The MPM-2 antibody inhibits mitogen-activated protein kinase activity by binding to an epitope containing phosphothreonine-183

Lasonolide A, a potent and reversible inducer of chromosome condensation

Lasonolide A (LSA) is a natural product with high and selective cytotoxicity against mesenchymal cancer cells, including leukemia, melanomas and glioblastomas. Here, we reveal that LSA induces rapid and reversible premature chromosome condensation (PCC) associated with cell detachment, plasma membrane smoothening and actin reorganization. PCC is induced at all phases of the cell cycle in proliferative cells as well as in circulating human lymphocytes in G 0. It is independent of Cdk1 signaling, associated with cyclin B downregulation and induced in cells at LSA concentrations that are three orders of magnitude lower than those required to block phosphatases 1 and 2A in vitro. At the epigenetic level, LSA-induced PCC is coupled with histone H3 and H1 hyperphosphorylation and deacetylation. Treatment with SAHA reduced LSA-induced PCC, implicating histone deacetylation as one of the PCC effector mechanisms. In addition, PCC is coupled with topoisomerase II (Top2) and Aurora A hyperphosphorylation and activation. Inhibition of Top2 or Aurora A partially blocked LSA-induced PCC. Our findings demonstrate the profound epigenetic alterations induced by LSA and the potential of LSA as a new cytogenetic tool. Based on the unique cellular effects of LSA, further studies are warranted to uncover the cellular target of lasonolide A ("TOL").

Large-scale identification of novel mitosis-specific phosphoproteins

Systematic identification of phosphoproteins is essential for understanding cellular signalling pathways since phosphorylation plays important roles in cellular regulation. Monoclonal antibody MPM-2 recognizes a discrete set of mitosis-specific phosphoproteins and constitutes a specific tool to investigate the significance of phosphorylation in cell cycle. However, due to the difficulties in identifying antigens revealed on immunoblot membrane, only minority of MPM-2 antigens have been identified. Here we originated proteomics approaches for large-scale identification of MPM-2 phosphoproteins. Mitotic extracts were run on several two-dimensional gel electrophoresis (2D) in parallel, and stained by Coomassie Blue. Each individual spot on one of the gels was excised, and proteins in it were further resolved by regular SDS-electrophoresis and blotted on membrane for MPM-2 stain. Counterparts of the positive proteins were selected on another parallel 2D gel and identified by mass-spectrometry. Using this strategy, 100 spots were excised from Coomassie-stained 2D gel and screened by 1D immunoblots for MPM-2 reactivity, and 22 proteins containing potential MPM-2 epitope were identified in addition to a known MPM-2 antigen, laminin-binding protein. These results were further validated by immunofluorescence, co-immunoprecipitation and in vitro phosphorylation assay. The identification of an unprecedented number of potential MPM-2 phosphoprotein antigens gives new insight into the range of proteins involved in the regulation of the early stages of cell division. Meanwhile, this strategy could be used wherever unknown antigens are explored, especially for antibodies that can recognize more than one antigen.

Anti-phosphopeptide antibody, P-STM as a novel tool for detecting mitotic phosphoproteins: identification of lamins A and C as two major targets

A polyclonal, phospho-epitope-specific antibody (P-STM) was generated to detect the activated p21-activated kinase 2 (PAK2), based on the regulatory autophosphorylation site Thr(402) of PAK2 [Yu et al., 1998]. In this report, we show that this antibody can also recognize many phosphoproteins in mitotic HeLa and A431 cells. Signal of these phosphoproteins emerged after treating the cells with nocodazole and okadaic acid, and was highly detected in G2-M phase transition of HeLa cells released from double thymidine block. Immunofluorescence analysis revealed that P-STM strongly stained HeLa cells at prometaphase and metaphase, but not at interphase and anaphase. Interestingly, this staining pattern was almost identical to that obtained by staining with MPM2, a monoclonal antibody known to react with phosphoproteins in mitotic HeLa cells. However, the phosphoproteins detected by the two antibodies are quite different. Two-dimensional gel electrophoresis (2DE) and tryptic peptide fingerprint analysis by matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry were employed to identify lamins A and C as two of the mitotic cell-specific phosphoproteins recognized by P-STM. Lamins A and C immunoprecipitated from nocodazole-treated cells, but not from untreated cells showed strong reactivity to P-STM, and this reactivity lost completely after protein phosphatase 2A treatment. In summary, our results show that P-STM represents a novel tool for detecting mitotic phosphoproteins, which are different from those recognized by MPM2, and that lamins A and C are the two prominent mitotic phosphoproteins detected by P-STM.

