Identification of a novel vertebrate cyclin: cyclin B3 shares properties with both A- and B-type cyclins.

Cyclins play a key role in controlling progression through the cell cycle. They act as regulatory subunits of p34cdc2/CDC28 and related cyclin-dependent protein kinases (cdks). In vertebrates, cyclins B1 and B2 function during M phase, whereas cyclin A is required for S phase as well as the G2 to M phase transition. Here, we describe the identification and characterization of a novel vertebrate cyclin, termed cyclin B3. The assignment of this cyclin to the B-type subfamily is based on its cDNA-derived sequence and its pattern of expression in synchronized cells, both suggesting a distant relationship to other B-type cyclins. Interestingly, however, cyclin B3 also displays properties that resemble those of A- rather than B-type cyclins. Specifically, cyclin B3 localizes to the cell nucleus throughout the cell cycle, and is able to associate in vivo with at least two kinase subunits, p34cdc2 and p33cdk2. Furthermore, deletion of 26 amino acids from the C-terminus of cyclin B3 impairs both its interaction with kinase catalytic subunits and its nuclear localization, reminiscent of recent results obtained with cyclin A. Based on these observations, we conclude that cyclin B3 may share functional properties with both A- and B-type cyclins.     

Effects of Cyclic AMP on Components of the Cell Cycle Machinery Regulating DNA Synthesis in Cultured Astrocytes

Cyclic AMP is a second messenger for various hormones that inhibits cell multiplication and DNA synthesis in cultured astrocytes. We examined the effects of increasing intracellular cyclic AMP on the catalytic (cdks) and regulatory (cyclins and ckis) components of cyclin-dependent protein kinases, which regulate progression of the cell cycle before completion of DNA synthesis, in primary cultured astrocytes and in an astrocytic cell line C.LT.T.1.1. The amount of cdk4 changed little during the cell cycle and was not affected by cyclic AMP. There was little cdk1 and cdk2 in quiescent cells, and their expression increased during the G1-S phases. Cyclic AMP strongly inhibited cdk1 and cdk2 expression. Transforming growth factor beta also inhibited cdk1 expression in primary astrocytes. Cyclic AMP did not affect the two ckis p27KIP1 and p21CIP1. There was little cyclin D1 in quiescent cells, but it increased during the G1 phase and was reduced by cyclic AMP. Cyclin E increased during the G1-S phases and was not affected by cyclic AMP in primary astrocytes. The amount of cyclin A was low in quiescent cells and increased during the G1-S phases. Expression of its mRNA and protein was inhibited by cyclic AMP. The protein kinase activities associated with complexes of cyclins and cdks were increased by growth factors and prevented by cyclic AMP. We conclude that cyclic AMP inhibits progression of the cell cycle in astrocytes at least by preventing the expression of the regulatory subunits, cyclins D1 and A, and catalytic subunits, cdk1 and cdk2, of cyclin-regulated protein kinases. Key Words: Cyclin-dependent protein kinases-Glial cells-Cyclic AMP.     

The expression and characterization of cyclin-dependent kinase 6 during the activation of murine G0 T-cells.

Recent evidence strongly suggest that the D type cyclins with cdk4 and cdk6 form holoenzymes that regulate cell cycle events earlier in G1 than cyclin E/cdk2 complexes which functions near the G1/S transition. In human T lymphocytes cdk6 has been shown to be the initial retinoblastoma protein kinase detectable at mid G1. Following activation of splenic derived murine G0T-cells, cdk6, cyclin D2 and D3 specific mRNAs were detected early in G1 and reached maximal levels prior to or near G1/S. The phosphorylation of retinoblastoma protein from T-cells was detected very early in G1 and was associated mainly with cdk6/cyclin D2 complexes which accounted for a minor portion of the total cellular cdk6 contained in the cytoplasmic fraction of T-cells and mostly in the catalytically inactive form.     

