[Relationship between estrogen receptor genotypes and female bone mineral density]

OBJECTIVE

To observe the relationship between polymorphyism of estrogen receptor (ER) gene and bone mineral density (BMD) in women.

METHODS

Seventy-eight postmenopausal women, 23 perimenopausal women, and 52 young women (aged 25-35 years) were enrolled into the study. BMD was measured by dual energy X-ray absorptiometry at lumbar spine, proximal femur, fore-arm and total body. We also examined the restriction fragment length polymorphism of the polymerase chain reaction product (PCR-RFLPs) of the ER gene with XbaI or PvuII enzyme in these women.

RESULT

There was no significant relationship between RFLP by PvuII or XbaI and BMD in 78 postmenopausal women. In young healthy women, the mean BMD at Ward triangle of genotype pp (0.823 +/- 0.095) g/cm2 was higher than that of genotype PP greater trochanter(0.665 +/- 0.071) g/cm2(P = 0.037). The BMD at total body, lumbar, hip, trochanter, and Ward of genotype xxpp were significantly higher than those of other genotypes (P < 0.05). But such differences were not found in postmenopausal women.

CONCLUSION

The genotype xxpp might indicate a higher BMD at total body, lumbar, hip, greater trochanter, and Ward in young women. There is no significant effect of ER genotype polymorphism defined by PvuII or XbaI on BMD in postmenopausal women.

Overexpression of Hp95 induces G1 phase arrest in confluent HeLa cells

Xp95, a protein recently identified in Xenopus laevis, is potentially involved in progesterone-induced Xenopus oocyte maturation. In this study, we cloned a human homologue of Xp95, designated Hp95, and examined the effect of its overexpression on the growth properties of human malignant HeLa cells which have lost the contact inhibition of cell proliferation. We observed that although HeLa cells did not undergo G1 phase arrest at any stage after confluence, they were able to downregulate their G1 phase CDK activities in response to confluence. When Hp95 was overexpressed in HeLa cells by transfection with a constitutive or an inducible expression vector containing a full-length Hp95 transgene, HeLa cells became able to undergo G1 phase arrest and form a monolayer culture after confluence. However, the G1 phase CDK activities in these Hp95 overexpressing cells were not inhibited further as compared to control cells after confluence. These results indicate that the defects in HeLa cells that cause the loss of contact inhibition of cell proliferation are in components downstream of the G1 phase CDKs and that overexpression of Hp95 counteracts some of these defects.

XCS-1, a maternally expressed gene product involved in regulating mitosis in Xenopus

The regulation of the cell cycle during early development is an important and complex biological process. We have cloned a cDNA, XCS-1, that may play an important role in regulating mitosis during early embryogenesis in Xenopus laevis. XCS-1 is a maternally expressed gene product that is the Xenopus homologue of the human cleavage signal protein (CS-1). XCS-1 transcripts were detected in oocytes with the titer decreasing just prior to the MBT. During development the XCS-1 protein was detected on the membrane and in the nucleus of blastomeres. It was also detected on the mitotic spindle in mitotic cells and on the centrosomes in interphase cells. Overexpression of myc-XCS-1 in Xenopus embryos resulted in abnormal mitoses with increased numbers of centrosomes, multipolar spindles, and abnormal distribution of chromosomes. Also, we observed incomplete cytokinesis resulting in multiple nuclei residing in the same cytoplasm with the daughter nuclei in different phases of the cell cycle. The phenotype depended on the presence of the N terminus of XCS-1 (aa 1–73) and a consensus NIMA kinase phosphorylation site (aa159-167). Mutations in this site affected the ability of the overexpressed XCS-1 protein to produce the phenotype. These results suggest that XCS-1 is a maternal factor playing an important role in the regulation of the cell cycle during early embryogenesis and that its function depends on its state of phosphorylation.