The Multifunctional Nuclear Protein p54nrb is Multiphosphorylated in Mitosis and Interacts with the Mitotic Regulator Pin1

The human protein p54nrb and its mouse homolog NonO have been implicated in a variety of nuclear processes including transcription, pre-mRNA processing, nuclear retention of edited RNA and DNA relaxation. We have identified p54nrb as an antigen of the phosphodependent monoclonal antibodies CC-3 and MPM-2 and shown that this protein is phosphorylated on multiple sites during mitosis. The use of the cyclin-dependent protein kinase inhibitor roscovitine and immunodepletion studies with an anti-cyclin B1 antibody established that Cdk1 was responsible for the phosphorylation of the carboxy-terminal extremity of p54nrb whereas a different kinase appeared to be involved in the generation of CC-3 epitope(s) in the amino-terminal moiety of the protein. Like many CC-3 and MPM-2 antigens, we show that p54nrb is a target of the peptidylprolyl isomerase Pin1, suggesting that it may be regulated by phosphorylation-dependent conformational changes as many other nuclear proteins upon entry into mitosis. In addition, site-directed mutagenesis indicated that the interaction of Pin1 with p54nrb was mediated by three threonine residues located in the proline-rich carboxy-terminal extremity of the protein. Our results also showed that Pin1 binding was favored when at least two of the three threonine residues were phosphorylated, suggesting a regulation mechanism based on multisite phosphorylation.

Multisite phosphorylation of Pin1-associated mitotic phosphoproteins revealed by monoclonal antibodies MPM-2 and CC-3

Background

The peptidyl-prolyl isomerase Pin1 recently revealed itself as a new player in the regulation of protein function by phosphorylation. Pin1 isomerizes the peptide bond of specific phosphorylated serine or threonine residues preceding proline in several proteins involved in various cellular events including mitosis, transcription, differentiation and DNA damage response. Many Pin1 substrates are antigens of the phosphodependent monoclonal antibody MPM-2, which reacts with a subset of proteins phosphorylated at the G2/M transition.

Results

As MPM-2 is not a general marker of mitotic phosphoproteins, and as most mitotic substrates are phosphorylated more than once, we used a different phosphodependent antibody, mAb CC-3, to identify additional mitotic phosphoproteins and eventual Pin1 substrates by combining affinity purification, MALDI-TOF mass spectrometry and immunoblotting. Most CC-3-reactive phosphoproteins appeared to be known or novel MPM-2 antigens and included the RNA-binding protein p54^nrb/nmt55, the spliceosomal protein SAP155, the Ki-67 antigen, MAP-1B, DNA topoisomerases II α and β, the elongation factor hSpt5 and the largest subunit of RNA polymerase II. The CC-3 mitotic antigens were also shown to be Pin1 targets. The fine CC-3- and MPM-2-epitope mapping of the RNA polymerase II carboxy-terminal domain confirmed that the epitopes were different and could be generated in vitro by distinct kinases. Finally, the post-mitotic dephosphorylation of both CC-3 and MPM-2 antigens was prevented when cellular Pin1 activity was blocked by the selective inhibitor juglone.

Conclusion

These observations indicate that the mitotic phosphoproteins associated with Pin1 are phosphorylated on multiple sites, suggesting combinatorial regulation of substrate recognition and isomerization.

Mitotic phosphorylation of Golgi reassembly stacking protein 55 by mitogen-activated protein kinase ERK2

The role of the mitogen-activated protein kinase kinase (MKK)/extracellular-activated protein kinase (ERK) pathway in mitotic Golgi disassembly is controversial, in part because Golgi-localized targets have not been identified. We observed that Golgi reassembly stacking protein 55 (GRASP55) was phosphorylated in mitotic cells and extracts, generating a mitosis-specific phospho-epitope recognized by the MPM2 mAb. This phosphorylation was prevented by mutation of ERK consensus sites in GRASP55. GRASP55 mitotic phosphorylation was significantly reduced, both in vitro and in vivo, by treatment with U0126, a potent and specific inhibitor of MKK and thus ERK activation. Furthermore, ERK2 directly phosphorylated GRASP55 on the same residues that generated the MPM2 phospho-epitope. These results are the first demonstration of GRASP55 mitotic phosphorylation and indicate that the MKK/ERK pathway directly phosphorylates the Golgi during mitosis.