Transforming activity of Fbxo7 is mediated specifically through regulation of cyclin D/cdk6

D cyclins (D1, D2 and D3) and their catalytic subunits (cyclin-dependent kinases cdk4 and cdk6) have a facilitating, but nonessential, role in cell cycle entry. Tissue-specific functions for D-type cyclins and cdks have been reported; however, the biochemical properties of these kinases are indistinguishable. We report that an F box protein, Fbxo7, interacted with cellular and viral D cyclins and distinguished among the cdks that bind D-type cyclins, specifically binding cdk6, in vitro and in vivo. Fbxo7 specifically regulated D cyclin/cdk6 complexes: Fbxo7 knockdown decreased cdk6 association with cyclin and its overexpression increased D cyclin/cdk6 activity and E2F activity. Fbxo7 interacted with p27, but its enhancement of cyclin D/cdk6 activity was p21/p27 independent. Fbxo7 overexpression transformed murine fibroblasts, rendering them tumorigenic in athymic nude mice. Transformed phenotypes were dependent on cdk6, as knockdown of cdk6 reversed them. Fbxo7 was highly expressed in epithelial tumors, but not in normal tissues, suggesting that it may have a proto-oncogenic role in human cancers.     

A family of human cdc2-related protein kinases.

The p34cdc2 protein kinase is known to regulate important transitions in the eukaryotic cell cycle. We have identified 10 human protein kinases based on their structural relation to p34cdc2. Seven of these kinases are novel and the products of five share greater than 50% amino acid sequence identity with p34cdc2. The seven novel genes are broadly expressed in human cell lines and tissues with each displaying some cell type or tissue specificity. The cdk3 gene, like cdc2 and cdk2, can complement cdc28 mutants of Saccharomyces cerevisiae, suggesting that all three of these protein kinases can play roles in the regulation of the mammalian cell cycle. The identification of a large family of cdc2-related kinases opens the possibility of combinatorial regulation of the cell cycle together with the emerging large family of cyclins.     

G1/S phase cyclin-dependent kinase overexpression perturbs early development and delays tissue-specific differentiation in Xenopus

Cell division and differentiation are largely incompatible but the molecular links between the two processes are poorly understood. Here, we overexpress G1/S phase cyclins and cyclin-dependent kinases in Xenopus embryos to determine their effect on early development and differentiation. Overexpression of cyclin E prior to the midblastula transition (MBT), with or without cdk2, results in a loss of nuclear DNA and subsequent apoptosis at early gastrula stages. By contrast, overexpressed cyclin A2 protein does not affect early development and, when stabilised by binding to cdk2, persists to tailbud stages. Overexpression of cyclin A2/cdk2 in post-MBT embryos results in increased proliferation specifically in the epidermis with concomitant disruption of skin architecture and delay in differentiation. Moreover, ectopic cyclin A2/cdk2 also inhibits differentiation of primary neurons but does not affect muscle. Thus, overexpression of a single G1/S phase cyclin/cdk pair disrupts the balance between division and differentiation in the early vertebrate embryo in a tissue-specific manner.     

PCR方法检测水稻中存在G蛋白基因家族

ＰＣＲ方法检测水稻中存在Ｇ蛋白基因家族陈忠英,王钧（中国科学院上海植物生理研究所,２００００３２）关键词Ｇ蛋白;聚合酶链式反应（ＰＣＲ）;水稻苗;ＤＮＡ序列分析植物细胞对各种外源和内源的刺激,如光、重力、病原、激素等都有灵敏复杂的反应,但对植物信息传...     

Inhibition of Cdk1 by Alsterpaullone and Thioflavopiridol Correlates with Increased Transit Time from Mid G2 Through Prophase

Current models suggest that cyclin B1/cdk1 regulates the G2 to M transition and that its activity is maximal during the period from prophase to metaphase in mammalian cells. Although data are lacking, the idea that cyclin B1/cdk1 regulates the transit time from prophase to metaphase is reasonable. Development of small molecule inhibitors of cyclin dependent kinases (cdk’s) as cancer therapeutics presents an opportunity to evaluate the effects of inhibiting cdk’s in asynchronous cell populations. Analysis of cdk1 inhibitors is complicated by their ability to inhibit other cdk’s in vitro at higher concentrations. In this study we measured the effects of two cdk1 inhibitors on S, G2, and M transit for Hela cells and correlated these effects on cyclin B1/cdk1 and cyclin A/cdk2 activities. Dose responses demonstrate that low concentrations of both compounds inhibited the activity of cdk1 but not cdk2 in HeLa cells. The partial loss of cdk1 activity at low doses induced a prophase accumulation during a 3 h period and an increased transit time through mitosis. In addition, both inhibitors lengthened the G2 transit time with progressively greater effect on mid and late G2. High doses of both inhibitors increased the S phase time, which correlated with the inhibition of cdk2 activity. These results suggest that cdk1-cyclin activity is rate limiting for cell cycle progression during a period from mid G2 through prophase.