[Comparing immunogenicity and efficacy of two hepatitis B vaccines in newborn infants of hepatitis B surface antigen (+)/hepatitis B e antigen (+) carrier mothers]

OBJECTIVE

To evaluate the effect of yeast recombinant HBV vaccine on protecting the newborns from HBV carrier mothers and to compare with plasma-derived vaccine.

METHODS

There were 117 neonates from HBsAg(+)/HBeAg(+) carrier mothers involved in the study. Study group [76 of them received RECOMBIVAX HB(5 mcg/0.5 ml)] and control group (41 neonates receive plasma derived vaccine) had blood samples collected and received vaccine at 0, 1, 6, 9 month after birth respectively. HBV-DNA, HBsAg/HBeAg, HBsAb were determined by PCR, ELISA, RIA independently.

RESULTS

(1) Children chronic HBV infection rates at 6, 9 month of age in study group were 7.89%, 5.26% respectively, which were lower than that of control group(14.63%, 17.07%). (2) The vaccine efficacy (VE) in study group was 93.82%, vs, that of control group was 79.92%. (3) The HBsAb positive rate in study group at 9 month of age was 94.74% which was significantly higher than that of control group(80.49%) (P < 0.05). (4) The difference of side-effect rates between the two groups was not significant.

CONCLUSION

Immunogenicity and efficacy of yeast-recombinant hepatitis B vaccine is better than that of plasma derived vaccine in newborn infants of HBsAg(+)/HBeAg(+) carrier mothers.

Identification and cloning of xp95, a putative signal transduction protein in Xenopus oocytes

A 95-kDa protein in Xenopus oocytes, Xp95, was shown to be phosphorylated from the first through the second meiotic divisions during progesterone-induced oocyte maturation. Xp95 was purified and cloned. The Xp95 protein sequence exhibited homology to mouse Rhophilin, budding yeast Bro1, and Aspergillus PalA, all of which are implicated in signal transduction. It also contained three conserved features including seven conserved tyrosines, a phosphorylation consensus sequence for the Src family of tyrosine kinases, and a proline-rich domain near the C terminus that contains multiple SH3 domain-binding motifs. We showed the following: 1) that both Xp95 isolated from Xenopus oocytes and a synthetic peptide containing the Src phosphorylation consensus sequence of Xp95 were phosphorylated in vitro by Src kinase and to a lesser extent by Fyn kinase; 2) Xp95 from Xenopus oocytes or eggs was recognized by an anti-phosphotyrosine antibody, and the relative abundance of tyrosine-phosphorylated Xp95 increased during oocyte maturation; and 3) microinjection of deregulated Src mRNA into Xenopus oocytes increased the abundance of tyrosine-phosphorylated Xp95. These results suggest that Xp95 is an element in a tyrosine kinase signaling pathway that may be involved in progesterone-induced Xenopus oocyte maturation.

Tumour amplified kinase STK15/BTAK induces centrosome amplification, aneuploidy and transformation

The centrosomes are thought to maintain genomic stability through the establishment of bipolar spindles during cell division, ensuring equal segregation of replicated chromosomes to two daughter cells. Deregulated duplication and distribution of centrosomes have been implicated in chromosome segregation abnormalities, leading to aneuploidy seen in many cancer cell types. Here, we report that STK15 (also known as BTAK and aurora2), encoding a centrosome-associated kinase, is amplified and overexpressed in multiple human tumour cell types, and is involved in the induction of centrosome duplication-distribution abnormalities and aneuploidy in mammalian cells. STK15 amplification has been previously detected in breast tumour cell lines and in colon tumours; here, we report its amplification in approximately 12% of primary breast tumours, as well as in breast, ovarian, colon, prostate, neuroblastoma and cervical cancer cell lines. Additionally, high expression of STK15 mRNA was detected in tumour cell lines without evidence of gene amplification. Ectopic expression of STK15 in mouse NIH 3T3 cells led to the appearance of abnormal centrosome number (amplification) and transformation in vitro. Finally, overexpression of STK15 in near diploid human breast epithelial cells revealed similar centrosome abnormality, as well as induction of aneuploidy. These findings suggest that STK15 is a critical kinase-encoding gene, whose overexpression leads to centrosome amplification, chromosomal instability and transformation in mammalian cells.