Mitotic phosphorylation of DNA topoisomerase II alpha by protein kinase CK2 creates the MPM-2 phosphoepitope on Ser-1469

DNA topoisomerase II alpha is required for chromatin condensation during prophase. This process is temporally linked with the appearance of mitosis-specific phosphorylation sites on topoisomerase IIalpha including one recognized by the MPM-2 monoclonal antibody. We now report that the ability of mitotic extracts to create the MPM-2 epitope on human topoisomerase II alpha is abolished by immunodepletion of protein kinase CK2. Furthermore, the MPM-2 phosphoepitope on topoisomerase II alpha can be generated by purified CK2. Phosphorylation of C-truncated topoisomerase II alpha mutant proteins conclusively shows, that the MPM-2 epitope is present in the last 163 amino acids. Use of peptides containing all conserved CK2 consensus sites in this region indicates that only the peptide containing Arg-1466 to Ala-1485 is able to compete with topoisomerase II alpha for binding of the MPM-2 antibody. Replacement of Ser-1469 with Ala abolishes the ability of the phosphorylated peptide to bind to the MPM-2 antibody while a peptide containing phosphorylated Ser-1469 binds tightly. Surprisingly, the MPM-2 phosphoepitope influences neither the catalytic activity of topoisomerase II alpha nor its ability to form molecular complexes with CK2 in vitro. In conclusion, we have identified protein kinase CK2 as a new MPM-2 kinase able to phosphorylate an important mitotic protein, topoisomerase II alpha, on Ser-1469.

Phosphorylation-dependent prolyl isomerization: a novel cell cycle regulatory mechanism

Protein phosphorylation by proline-directed protein kinases plays an essential role in triggering a programmed set of cell cycle events. We have recently isolated an essential and conserved mitotic regulator, Pin1. Pin1 is a phosphorylation-dependent prolyl isomerase that specifically isomerizes the phosphorylated serine/threonine-proline bond. Pin1 also binds and regulates the function of a conserved set of mitosis-specific phosphoproteins. These results suggest phosphorylation-dependent prolyl isomerization to be a novel cell cycle regulatory mechanism. This new post-translational regulation may allow the general increase in protein phosphorylation to be converted into the organised and programmed set of structural modifications that occur during mitosis. In addition, since inhibition of Pin1 induces mitotic arrest and apoptosis, Pin1 may be a potential new drug target.

Cell cycle-dependent phosphorylation of centrosomes: localization of phosphopeptide specific antibodies to the centrosome

The microtubule nucleation capacity of the centrosome increases dramatically as cells progress from interphase into mitosis. The increase in nucleation capacity of the centrosome correlates with the cell cycle-dependent localization of the mitotic protein monoclonal-2 (MPM-2) phosphoepitope-specific antibody to the mitotic centrosome. Therefore, the phosphorylation state of centrosomal components may regulate the microtubule nucleation capacity of this organelle during mitosis. Neither the identity of the MPM-2 kinase(s) nor all of the MPM-2-reactive phosphoproteins associated with the centrosome have been fully elucidated. Only recently have the characteristics of the MPM-2 epitope site been defined, and we used this information to prepare polyclonal antibodies against synthetic phosphopeptides containing potential MPM-2 epitopes derived from the sequences of two MPM-2-reactive proteins, topoisomerase II, and microtubule associated protein 1B (MAP1B). We demonstrate that these phosphopeptide-specific antibodies also localize to the centrosome in a cell cycle-dependent fashion. Thus, polyclonal antibodies have been generated against defined phosphopeptides that reiterate many of the immunofluorescence staining properties exhibited by the MPM-2 antibody. These new phosphopeptide-specific antibodies will provide additional probes to examine the phosphorylation of centrosomal components and the functional consequences of their phosphorylation during mitosis.

