Mitosis-associated kinase

Mineral fibers, particularly asbestos, can activate protein kinase C and increase expression of ornithine decarboxylase, the proto-oncogenes, c-fos, and c-jun (18-20), and the nuclear transcription factor, NF-kB (21). All these events are associated with cell proliferation and, potentially, with neoplastic transformation. Recently, Zanella et al. (22) have shown that asbestos activates the mitosis-associated kinase (MAP) system, apparently through interaction with the epidermal growth factor receptor, probably implying that some of these events are mediated by signal transduction pathways that start on the cell surface, rather than by fiber uptake, although this point needs to be directly examined. 

In the filamentous fungus, Aspergillus nidulans, NIMA (never in mitosis, gene A) kinase phosphorylates H3 at Ser-10 [48]. At mitosis NIMA kinase association with chromatin increases and following metaphase NIMA locates to the mitotic spindle and spindle pole bodies. A human NIMA-related kinase (Nek6) was identified as a putative mitotic HI and H3 kinase [49]. 

Zecevic M, Catling AD, Eblen ST et al 1998 Active MAP kinase in mitosis localization at kinetochores and association with the motor protein CENP-E. J Cell Biol 142 1547—1558... 

Histone H3 (Til) phosphorylation occurs during mitosis by Dlk/ZIP kinase (Dlk Death-associated protein (DAP)-like kinase, ZIP Zipper interacting protein kinase) (Preuss et al, 2003) (Table 1). Histone H3 at Serine 28 is phosphorylated by Aurora B kinase at mitosis and this phosphorylation coincides with chromosome condensation (Goto et al., 1999, Goto et al, 2002) (Fig. 2), (Table 1). Histone H3 (S28) phosphorylation initiates at prophase, whereas histone H3 (SIO) phosphorylation initiates during the late G2 phase (Hendzel et al, 1997). 

In interphase, microtubules are stabilized by several kinds of proteins that are found all along microtubules and are called MAPs. They tend to have repeating domains, which allow each MAP molecule to associate with more than one tubulin dimer. This produces a doubly effective method of controlling assembly, in that the conformations of several tubulin dimers may be individually stabilized and the stabilized subunits are also cross-linked. The binding of these structural MAPs is in turn controlled by kinases and phosphatases (Cassimeris and Spittle, 2001). During mitosis they are phosphorylated and detach from tubulin, whose assembly and disassembly comes under the control of proteins that operate more at the ends of microtubules. Differentiated cells, such as neurons, do not divide. However, as microtubules and MAPs are slowly transported along axons (Baas and Buster, 2004), the MAPs maybe phosphorylated in particular places, at times when structural plasticity is required for making synapses or other contacts. 

The increase in nuclear cyclin B/CDKl activity promotes phosphorylation of nuclear substrates that are necessary for mitosis, such as nuclear envelope breakdown, spindle formation, chromatin condensation, and restmcturing of the Golgi and endoplasmic reticulum (85, 86). Numerous cyclin B/CDKl substrates have been dehned, which include nuclear lamins, nucleolar proteins, centrosomal proteins, components of the nuclear pore complex, and microtubule-associated proteins (87-89). Cyclin B/CDKl complexes also phosphorylate MCM4 to block replication of DNA, the TFIIH subunit of RNA polymerase II to inhibit transcription, and the ribosomal S6 protein kinase to prevent translation during mitosis (90-92). 

Long JJ, Leresche A, Kriwacki RW, Gottesfeld JM. Repression of TFIIH transcriptional activity and TFIIH-associated cdk7 kinase activity at mitosis. Mol. Cell. Biol. 1998 18 1467-1476. 

Figure 4 Correction of improper chromosome attachments by activation of Aurora kinase (45). (a) Assay schematic, (i) Treatment with the Eg5 inhibitor monastrol arrests cells in mitosis with monopolar spindles, in which sister chromosomes often are both attached to the single spindle pole, (ii) Hesperadin, an Aurora kinase inhibitor, is added as monastrol is removed. As the spindle bipolarizes with Aurora kinase inhibited, attachment errors fail to correct so that some sister chromosomes are still attached to the same pole of the bipolar spindle, (iii) Removal of hesperadin activates Aurora kinase. Incorrect attachments are destabilized by disassembling the microtubule fibers, which pulls the chromosomes to the pole, whereas correct attachments are stable, (iv) Chromosomes move from the pole to the center of the spindle as correct attachments form, (b) Structures of the Eg5 inhibitor monastrol and two Aurora kinase inhibitors, hesperadin and AKI-1. (c) Spindles were fixed after bipolarization either in the absence (i) or presence (ii) of an Aurora kinase inhibitor. Arrows indicate sister chromosomes that are both attached to the same spindle pole. Projections of multiple image planes are shown, with optical sections of boxed regions (1 and 2) to highlight attachment errors. Scale bars 5 xm. (d) After the removal of hesperadin, GFP-tubulin (top) and chromosomes (bottom) were imaged live by three-dimensional confocal fluorescence microcopy and DIC, respectively. Arrow and arrowhead show two chromosomes that move to the spindle pole (marked by circle in DIC images) as the associated kinetochore-microtubule fibers shorten and that then move to the center of the spindle. Time (minutes seconds) after the removal of hesperadin. Scale bar 5 (cm.