Cloning and sequence analysis of a pituitary prolactin cDNA from the brushtail possum (Trichosurus vulpecula)

Overlapping cDNA partial clones of pituitary prolactin from the marsupial brushtail possum (Trichosurus vulpecula) were isolated and sequenced. The nucleotide and deduced amino acid sequences showed high sequence identity with pig prolactin (84.3 and 92.5%, respectively) and all of the expected structural features of a quadruped prolactin. A prolactin gene tree was constructed and rates of evolution calculated for possum along with several mammalian and nonmammalian prolactins. Possum prolactin was most closely linked to the prolactins of eutherian mammals but branched from the main mammalian line well before the eutherian prolactins. The prolactin/GH family shows variable rates of evolution ranging from 0.3 substitutions/amino acid site/year x 10(9) for pig prolactin to 7.0 substitutions/ amino acid site/year x 10(9) for the mouse. Since divergence from the eutherian mammals, possum prolactin has shown a slow rate of evolution (0.2 substitutions/ amino acid site/year x 10(9)). As expected, the prolactin gene was expressed in the possum pituitary gland but not in the liver, lung, kidney, heart, or mammary gland.

The G2 block induced by DNA damage: a caffeine-resistant component independent of Cdc25C, MPM-2 phosphorylation, and H1 kinase activity

Treatment of cells with agents that cause DNA damage often results in a delay in G2. There is convincing evidence showing that inhibition of p34cdc2 kinase activation is involved in the DNA damage-induced G2 delay. In this study, we have demonstrated the existence of an additional pathway, independent of the p34cdc2 kinase activation pathway, that leads to a G2 arrest in etoposide-treated cells. Both the X-ray-induced and the etoposide-induced G2 arrest were associated with inhibition of the p34cdc2 H1 kinase activation pathway as judged by p34cdc2 H1 kinase activity and phosphorylation of cdc25C. Caffeine treatment restored these activities after either of the treatments. However, the etoposide-treated cells did not resume cycling, revealing the presence of an alternative pathway leading to a G2 arrest. To explore the possibility that this additional pathway involved phosphorylation of the MPM-2 epitope that is shared by a large family of mitotic phosphoproteins, we monitored the phosphorylation status of the MPM-2 epitope after DNA damage and after treatment with caffeine. Phosphorylation of the MPM-2 epitope was depressed in both X-ray and etoposide-treated cells, and the depression was reversed by caffeine in both cases. The results indicate that the pathway affecting MPM-2 epitope phosphorylation is involved in the G2 delay caused by DNA damage. However, it is not part of the caffeine-insensitive pathway leading to a G2 block seen in etoposide-treated cells.

A phosphatase activity in Xenopus oocyte extracts preferentially dephosphorylates the MPM-2 epitope

MPM-2 antigens are a large family of mitotic phosphoproteins that contain similar phosphoepitopes recognized by the anti-phosphoepitope antibody MPM-2 (MPM-2 epitopes). These proteins are phosphorylated during M phase induction and dephosphorylated from the onset of anaphase through interphase. Since biochemical characterization of the MPM-2 epitope phosphatase requires a specific assay for its activity, we tested different methods for measurement of the MPM-2 epitope phosphatase activity in crude cell lysates. First, an ELISA-based assay was designed that measured the phosphatase-induced reduction of the MPM-2 reactivity in crude M phase cell lysates. Using this assay to follow the phosphatase activity during sequential chromatography of Xenopus oocyte extracts, one predominant peak of phosphatase activity was detected which was separated from the majority of PP1 and PP2A activities. This phosphatase activity dephosphorylated the MPM-2 epitope on multiple MPM-2 antigens. The second method measured dephosphorylation of cdc25, a known MPM-2 antigen. Two major peaks of cdc25 dephosphorylating activities were detected during the sequential chromatography, one that copurified with the major peak of MPM-2 epitope phosphatase activity, and the other with the major peak of PP2A activity. Finally, we examined whether GST-MPM2, a fusion protein between glutathione S-transferase and a 19-residue peptide that contained two representative MPM-2 epitope sequences, could be dephosphorylated efficiently and specifically by the major MPM-2 epitope phosphatase activity in Xenopus oocyte extracts. Neither the crude extract nor the partially purified MPM-2 epitope phosphatase activity efficiently dephosphorylated the MPM-2 epitope on GST-MPM2. These results demonstrate that the ELISA-based assay preferentially detects the MPM-2 epitope phosphatase activity in crude cell lysates which may represent a physiological MPM-2 epitope phosphatase.

