with Aurora B kinase, and this phosphorylation prevented the inhibition of GAP activity by PRC1. Cells overexpressing a phosphorylation mimic mutant of MgcRacGAP exhibited an abnormality of spindle morphology in the metaphase. Cdc42 showed high activity and was localized to the mitotic spindles and centrosomes during the metaphase. We propose that PRC1 down-regulates the GAP activity of MgcRac-GAP during the metaphase and thereby contributes to the correct formation of the spindle.

MgcRacGAP regulates cortical activity through RhoA during cytokinesis

Although Rho GTPases regulate multiple cellular events, their role in cell division is still obscure. Here we show that expression of a GTPase-activating protein (GAP)-deficient mutant (R386A) of the Rho regulator MgcRacGAP induces abnormal cortical activity during cytokinesis in U2OS cells. Multiple large blebs were observed in cells expressing MgcRacGAP R386A from the onset of anaphase to the late stage of cell division. When mitotic blebbing was excessive, cytokinesis was inhibited, and cells with micronuclei were generated. It has been reported that blebbing is caused by abnormal cortical activity. The MgcRacGAP R386A-induced abnormal cortical activity was inhibited by the dominant negative form of RhoA, but not Rac1 or Cdc42. Moreover, expression of constitutively active RhoA also induced drastic cortical activity during cytokinesis. Unlike apoptotic blebbing, MgcRacGAP R386A-induced blebbing was not inhibited by the ROCK inhibitor Y-27632, suggesting that MgcRacGAP regulates cortical activity during cytokinesis through a novel signaling pathway. We propose that MgcRacGAP plays a pivotal role in cytokinesis by regulating cortical movement through RhoA.

Dynamic association of the mammalian insulator protein CTCF with centrosomes and the midbody

CTCF is a highly conserved, ubiquitously expressed DNA-binding protein that has widespread capabilities in gene regulation. CTCF plays important roles in cell growth regulatory processes and epigenetic functions. Ectopic expression of CTCF results in severe cell growth inhibition at multiple points within the cell cycle, indicating that CTCF levels must be stringently monitored. We have investigated the subcellular localization of CTCF in detail. Interestingly, we observe that CTCF shows a dynamic cell cycle-dependent distribution. Immunofluorescent staining reveals that in interphase CTCF is a nuclear protein, which is mainly excluded from the nucleolus. Strikingly, CTCF is associated with the centrosome during mitosis, especially from metaphase to anaphase. At telophase, CTCF dissociates from the centrosome and localizes to the midbody and the reformed nuclei. The association of CTCF with centrosomes and the midbody is further confirmed by biochemical fractionation. Moreover, subcellular fractions of CTCF show cell cycle and organelle-specific posttranslational modifications, suggesting different roles for CTCF at different stages of the cell cycle.

p190RhoGAP is cell cycle regulated and affects cytokinesis

p190RhoGAP (p190), a Rho family GTPase-activating protein, regulates actin stress fiber dynamics via hydrolysis of Rho-GTP. Recent data suggest that p190 also regulates cell proliferation. To gain insights into the cellular process(es) affected by p190, we altered its levels by conditional or transient overexpression. Overexpression of p190 resulted in a multinucleated phenotype that was dependent on the GTPase-activating protein domain. Confocal immunofluorescence microscopy revealed that both endogenous and exogenous p190 localized to the newly forming and contracting cleavage furrow of dividing cells. However, overexpression of p190 resulted in abnormal positioning of the furrow specification site and unequal daughter cell partitioning, as well as faulty furrow contraction and multinucleation. Furthermore, levels of endogenous p190 protein were transiently decreased in late mitosis via an ubiquitin-mediated degradation process that required the NH₂-terminal GTP-binding region of p190. These results suggest that a cell cycle–regulated reduction in endogenous p190 levels is linked to completion of cytokinesis and generation of viable cell progeny.