Thus, the differential expression of genes is required for the production of different proteins because each protein controls a distinct function. The function of many proteins is listed in Table 1.1. In addition, the protein profile of a cell can vary depending on the different kinds of modification of the same protein such modifications of protein may involve acetylation, phosphorylation, glycosylation, or association with lipid or carbohydrate molecules. These modifications in proteins occur as posttranslational events and alter the function of proteins. One example is the mitosis activator protein (MAP) kinase protein controlling the mitosis this protein is activated by phosphorylation to give MAP Kinase (MAPK), MAP kinase kinase (MAPKK), and MAP kinase kinase kinase (MAPKKK). The role of protein modification in the control of cellular activity is discussed later in this book. 

The association of ATR with replication forks is thought to activate its protein kinase activity, leading to the phosphorylation and activation of the Chkl kinase. Active Chkl then phosphorylates and inactivates the Cdc25 phosphatase (Cdc25C in vertebrates), which normally removes the inhibitory phosphate from CDKs that function during mitosis. As a result, the cyclln A/B-CDKl complexes remain inhibited and cannot phosphorylate targets required to initiate mitosis. ATR continues to initiate this protein kinase cascade until all replication forks complete DNA replication and disassemble. 

In cultured tumor cells, resistance to taxanes is associated in some lines with increased expression of the mdr-1 gene and its product, the P-glycoprotein other resistant cells have P-tubulin mutations, and these latter cells may display heightened sensitivity to vinca alkaloids. Other cell lines display an increase in survivin, an antiapoptotic factor or aurora kinase, an enzyme that promotes completion of mitosis. The basis of clinical drug resistance is not known. Cell death occurs by apoptosis, but the effectiveness of paclitaxel against experimental tumors does not depend on an intact p53 gene product. 

Yeast control - cdc2 (from yeast) is a serine/threonine kinase whose activation requires association with specific small proteins called cyclins. In yeast, one cyclin activates cdc2 at the start of S phase, another reactivates it at the beginning of mitosis. 

Cyclins/cyclin-dependent kinases in higher eukaryotes - In higher eukaryotes, there are several "cyclin-dependent kinases" and a number of cyclins to associate with them. Each transition in the cell cycle appears to have a unique cyclin/kinase complex as its trigger. A simplified view of the roles of these proteins in mammalian cells is shown in Figure 28.16. The cyclin-dependent kinase CDK2 is involved in the entrance to S-phase, and cdc2, with cyclins A and B, regulates mitosis. 

A single oscillahon in is kinase activity induced by a B-type cyclin can promote both replication and mitosis. Flowever, in S.cerevisiae there are 14 different cyclinlike proteins, and their individual funchons are not clear. signal that is sent to the ORCs is likewise unclear.3i3 However, theoretical models involving Eq. 26-3 and many additional components have been proposed. Multiple phosphorylations may occur, some on the RPA initiator protein. Many proteins required for replication, including DNA polymerase and primase, are associated with the nuclear matrix. i The nuclear membrane may also be important in controlling replication. 

Finally, cytokinins were reported to activate the p34 protein kinase in tobacco pith cells [93]. Cells not supplied with cytokinin (benzylaminopurine) became arrested in Gj phase of the mitotic cycle and contained inactive kinase. The kinase activation process, which appears to involve the phosphorylation of tyrosine at sites within the kinase molecule, could be relevant for the remodelling of the cytoskeleton prior to mitosis. Activated p34 protein kinase was shown specifically to bind to and also to disintegrate MTs of pre-prophase bands [94,95]. Also relevant in this respect is the finding that plant cyclins associate with various MT arrays during the mitotic cycle, including the preprophase band [96]. These reports indicate that advances in understanding of the controls of MT activity by cytokinins are at last in prospect. 

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