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.

Molecular cloning of a splice variant of Caenorhabditis elegans YNK1, a putative element in signal transduction

YNK1 is a 98.3-kDa protein whose sequence was originally deduced from a genomic sequence in Caenorhabditis elegans. It was recently found that YNK1 is homologous to three different proteins implicated in signal transduction, suggesting that YNK1 is a signal transduction protein. In this report we describe the isolation of a full-length cDNA that encodes a splice variant of YNK1, designated YNK1a. We also present evidence that both YNK1 and YNK1a transcripts exist in vivo. Furthermore, using an antibody raised against a YNK1a recombinant protein, we demonstrate that the YNK1 protein is expressed in vivo throughout development.

MPM-2 epitope sequence is not sufficient for recognition and phosphorylation by ME kinase-H

Monoclonal antibody MPM-2 recognizes a large family of mitotic phosphoproteins in a phosphorylation-dependent manner. The antigenic phosphoepitope, designated the MPM-2 epitope, putatively consists of hydrophobic residue-Thr/Ser-Pro-hydrophobic residue-uncharged/basic residue. In this study, we addressed whether this sequence motif contains all the information necessary for recognition and phosphorylation by the kinase that phosphorylates most MPM-2 antigens. A fusion protein between glutathione S-transferase and a 19-residue peptide that contained two representative MPM-2 epitope sequences overlapping with two potential MAP kinase phosphorylation sites was constructed. Both the MPM-2 epitope sequences in the fusion protein (GST-MPM2) were phosphorylated by Xenopus egg extract, making the fusion protein MPM-2 reactive. However, while MAP kinase phosphorylated both the MPM-2 epitope sequences, neither ME kinase-H, a good candidate for a major MPM-2 epitope kinase, nor mitotic cdc2 kinase, which is known to phosphorylate certain MPM-2 antigens in vitro, phosphorylated GST-MPM2 to any significant extent. Furthermore, depletion of MAP kinase activity removed most, if not all, of the GST-MPM2 phosphorylating activity from crude Xenopus egg extracts. These results suggest that additional or different structural information than that provided by the deduced MPM-2 epitope sequence is required for recognition and phosphorylation by ME kinase-H or other major MPM-2 epitope kinases. They also offer a valid explanation for selective phosphorylation of certain MPM-2 antigens by MAP kinase as well as selective recognition of certain phosphorylated MAP kinase substrates by MPM-2.

Mitogen-activated protein kinase and cyclin B/Cdc2 phosphorylate Xenopus nuclear factor 7 (xnf7) in extracts from mature oocytes. Implications for regulation of xnf7 subcellular localization