A role for the spectrin superfamily member Syne-1 and kinesin II in cytokinesis

Expression of a dominant negative fragment of the spectrin family member Syne-1 causes an accumulation of binucleate cells, suggesting a role for this protein in cytokinesis. An association of this fragment with the C-terminal tail domain of the kinesin II subunit KIF3B was identified by yeast two-hybrid and co-precipitation assays, suggesting that the role of Syne-1 in cytokinesis involves an interaction with kinesin II. In support of this we found that (1) expression of KIF3B tail domain also gives rise to multinucleate cells, (2) both Syne-1 and KIF3B localize to the central spindle and midbody during cytokinesis in a detergent resistant and ATP sensitive manner and (3) Syne-1 localization is blocked by expression of KIF3B tail. Also, membrane vesicles containing syntaxin associate with the spindle midbody with identical properties. We conclude that Syne-1 and KIF3B function together in cytokinesis by facilitating the accumulation of membrane vesicles at the spindle midbody.

Cytokinesis: a logical GAP

Two complexes localize to the central spindle and regulate completion of cytokinesis: one, centralspindlin, contains a kinesin-like protein and a Rho-family GAP; the second contains Aurora B kinase. Aurora B kinase is known to regulate localization of centralspindlin and may regulate the activity of the RhoGAP component of centralspindlin.

The mammalian passenger protein TD-60 is an RCC1 family member with an essential role in prometaphase to metaphase progression

Passenger proteins migrate from inner centromeres to the spindle midzone during late mitosis, and those described to date are essential both for proper chromosome segregation and for completion of cell cleavage. We have purified and cloned the human passenger protein TD-60, and we here report that it is a member of the RCC1 family and that it binds preferentially the nucleotide-free form of the small G protein Rac1. Using siRNA, we further demonstrate that the absence of TD-60 substantially suppresses overall spindle assembly, blocks cells in prometaphase, and activates the spindle assembly checkpoint. These defects suggest TD-60 may have a role in global spindle assembly or may be specifically required to integrate kinetochores into the mitotic spindle. The latter is consistent with a TD-60 requirement for recruitment of the passenger proteins survivin and Aurora B, and suggests that like other passenger proteins, TD-60 is involved in regulation of cell cleavage.

Activity of Rho-family GTPases during cell division as visualized with FRET-based probes

Rho-family GTPases regulate many cellular functions. To visualize the activity of Rho-family GTPases in living cells, we developed fluorescence resonance energy transfer (FRET)-based probes for Rac1 and Cdc42 previously (Itoh, R.E., K. Kurokawa, Y. Ohba, H. Yoshizaki, N. Mochizuki, and M. Matsuda. 2002. Mol. Cell. Biol. 22:6582-6591). Here, we added two types of probes for RhoA. One is to monitor the activity balance between guanine nucleotide exchange factors and GTPase-activating proteins, and another is to monitor the level of GTP-RhoA. Using these FRET probes, we imaged the activities of Rho-family GTPases during the cell division of HeLa cells. The activities of RhoA, Rac1, and Cdc42 were high at the plasma membrane in interphase, and decreased rapidly on entry into M phase. From after anaphase, the RhoA activity increased at the plasma membrane including cleavage furrow. Rac1 activity was suppressed at the spindle midzone and increased at the plasma membrane of polar sides after telophase. Cdc42 activity was suppressed at the plasma membrane and was high at the intracellular membrane compartments during cytokinesis. In conclusion, we could use the FRET-based probes to visualize the complex spatio-temporal regulation of Rho-family GTPases during cell division.