Regulation of cell division and the cytoskeleton by mitogen-activated protein kinases in higher plants

The microtubule-associated protein 2 kinase (MAP2-kinase), now better known as mitogen-activated protein kinase (MAPK), was initially discovered in association with the cytoskeleton, and was later also implicated in cell division. The importance of mitogenic stimulation in plant development roused interest in finding the plant homologues of MAPKs. However, data on plant MAPKs in cell division are rather sparse and fragmentary. Therefore we place the available information on cell cycle control of MAPKs in plants into a broader context. We discuss four aspects of cell division control: cell proliferation and the G1/S-phase transition, G2-phase and mitosis, cytokinesis, and cytoskeletal reorganisation. Future work will reveal to what extent plants use signalling pathways that are similar or different to those of animal or yeast cells in regulating cell divisions.

A hyperphosphorylated form of RNA polymerase II is the major interphase antigen of the phosphoprotein antibody MPM-2 and interacts with the peptidyl-prolyl isomerase Pin1

The monoclonal antibody MPM-2 recognizes a subset of M phase phosphoproteins in a phosphorylation-dependent manner. It is believed that phosphorylation at MPM-2 antigenic sites could regulate mitotic events since most of the MPM-2 antigens identified to date have M phase functions. In addition, many of these proteins are substrates of the mitotic regulator Pin1, a peptidyl-prolyl isomerase which is present throughout the cell cycle and which is thought to alter its mitotic targets by changing their conformation. In interphase cells, most MPM-2 reactivity is confined to nuclear speckles. We report here that a hyperphosphorylated form of the RNA polymerase II largest subunit is the major MPM-2 interphase antigen. These findings were made possible by the availability of another monoclonal antibody, CC-3, that was previously used to identify a 255 kDa nuclear matrix protein associated with spliceosomal components as a hyperphosphorylated form of the RNA polymerase II largest subunit. MPM-2 recognizes a phosphoepitope of the large subunit that becomes hyperphosphorylated upon heat shock in contrast to the phosphoepitope defined by CC-3, whose reactivity is diminished by the heat treatment. Therefore, these two antibodies may discriminate between distinct functional forms of RNA polymerase II. We also show that RNA polymerase II large subunit interacts with Pin1 in HeLa cells. Pin1 may thus regulate transcriptional and post-transcriptional events by catalyzing phosphorylation-dependent conformational changes of the large RNA polymerase II subunit.

Essential mitotic functions of DNA topoisomerase IIalpha are not adopted by topoisomerase IIbeta in human H69 cells

Unique functions of mammalian DNA-topoisomerases IIalpha and -beta are suggested by their distinct cellular distribution and chromatin binding at mitosis. Here, we studied H69-VP cells that, due to a homozygous mutation, express topoisomerase IIalpha mostly outside the nucleus. In these cells topoisomerase IIbeta showed a normal nuclear localization. However, at mitosis it diffused away from the chromatin despite the nuclear lack of the alpha-isoform. 80% of these cells performed chromosome condensation and disjunction with the aid of cytosolic topoisomerase IIalpha, which bound to the mitotic chromatin with low affinity. However, the genotype of these cells was highly polyploid indicating an increased rate of non-disjunction. In 20% of the mutant cells neither topoisomerase II isoform was bound to the mitotic chromatin, which appeared as an unstructured DNA spheroid unable to undergo disjunction and cytokinesis. Parental H69 cells expressing topoisomerase IIalpha inside the nucleus exhibited high affinity binding of the enzyme to the mitotic chromatin. Their genotype was mostly diploid and stable. We conclude (i) that high affinity chromatin binding of topoisomerase IIalpha is essential for chromosome condensation/disjunction and (ii) that topoisomerase IIbeta does not adopt these functions.

Modification of the E-cadherin-catenin complex in mitotic Madin-Darby canine kidney epithelial cells

One of the hallmarks of polarized epithelial cells undergoing mitosis is their rounded morphology. This phenotype correlates with a reduced cell-substratum adhesion, apparently caused by a modulation of integrin function. However, it is still unclear whether the cadherin-mediated cell-cell adhesion is affected as well. To address this question, the cadherin complex was analyzed in different cell cycle stages of Madin-Darby canine kidney cells. By immunofluorescence, mitotic Madin-Darby canine kidney cells showed an increased staining of E-cadherin and the catenins (alpha-catenin, beta-catenin, plakoglobin, p120(ctn)) in the cytosol, suggesting a reorganization of the cadherin-catenin complex during mitosis. Biochemical analysis revealed that the overall amount of these components, as well as the proportion of the complex associated with the actin cytoskeleton, remained unchanged in mitotic cells. However, we found evidence for an internalization of E-cadherin during mitosis. In addition, the cadherin-catenin complex was analyzed for mitosis-specific changes in phosphorylation. We report a decrease in the tyrosine phosphorylation of beta-catenin, plakoglobin, and p120(ctn) during mitosis. Moreover, we observed a mitosis-specific Ser/Thr-phosphorylation of p120(ctn), as detected by the MPM-2 antibody. Hence, the cadherin/catenin complex is a target for different posttranslational modifications during mitosis, which may also have a profound impact on cadherin-mediated cell-cell adhesion.