Xenopus nuclear factor 7 (xnf7) is a maternally expressed putative transcription factor that exhibits phosphorylation-dependent changes in subcellular localization during early Xenopus development. Xnf7 is localized to the germinal vesicle (nucleus) of immature oocytes in a hypophosphorylated state. Xnf7 is phosphorylated during oocyte maturation and released to the cytoplasm. The protein is retained in the cytoplasm during early embryonic cleavage stages but returns to nuclei at the mid-blastula transition. Xnf7 is phosphorylated at two sites during oocyte maturation, designated P1, consisting of one threonine at position 103, and P2, consisting of three clustered threonines at positions 209, 212, and 218. Phosphorylation of both sites is important in regulating xnf7 localization. The P1 site can be phosphorylated by cyclin B/Cdc2 in vitro. To further understand the mechanisms regulating subcellular localization of xnf7 during early development, kinases capable of catalyzing phosphorylation of the P2 site were purified from mature oocyte extracts. We found that mitogen-activated protein kinase phosphorylated Thr212 and cyclin B/Cdc2 phosphorylated Thr 209 and Thr212. No other kinase in mature oocyte extracts phosphorylated the xnf7 P2 site to a significant extent. These results implicate mitogen-activated protein kinase and cyclin B/Cdc2 in regulating xnf7 localization during oocyte maturation. This also suggests that localization of xnf7 may be regulated by multiple kinase activation pathways.

[Effects of herpes simplex virus thymidine kinase gene transduction and prodrug on ovarian cancer cell]

OBJECTIVE

To evaluate the efficacy of gene therapy with the herpes simplex virus thymidine kinase (HSV-TK) gene transference followed by ganciclovir (GCV) therapy in ovarian cancer.

METHODS

HSV-TK gene was inserted into the hind III site of pLNSX vector and recombinant plasmid (pLNS/HSV-TK) was constracted. Then, the recombinant plasmid was transfered into PA317 packaged cell and the producer cell line of recombinant retrovirus vector pLNS/HSV-TK was established. The cytotoxicity efficacy of HSV-TK/GCV system to AO cell line was evaluated by microcucture tetrajolium test (MTT) method.

RESULTS

The recombinant retroviral vector pLNS/HSK-TK can transfer HSV-SK gene into the genome of AO cell line and make it sensitive to GCV. The growth inhibitory rate of AO cell line transfered by pLNS/HSV-TK was 98.0% in the medium containing 400 mumol GVC.

CONCLUSION

The ovarian cancer cell line transfered by HSV-TK gene regressed rapidly in response to the GCV therapy.

[Twenty-eight alphavirus strains isolated from mosquitoes and ticks captured from Hainan Island, China]

Virus isolation was carried out from mosquitoes and ticks captured in nature from Hainan Island of China from 1983-1988. Twenty-eight non-dengue and non-Japanese encephalitis viral strains were isolated from different places, of which 27 strains were recovered from different species of mosquitoes and 1 strain from ticks. At the Division of Vector-Bore Viral Diseases, National Center for Infectious Diseases, Centers for Disease Control, USA, identification of virus family and virus genus on these 28 viral strains was conducted by the use of a complete set of internationally standardized antibodies against arboviruses and by means of indirect immunofluorescence. Results proved that all these 28 viral strains belonged to the Genus Alphavirus of the Family Togaviridae.

Systematic identification of mitotic phosphoproteins

BACKGROUND

Cyclin-dependent kinases (CDKs) are thought to initiate and coordinate cell division processes by sequentially phosphorylating key targets; in most cases these substrates remain unidentified.

RESULTS

Using a screen that scores for phosphorylation of proteins, which were translated from pools of cDNA plasmids in vitro, by either phosphoepitope antibody recognition or electrophoretic mobility shifts, we have identified 20 mitotically phosphorylated proteins from Xenopus embryos, 15 of which have sequence similarity to other proteins. Of these proteins, five have previously been shown to be phosphorylated during mitosis (epithelial-microtubule associated protein-115, Oct91, Elongation factor 1gamma, BRG1 and Ribosomal protein L18A), five are related to proteins postulated to have roles in mitosis (epithelial-microtubule associated protein-115, Schizosaccharomyces pombe Cdc5, innercentrosome protein, BRG1 and the RNA helicase WM6), and nine are related to transcription factors (BRG1, negative co-factor 2alpha, Oct91, S. pombe Cdc5, HoxD1, Sox3, Vent2, and two isoforms of Xbr1b). Of 16 substrates tested, 14 can be directly phosphorylated in vitro by the mitotic CDK, cyclin B-Cdc2, although three of these may be physiological substrates of other kinases activated during mitosis.