M phase phosphoprotein 1 is a human plus-end-directed kinesin-related protein required for cytokinesis

The human M phase phosphoprotein 1 (MPP1), previously identified through a screening of a subset of proteins specifically phosphorylated at the G2/M transition (Matsumoto-Taniura, N., Pirollet, F., Monroe, R., Gerace, L., and Westendorf, J. M. (1996) Mol. Biol. Cell 7, 1455-1469), is characterized as a plus-end-directed kinesin-related protein. Recombinant MPP1 exhibits in vitro microtubule-binding and microtubule-bundling properties as well as microtubule-stimulated ATPase activity. In gliding experiments using polarity-marked microtubules, MPP1 is a slow molecular motor that moves toward the microtubule plus-end at a 0.07 microm/s speed. In cycling cells, MPP1 localizes mainly to the nuclei in interphase. During mitosis, MPP1 is diffuse throughout the cytoplasm in metaphase and subsequently localizes to the midzone to further concentrate on the midbody. MPP1 suppression by RNA interference induces failure of cell division late in cytokinesis. We conclude that MPP1 is a new mitotic molecular motor required for completion of cytokinesis.

Localization of Pavarotti-KLP in living Drosophila embryos suggests roles in reorganizing the cortical cytoskeleton during the mitotic cycle

Pav-KLP is the Drosophila member of the MKLP1 family essential for cytokinesis. In the syncytial blastoderm embryo, GFP-Pav-KLP cyclically associates with astral, spindle, and midzone microtubules and also to actomyosin pseudocleavage furrows. As the embryo cellularizes, GFP-Pav-KLP also localizes to the leading edge of the furrows that form cells. In mononucleate cells, nuclear localization of GFP-Pav-KLP is mediated through NLS elements in its C-terminal domain. Mutants in these elements that delocalize Pav-KLP to the cytoplasm in interphase do not affect cell division. In mitotic cells, one population of wild-type GFP-Pav-KLP associates with the spindle and concentrates in the midzone at anaphase B. A second is at the cell cortex on mitotic entry and later concentrates in the region of the cleavage furrow. An ATP binding mutant does not localize to the cortex and spindle midzone but accumulates on spindle pole microtubules to which actin is recruited. This leads either to failure of the cleavage furrow to form or later defects in which daughter cells remain connected by a microtubule bridge. Together, this suggests Pav-KLP transports elements of the actomyosin cytoskeleton to plus ends of astral microtubules in the equatorial region of the cell to permit cleavage ring formation.

Liver cell polyploidization: a pivotal role for binuclear hepatocytes

Polyploidy is a general physiological process indicative of terminal differentiation. During liver growth, this process generates the appearance of tetraploid (4n) and octoploid (8n) hepatocytes with one or two nuclei. The onset of polyploidy in the liver has been recognized for quite some time; however, the cellular mechanisms that govern it remain unknown. In this report, we observed the sequential appearance during liver growth of binuclear diploid (2 x 2n) and mononuclear 4n hepatocytes from a diploid hepatocyte population. To identify the cell cycle modifications involved in hepatocyte polyploidization, mitosis was then monitored in primary cultures of rat hepatocytes. Twenty percent of mononuclear 2n hepatocytes failed to undergo cytokinesis with no observable contractile movement of the ring. This process led to the formation of binuclear 2 x 2n hepatocytes. This tetraploid condition following cleavage failure did not activate the p53-dependent checkpoint in G1. In fact, binuclear hepatocytes were able to proceed through S phase, and the formation of a bipolar spindle during mitosis constituted the key step leading to the genesis of two mononuclear 4n hepatocytes. Finally, we studied the duplication and clustering of centrosomes in the binuclear hepatocyte. These cells exhibited two centrosomes in G1 that were duplicated during S phase and then clustered by pairs at opposite poles of the cell during metaphase. This event led only to mononuclear 4n progeny and maintained the tetraploidy status of hepatocytes.