A cell cycle regulated MAP kinase with a possible role in cytokinesis in tobacco cells

Mitogen-activated protein (MAP) kinases have been demonstrated to have a role in meiosis but their involvement in mitotic events is less clear. Using a peptide antibody raised against the tobacco MAP kinase p43(Ntf6) and extracts from synchronized tobacco cell suspension cultures, we show that this kinase is activated specifically during mitosis. Entry into mitosis appears to be necessary for the activation of the kinase, which occurs as a post-translational event. The activation of the kinase occurs in late anaphase/early telophase. The p43(Ntf6) protein shows a transient localization to the cell plate in anaphase cells, in the middle of the two microtubule arrays characteristic of the phragmoplast, a plant-specific structure involved in laying down the new cell wall. The combined data support a role for the MAP kinase p43(Ntf6) in cytokinesis in tobacco cells.

Activated polo-like kinase Plx1 is required at multiple points during mitosis in Xenopus laevis

Entry into mitosis depends upon activation of the dual-specificity phosphatase Cdc25C, which dephosphorylates and activates the cyclin B-Cdc2 complex. Previous work has shown that the Xenopus polo-like kinase Plx1 can phosphorylate and activate Cdc25C in vitro. In the work presented here, we demonstrate that Plx1 is activated in vivo during oocyte maturation with the same kinetics as Cdc25C. Microinjection of wild-type Plx1 into Xenopus oocytes accelerated the rate of activation of Cdc25C and cyclin B-Cdc2. Conversely, microinjection of either an antibody against Plx1 or kinase-dead Plx1 significantly inhibited the activation of Cdc25C and cyclin B-Cdc2. This effect could be reversed by injection of active Cdc25C, indicating that Plx1 is upstream of Cdc25C. However, injection of Cdc25C, which directly activates cyclin B-Cdc2, also caused activation of Plx1, suggesting that a positive feedback loop exists in the Plx1 activation pathway. Other experiments show that injection of Plx1 antibody into early embryos, which do not require Cdc25C for the activation of cyclin B-Cdc2, resulted in an arrest of cleavage that was associated with monopolar spindles. These results demonstrate that in Xenopus laevis, Plx1 plays important roles both in the activation of Cdc25C at the initiation of mitosis and in spindle assembly at late stages of mitosis.

The essential mitotic peptidyl-prolyl isomerase Pin1 binds and regulates mitosis-specific phosphoproteins

Phosphorylation of mitotic proteins on the Ser/Thr-Pro motifs has been shown to play an important role in regulating mitotic progression. Pin1 is a novel essential peptidyl-prolyl isomerase (PPIase) that inhibits entry into mitosis and is also required for proper progression through mitosis, but its substrate(s) and function(s) remain to be determined. Here we report that in both human cells and Xenopus extracts, Pin1 interacts directly with a subset of mitotic phosphoproteins on phosphorylated Ser/Thr-Pro motifs in a phosphorylation-dependent and mitosis-specific manner. Many of these Pin1-binding proteins are also recognized by the monoclonal antibody MPM-2, and they include the important mitotic regulators Cdc25, Myt1, Wee1, Plk1, and Cdc27. The importance of this Pin1 interaction was tested by constructing two Pin1 active site point mutants that fail to bind a phosphorylated Ser/Thr-Pro motif in mitotic phosphoproteins. Wild-type, but not mutant, Pin1 inhibits both mitotic division in Xenopus embryos and entry into mitosis in Xenopus extracts. We have examined the interaction between Pin1 and Cdc25 in detail. Pin1 not only binds the mitotic form of Cdc25 on the phosphorylation sites important for its activity in vitro and in vivo, but it also inhibits its activity, offering one explanation for the ability of Pin1 to inhibit mitotic entry. In a separate paper, we have shown that Pin1 is a phosphorylation-dependent PPIase that can recognize specifically the phosphorylated Ser/Thr-Pro bonds present in mitotic phosphoproteins. Thus, Pin1 likely acts as a general regulator of mitotic proteins that have been phosphorylated by Cdc2 and other mitotic kinases.