CONCLUSIONS

Examination of this broad set of mitotic phosphoproteins has allowed us to draw three conclusions about how the activation of CDKs regulates cell-cycle events. First, Cdc2 itself appears to directly phosphorylate most of the mitotic phosphoproteins. Second, during mitosis most of the substrates are phosphorylated more than once and a number may be targets of multiple kinases, suggesting combinatorial regulation. Third, the large fraction of mitotic phosphoproteins that are presumptive transcription factors, two of which have been previously shown to dissociate from DNA during mitosis, suggests that an important function of mitotic phosphorylation is to strip the chromatin of proteins associated with gene expression.

[Effects of levonorgestrel subdermal implants on estradiol and progesterone receptors of endometrium and ovarian tissues in rats]

OBJECTIVE

To study the influence of subdermal implantation of levonorgestrel (LNG) on the levels of estradiol receptor (ER) and progesterone receptor (PR) in ovaries and endometrium of rats.

METHODS

45 female mature rats were divided into 3 groups: control group, silicon tube group and LNG implants group. Using single-point determination at saturating concentration method, ER and PR levels of both ovaries and endometrium were measured at 15, 30 and 45 days after subdermal implantation of LNG or silicon tube, and compared with that of the control group.

RESULTS

ER and PR levels in LNG group were significantly lower than those in the other 2 groups (P < 0.01). The concentration of ER and PR decreased gradually as implantation prolonged.

CONCLUSION

subdermal implantation of LNG resulted in the reduction of ER and PR levels in ovaries and endometrium of rats, which may be related to the mechanism of contraception and breakthrough bleeding after implantation.

Finely tuned regulation of cytoplasmic retention of Xenopus nuclear factor 7 by phosphorylation of individual threonine residues

Xenopus nuclear factor 7 (xnf7) is a maternal gene product that functi ons in dorsal/ventral patterning of the embryo. The xnf7 protein is stored in the oocyte nucleus germinal vesicle in a hypophosphorylated state. At oocyte maturation, xnf7 is hyperphosphorylated and released into the cytoplasm, where it is anchored until the midblastula stage, where it is dephosphorylated and enters the nucleus. We demonstrated that cytoplasmic anchoring of xnf7 was regulated by changes in the phosphorylation status of four threonines within two sites, site 1 (Thr-103) and site 2 (Thr-209, Thr-212, and Thr-218), which function in an additive manner. A mutant form of xnf7 (xnf7thr-glu) in which the threonines at sites 1 and 2 were mutated to glutamic acids to mimic a permanent state of phosphorylation was retained in the cytoplasm in oocytes and embryos through the gastrula stage. The cytoplasmic form of xnf7 was detected in a large 670-kDa protein complex probably consisting of xnf7 and several other unknown protein components. Anchoring of xnf7 was not dependent on association with either microtubule or microfilament components of the cytoskeleton, since treatment with cytochalasin B and nocodazole did not affect cytoplasmic retention. Both wild-type xnf7 and xnf7thr-glu form dimers in the yeast two-hybrid system; however, homodimerization was not required for cytoplasmic retention. We suggest that the cytoplasmic retention of xnf7 depends on the phosphorylation state of the protein whereas the cytoplasmic anchoring machinery appears to be constitutively present in oocytes and throughout development until the gastrula stage.

cdc25 is one of the MPM-2 antigens involved in the activation of maturation-promoting factor