Rho family GTPase Rnd2 interacts and co-localizes with MgcRacGAP in male germ cells

The male-germ-cell Rac GTPase-activating protein gene (MgcRacGAP) was initially described as a human RhoGAP gene highly expressed in male germ cells at spermatocyte stage, but exhibits significant levels of expression in most cell types. In somatic cells, MgcRacGAP protein was found to both concentrate in the midzone/midbody and be required for cytokinesis. As a RhoGAP, MgcRacGAP has been proposed to down-regulate RhoA, which is localized to the cleavage furrow and midbody during cytokinesis. Due to embryonic lethality in MgcRacGAP -null mutant mice and to the lack of an in vitro model of spermatogenesis, nothing is known regarding the role and mode of action of MgcRacGAP in male germ cells. We have analysed the expression, subcellular localization and molecular interactions of MgcRacGAP in male germ cells. Whereas MgcRacGAP was found only in spermatocytes and early spermatids, the widespread RhoGTPases RhoA, Rac1 and Cdc42 (which are, to various extents, in vitro substrates for MgcRacGAP activity) were, surprisingly, not detected at these stages. In contrast, Rnd2, a Rho family GTPase-deficient G-protein was found to be co-expressed with MgcRacGAP in spermatocytes and spermatids. MgcRacGAP was detected in the midzone of meiotic cells, but also, unexpectedly, in the Golgi-derived pro-acrosomal vesicle, co-localizing with Rnd2. In addition, a stable Rnd2-MgcRacGAP molecular complex could be evidenced by glutathione S-transferase pull-down and co-immunoprecipitation experiments. We conclude that Rnd2 is a probable physiological partner of MgcRacGAP in male germ cells and we propose that MgcRacGAP, and, quite possibly, other RhoGAPs, may participate in signalling pathways involving Rnd family proteins.

Dispatch. Cell division: the place and time of cytokinesis

Recent studies have provided evidence that, during cytokinesis, activation of the Pbl-Rho1 pathway by a protein complex located at the spindle midzone, and inhibition of this pathway by two mitotic cyclins, may be major contributing factors controlling the place and timing of the cleavage furrow.

Phosphorylation by aurora B converts MgcRacGAP to a RhoGAP during cytokinesis

Cell division is finely controlled by various molecules including small G proteins and kinases/phosphatases. Among these, Aurora B, RhoA, and the GAP MgcRacGAP have been implicated in cytokinesis, but their underlying mechanisms of action have remained unclear. Here, we show that MgcRacGAP colocalizes with Aurora B and RhoA, but not Rac1/Cdc42, at the midbody. We also report that Aurora B phosphorylates MgcRacGAP on serine residues and that this modification induces latent GAP activity toward RhoA in vitro. Expression of a kinase-defective mutant of Aurora B disrupts cytokinesis and inhibits phosphorylation of MgcRacGAP at Ser387, but not its localization to the midbody. Overexpression of a phosphorylation-deficient MgcRacGAP-S387A mutant, but not phosphorylation-mimic MgcRacGAP-S387D mutant, arrests cytokinesis at a late stage and induces polyploidy. Together, these findings indicate that during cytokinesis, MgcRacGAP, previously known as a GAP for Rac/Cdc42, is functionally converted to a RhoGAP through phosphorylation by Aurora B.

The protein tyrosine phosphatase PTP-BL associates with the midbody and is involved in the regulation of cytokinesis

PTP-BL is a highly modular protein tyrosine phosphatase of unknown function. It consists of an N-terminal FERM domain, five PDZ domains, and a C-terminally located tyrosine phosphatase domain. Here we show that PTP-BL is involved in the regulation of cytokinesis. We demonstrate localization of endogenous PTP-BL at the centrosomes during inter- and metaphase and at the spindle midzone during anaphase. Finally PTP-BL is concentrated at the midbody in cytokinesis. We show that PTP-BL is targeted to the midbody and centrosome by a specific splicing variant of the N-terminus characterized by an insertion of 182 amino acids. Moreover, we demonstrate that the FERM domain of PTP-BL is associated with the contractile ring and can be cosedimented with filamentous actin, whereas the N-terminus can be cosedimented with microtubules. We demonstrate that elevating the expression level of wild-type PTP-BL or expression of PTP-BL with an inactive tyrosine phosphatase domain leads to defects in cytokinesis and to the generation of multinucleate cells. We suggest that PTP-BL plays a role in the regulation of cytokinesis.