Sequence-specific and phosphorylation-dependent proline isomerization: a potential mitotic regulatory mechanism

Pin1 is an essential and conserved mitotic peptidyl-prolyl isomerase (PPIase) that is distinct from members of two other families of conventional PPIases, cyclophilins and FKBPs (FK-506 binding proteins). In response to their phosphorylation during mitosis, Pin1 binds and regulates members of a highly conserved set of proteins that overlaps with antigens recognized by the mitosis-specific monoclonal antibody MPM-2. Pin1 is here shown to be a phosphorylation-dependent PPIase that specifically recognizes the phosphoserine-proline or phosphothreonine-proline bonds present in mitotic phosphoproteins. Both Pin1 and MPM-2 selected similar phosphorylated serine-proline-containing peptides, providing the basis for the specific interaction between Pin1 and MPM-2 antigens. Pin1 preferentially isomerized proline residues preceded by phosphorylated serine or threonine with up to 1300-fold selectivity compared with unphosphorylated peptides. Pin1 may thus regulate mitotic progression by catalyzing sequence-specific and phosphorylation-dependent proline isomerization.

Partial characterization of the MPM-2 phosphoepitope

The MPM-2 monoclonal antibody recognizes a distinctive group of proteins that are associated with structural components of the mitotic apparatus. These proteins become phosphorylated and MPM-2 reactive during M-phase and appear to be required for both the onset and completion of M-phase. Based upon the analysis of reported MPM-2 reactive sequences, we have developed a model for the essential elements that comprise the MPM-2 epitope. This model was tested by employing a series of synthetic phosphopeptides. We show here that a 14 amino acid synthetic phosphopeptide, derived from a potential MPM-2 site on human DNA topoisomerase II, is recognized by the MPM-2 antibody. This phosphopeptide was sufficient to compete for MPM-2 antibody recognition of (1) an isolated native mitotic MPM-2 antigen on dot blots, (2) proteins on immunoblots of mitotic cell lysates, and (3) specific immunostaining of mitotic cells. These results indicated that the topoisomerase peptide contained all of the essential elements of the MPM-2 epitope. By substituting selected amino acids with alanine, we were able to examine the contribution of different amino acids to the binding between the MPM-2 antibody and the epitope. Changing the amino acid that was adjacent to the phosphorylated threonine residue on the C-terminal side (the +1 position) had no effect on MPM-2 antibody binding. However, substitution of aromatic amino acids at either the -2 or +2 positions reduced antibody recognition. The aromatic amino acid at the -2 position appeared to be the most critical residue of those tested that influenced antibody binding. These results provide information required for the molecular definition of the MPM-2 epitope and should aid in the identification of potential MPM-2 reactive sites on other mitotic phosphoproteins.

Identification of novel M phase phosphoproteins by expression cloning

Using an expression cloning technique, we isolated cDNAs for eight M phase phosphoproteins (MPPs 4-11). We then used affinity-purified antibodies to fusion proteins to characterize the intracellular localization and some biochemical properties of these proteins and two others that we identified previously (MPPs 1-2). Each antibody immunoprecipitated one or two protein species of a characteristic size ranging from 17,000 to 220,000 Da. Each MPP, when immunoprecipitated from lysates of M phase cells, was reactive with MPM2, a monoclonal antibody that recognizes a group of related M phase phosphorylation sites, including F-phosphoT-P-L-Q. This reactivity indicated that all the MPPS encoded genuine M phase phosphoproteins. When antibodies to the MPPS were used for immunofluorescence microscopy, each anti-MPP gave a characteristic pattern of localization. In interphase, several of the MPPs were nuclear proteins, whereas others were cytoplasmic or distributed throughout the cell. Three MPPS were strikingly localized to interphase structures: MPP7 to centers of DNA replication, MPP9 to the Golgi complex, and MPP10 to nucleoli. In mitosis, most of the MPPs were distributed throughout the cells. Further studies of the 10 MPPs, most of which are previously undescribed, are expected to provide new understandings of the process of cell division.