MPM-2 antigens, a discrete set of phosphoproteins that contain similar phosphoepitopes recognized by the monoclonal antibody MPM-2, are phosphorylated during M-phase induction. Our previous studies suggested that certain MPM-2 antigens are involved in the appearance of maturation-promoting factor (MPF) activity. Because the central mitotic regulator cdc2 kinase has been shown to exhibit MPF activity, we explored the possibility that certain MPM-2 antigens are regulators of cdc2 kinase. We found that MPM-2 binding of its antigens would inhibit the autoamplification of cdc2 kinase in Xenopus oocytes and interfere with cyclin-activation of cdc2 kinase in Xenopus interphase egg extract. Immunodepletion of MPM-2 antigens from cyclin-induced M-phase egg extract caused the inactivation of cdc2 kinase, which was accompanied by an inhibitory phosphorylation of p34cdc2 on Thr 14 and Tyr 15, indicating that at least one MPM-2 antigen is a positive regulator of p34cdc2 dephosphorylation. We then showed that cdc25 from M-phase arrested egg extract is an MPM-2 antigen. These results suggest that phosphorylation of the epitope recognized by MPM-2 may be a crucial event in the activation of cdc25 and that the kinase(s) that phosphorylates this MPM-2 epitope may be an important regulator of cdc2 kinase activation.

At least two kinases phosphorylate the MPM-2 epitope during Xenopus oocyte maturation

MPM-2 antigens, a discrete set of phosphoproteins that contain similar phosphoepitopes (the MPM-2 epitope), are associated with various mitotically important structures. The central mitotic regulator cdc2 kinase has been proposed to induce M-phase by phosphorylating many proteins which might include the MPM-2 antigens. To clarify the relationship of cdc2 kinase and the MPM-2 antigens, we developed an in vitro assay that enabled us to specifically detect the kinases that phosphorylate the MPM-2 epitope (ME kinases) in crude cell extracts. Two different ME kinase activities were identified in unfertilized Xenopus eggs, neither of which was cdc2 kinase, but both appeared to be activated by the introduction of cdc2 kinase into oocytes or oocyte extract. The two ME kinases differed in molecular size, substrate specificity, peptide components, and MPM-2 reactivity. The larger one, ME kinase-H, phosphorylated several MPM-2 antigens, while the smaller one, ME kinase-L, phosphorylated mainly one. We purified ME kinase-L to near homogeneity by sequential chromatography and showed that it has the characteristics of the 42-kD microtubule-associated protein (MAP) kinase. Our results support the previous finding that MAP kinase is activated during Xenopus oocyte maturation and suggest that MAP kinase may contribute to oocyte maturation induction by phosphorylating one subtype of MPM-2 epitope.

Multiple forms of maturation-promoting factor in unfertilized Xenopus eggs

Maturation-promoting factor (MPF), which is functionally defined by its ability to induce frog oocyte maturation independent of protein synthesis, is hypothesized to be the mitotic inducer in eukaryotic cells. Previous studies have demonstrated that the cdc2 protein kinase complex (p34cdc2-cyclin) meets the criteria for MPF. In the present study, we show that MPF activity in extracts of unfertilized Xenopus eggs can be resolved into three fractions by Q-Sepharose chromatography. Of the total MPF activity recovered, approximately 20% was in the flow-through fraction that was accounted for by the cdc2 kinase complex, approximately 40% was in the 0.2 M NaCl eluate, and the remaining approximately 40% was in the 0.5 M NaCl eluate. Neither eluate contained cdc2 kinase, but each could activate cdc2 kinase upon microinjection into Xenopus oocytes. The MPF activity in the two eluates, but not in the flow-through fraction, could be depleted by the mitosis-specific monoclonal antibody MPM-2. This antibody has been shown to inhibit Xenopus oocyte maturation and deplete MPF activity from mature oocyte extract but does not recognize the cdc2 kinase complex. The three MPFs differed in apparent molecular size, H1 kinase activity, and stability at 4 degrees C. We propose that MPF activity in unfertilized Xenopus eggs resides in at least three different molecular species, the combined activities of which may be required for autoamplification of MPF.