A RhoGEF and Rho family GTPase-activating protein complex links the contractile ring to cortical microtubules at the onset of cytokinesis

The mechanism that positions the cytokinetic contractile ring is unknown, but derives from the spindle midzone. We show that an interaction between the Rho GTP exchange factor, Pebble, and the Rho family GTPase-activating protein, RacGAP50C, connects the contractile ring to cortical microtubules at the site of furrowing in D. melanogaster cells. Pebble regulates actomyosin organization, while RacGAP50C and its binding partner, the Pavarotti kinesin-like protein, regulate microtubule bundling. All three factors are required for cytokinesis. As furrowing begins, these proteins colocalize to a cortical equatorial ring. We propose that RacGAP50C-Pavarotti complexes travel on cortical microtubules to the cell equator, where they associate with the Pebble RhoGEF to position contractile ring formation and coordinate F-actin and microtubule remodeling during cytokinesis.

Essential role of citron kinase in cytokinesis of spermatogenic precursors

During spermatogenesis, the first morphological indication of spermatogonia differentiation is incomplete cytokinesis, followed by the assembly of stable intercellular cytoplasmic communications. This distinctive feature of differentiating male germ cells has been highly conserved during evolution, suggesting that regulation of the cytokinesis endgame is a crucial aspect of spermatogenesis. However, the molecular mechanisms underlying testis-specific regulation of cytokinesis are still largely unknown. Citron kinase is a myotonin-related protein acting downstream of the GTPase Rho in cytokinesis control. We previously reported that Citron kinase knockout mice are affected by a complex neurological syndrome caused by cytokinesis block and apoptosis of specific neuronal precursors. In this report we show that, in addition, these mice display a dramatic testicular impairment, with embryonic and postnatal loss of undifferentiated germ cells and complete absence of mature spermatocytes. By contrast, the ovaries of mutant females appear essentially normal. Developmental analysis revealed that the cellular depletion observed in mutant testes is caused by increased apoptosis of undifferentiated and differentiating precursors. The same cells display a severe cytokinesis defect, resulting in the production of multinucleated cells and apoptosis. Our data indicate that Citron kinase is specifically required for cytokinesis of the male germ line.

Identification of genes periodically expressed in the human cell cycle and their expression in tumors

The genome-wide program of gene expression during the cell division cycle in a human cancer cell line (HeLa) was characterized using cDNA microarrays. Transcripts of >850 genes showed periodic variation during the cell cycle. Hierarchical clustering of the expression patterns revealed coexpressed groups of previously well-characterized genes involved in essential cell cycle processes such as DNA replication, chromosome segregation, and cell adhesion along with genes of uncharacterized function. Most of the genes whose expression had previously been reported to correlate with the proliferative state of tumors were found herein also to be periodically expressed during the HeLa cell cycle. However, some of the genes periodically expressed in the HeLa cell cycle do not have a consistent correlation with tumor proliferation. Cell cycle-regulated transcripts of genes involved in fundamental processes such as DNA replication and chromosome segregation seem to be more highly expressed in proliferative tumors simply because they contain more cycling cells. The data in this report provide a comprehensive catalog of cell cycle regulated genes that can serve as a starting point for functional discovery. The full dataset is available at http://genome-www.stanford.edu/Human-CellCycle/HeLa/.

Human RhoGAP domain-containing proteins: structure, function and evolutionary relationships

Proteins containing a RhoGAP (Rho GTPase activating protein) domain usually function to catalyze the hydrolysis of GTP that is bound to Rho, Rac and/or Cdc42, inactivating these regulators of the actin cytoskeleton. Using database searches, at least 53 distinct RhoGAP domain-containing proteins are likely to be encoded in human DNA. Phylogenetic analysis of only the RhoGAP domains divides these proteins into distinct families that appear to be functionally related. We also review the current understanding of the structure and likely functions of these human proteins. The presence of RhoGAP domains in a number of different human proteins suggests that cytoskeletal changes, regulated by Rho GTPase, may be integrated with many different signaling pathways.