A novel M phase-specific H1 kinase recognized by the mitosis-specific monoclonal antibody MPM-2

At the onset of mitosis, eukaryotic cells display an abrupt increase in a Ca2(+)- and cyclic nucleotide-independent histone H1 kinase activity, referred to as growth-associated or M phase-specific H1 kinase. The molecular basis for this activity is generally attributed to a kinase complex that consists of the p34cdc2 protein and cyclin, and exhibits maturation-promoting factor (MPF) activity. In the present study, we show that more than one kinase contributes to M phase-specific H1 kinase activity. When mature Xenopus oocyte extract prepared with ATP gamma S and NaF was fractionated by gel filtration, two prominent peaks of H1 kinase activity were detected, with apparent molecular masses of 600 and 150 kDa. The 150-kDa kinase copurified with the p34cdc2 protein and was immobilized by the suc 1 gene product p13 and anti-cyclin B2, which are specific for the cdc2 kinase complex. However, the 600-kDa kinase did not satisfy any of these criteria, thus identifying it as a novel M phase-specific H1 kinase. Only the 600-kDa kinase was recognized by the mitosis-specific monoclonal antibody, MPM-2, which inhibits Xenopus oocyte maturation and immunodepletes MPF activity. Furthermore, not only did the full activation of this kinase (MPM-2 kinase) coincide with the activation of MPF during the cell cycle, but also MPM-2 kinase-positive fractions obtained by gel filtration accelerated progesterone-induced oocyte maturation. It is, therefore, likely that MPM-2 kinase is a positive regulator in the M phase induction pathway.

Mitosis-specific monoclonal antibody MPM-2 inhibits Xenopus oocyte maturation and depletes maturation-promoting activity

MPM-2, a monoclonal antibody specific for cells in mitosis, recognizes a family of proteins that share a common phosphorylated epitope. In this study we have shown that during the maturation of Xenopus laevis oocytes induced by progesterone, phosphorylation of MPM-2 antigens coincided with the appearance of MPF activity. When MPM-2 (0.7-1.4 micrograms per oocyte) was injected into oocytes prior to progesterone stimulation, MPF activity failed to appear and induction of maturation was inhibited as judged by both germinal-vesicle breakdown and white-spot formation. Further, MPM-2 was able to neutralize as well as immunodeplete MPF activity from mitotic HeLa cell and mature oocyte extracts. These results suggest that MPM-2 recognizes either MPF itself or a protein(s) that regulates MPF activity and that the kinase that phosphorylates MPM-2 antigens may be a key component in the regulation of M-phase induction.

Threonine phosphorylation is associated with mitosis in HeLa cells

Phosphorylation and dephosphorylation of proteins play an important role in the regulation of mitosis and meiosis. In our previous studies we have described mitosis-specific monoclonal antibody MPM-2 that recognizes a family of phosphopeptides in mitotic cells but not in interphase cells. These peptides are synthesized in S phase but modified by phosphorylation during G2/mitosis transition. The epitope for the MPM-2 is a phosphorylated site. In this study, we attempted to determine which amino acids are phosphorylated during the G2-mitosis (M) transition. We raised a polyclonal antibody against one of the antigens recognized by MPM-2, i.e. a protein of 55 kDa, that is present in interphase cells but modified by phosphorylation during mitosis. This antibody recognizes the p55 protein in both interphase and mitosis while it is recognized by the monoclonal antibody MPM-2 only in mitotic cells. Phosphoamino acid analysis of protein p55 from 32P-labeled S-phase and M-phase HeLa cell extracts after immunoprecipitation with anti-p55 antibodies revealed that threonine was extensively phosphorylated in p55 during G2-M but not in S phase, whereas serine was phosphorylated during both S and M phases. Tyrosine was not phosphorylated. Identical results were obtained when antigens recognized by MPM-2 were subjected to similar analysis. As cells completed mitosis and entered G1 phase phosphothreonine was completely dephosphorylated whereas phosphoserine was not. These results suggest that phosphorylation of threonine might be specific to some of the mitosis-related events.