Identification of genes periodically expressed in the human cell cycle and their expression in tumors

The genome-wide program of gene expression during the cell division cycle in a human cancer cell line (HeLa) was characterized using cDNA microarrays. Transcripts of >850 genes showed periodic variation during the cell cycle. Hierarchical clustering of the expression patterns revealed coexpressed groups of previously well-characterized genes involved in essential cell cycle processes such as DNA replication, chromosome segregation, and cell adhesion along with genes of uncharacterized function. Most of the genes whose expression had previously been reported to correlate with the proliferative state of tumors were found herein also to be periodically expressed during the HeLa cell cycle. However, some of the genes periodically expressed in the HeLa cell cycle do not have a consistent correlation with tumor proliferation. Cell cycle-regulated transcripts of genes involved in fundamental processes such as DNA replication and chromosome segregation seem to be more highly expressed in proliferative tumors simply because they contain more cycling cells. The data in this report provide a comprehensive catalog of cell cycle regulated genes that can serve as a starting point for functional discovery. The full dataset is available at http://genome-www.stanford.edu/Human-CellCycle/HeLa/.

Vav3 is regulated during the cell cycle and effects cell division

Vav3 is a member of the family of guanine nucleotide exchange factors implicated in the regulation of Rho GTPases. Although the exact in vivo function of Vav3 is unknown, evidence from several studies indicates a role distinct from Vav2 or Vav1. Here we report that the expression of Vav3 is regulated during the cell cycle. Strikingly, Vav3 was transiently up-regulated in HeLa cells during mitosis, whereas enforced expression of Vav3 perturbed cytokinesis and led to the appearance of multinucleated cells. These effects of Vav3 were RhoA-dependent, required phosphorylation of the regulatory tyrosine 173, but were not enhanced by N-terminal truncations. Thus, this report establishes that expression of Vav3 is strictly regulated in a cell cycle-dependent manner and implicates Vav3 in the control of cytokinesis.

Animal cell cytokinesis

Cytokinesis creates two daughter cells endowed with a complete set of chromosomes and cytoplasmic organelles. This conceptually simple event is mediated by a complex and dynamic interplay between the microtubules of the mitotic spindle, the actomyosin cytoskeleton, and membrane fusion events. For many decades the study of cytokinesis was driven by morphological studies on specimens amenable to physical manipulation. The studies led to great insights into the cellular structures that orchestrate cell division, but the underlying molecular machinery was largely unknown. Molecular and genetic approaches have now allowed the initial steps in the development of a molecular understanding of this fundamental event in the life of a cell. This review provides an overview of the literature on cytokinesis with a particular emphasis on the molecular pathways involved in the division of animal cells.

Central spindle assembly and cytokinesis require a kinesin-like protein/RhoGAP complex with microtubule bundling activity

A late step in cytokinesis requires the central spindle, which forms during anaphase by the bundling of antiparallel nonkinetochore microtubules. Microtubule bundling and completion of cytokinesis require ZEN-4/CeMKLP-1, a kinesin-like protein, and CYK-4, which contains a RhoGAP domain. We show that CYK-4 and ZEN-4 exist in a complex in vivo that can be reconstituted in vitro. The N terminus of CYK-4 binds the central region of ZEN-4, including the neck linker. Genetic suppression data prove the functional significance of this interaction. An analogous complex, containing equimolar amounts of a CYK-4 ortholog and MKLP-1, was purified from mammalian cells. Biochemical studies indicate that this complex, named centralspindlin, is a heterotetramer. Centralspindlin, but not its individual components, strongly promotes microtubule bundling in vitro.

The pebble GTP exchange factor and the control of cytokinesis

Several G proteins of the Rho family have been shown to be required for cytokinesis. The activity of these proteins is regulated by GTP exchange factors (GEFs), which stimulate GDP/GTP exchange, and by GTPase activating proteins (GAPs), which suppress activity by stimulating the intrinsic GTPase activity. The role of Rho family members during cytokinesis is likely to be determined by their spatial and temporal interactions with these factors. Here we focus on the role of the pebble (pbl) gene of Drosophila melanogaster, a RhoGEF that is required for cytokinesis. We summarise the evidence that the primary target of PBL is Rho1 and describe genetic approaches to elucidating the function of PBL and identifying other components of the PBL-activated Rho signalling pathway.

Role of MgcRacGAP/Cyk4 as a regulator of the small GTPase Rho family in cytokinesis and cell differentiation

To identify the key molecules that regulate differentiation of hematopoietic cells, we carried out retrovirus-mediated functional screening for cDNAs whose expression suppresses IL-6-induced differentiation of mouse myeloid leukemic M1 cells. From this screening, we obtained a full length cDNA encoding a mouse homologue of human MgcRacGAP. Overexpression of the anti-sense MgcRacGAP profoundly inhibited IL-6-induced macrophage-differentiation of M1 cells. On the other hand, overexpression of the full-length form of MgcRacGAP alone enhanced macrophage differentiation of M1 cells in response to IL-6, and induced macrophage differentiation of HL-60 leukemic cells. To determine how this protein regulates differentiation and proliferation, an antibody against MgcRacGAP was prepared. Immunohistochemical studies revealed that MgcRacGAP mainly localizes in the nucleus in interphase, accumulates on the mitotic spindle in metaphase, and is condensed in the midbody during cytokinesis. Overexpression of an N-terminal domain deletion mutant, which lacks the ability to localize to the midbody through association with tubulins, or a GAP-inactive mutant resulted in the formation of multinucleated cells in HeLa cells as well as in hemopoietic cells. Interestingly, MgcRacGAP in the midbody was phosphorylated probably on serine and threonine residues. These results indicate that MgcRacGAP regulates cytokinesis and cellular differentiation as a regulator of Rho family of GTPase and suggest that this process is controlled by some serine/threonine kinases.

Tat1, a novel sulfate transporter specifically expressed in human male germ cells and potentially linked to rhogtpase signaling

RhoGTPases (Rho, Rac, and Cdc42) are known to regulate multiple functions, including cell motility, adhesion, and proliferation; however, the signaling pathways underlying these pleiotropic effects are far from fully understood. We have recently described a new RhoGAP (GTPase activating protein for RhoGTPases) gene, MgcRacGAP, primarily expressed in male germ cells, at the spermatocyte stage. We report here the isolation, through two-hybrid cloning, of a new partner of MgcRacGAP, very specifically expressed in the male germ line and showing structural features of anion transporters. This large protein (970 amino acids and a predicted size of 109 kDa), we provisionally designated Tat1 (for testis anion transporter 1), is closely related to a sulfate permease family comprising three proteins in human (DRA, Pendrin, and DTD); it is predicted to be an integral membrane protein with 14 transmembrane helices and intracytoplasmic NH(2) and COOH termini. In situ hybridization studies demonstrate that Tat1 and MgcRacGAP genes are coexpressed in male germ cells at the spermatocyte stage. On testis sections, Tat1 protein can be immunodetected in spermatocytes and spermatids associated with plasma membrane. Two-hybrid and in vitro binding assays demonstrate that MgcRacGAP stably interacts through its NH(2)-terminal domain with the Tat1 COOH-terminal region. Expression of Tat1 protein in COS7 cells generates a 4,4'-diisothiocyano-2,2'-disulfonic acid stilbene and chloride-sensitive sulfate transport. Therefore we conclude that Tat1 is a novel sulfate transporter specifically expressed in spermatocytes and spermatids and interacts with MgcRacGAP in these cells. These observations raise the possibility of a new regulatory pathway linking sulfate transport to Rho signaling in male germ cells.