Cyclin Table

Othet cyclins and CDKs are involved in different aspects of cell cycle progression (Table 36-7). Cychn E and CDK2 form a complex in late Gl. Cychn E is tapidly degraded, and the released CDK2 then fotms a complex with cyclin A. This sequence is necessaty fot the initiation of DNA synthesis in S phase. A complex between cychn B and CDKl is tate-hmiting fot the G2/M transition in eukatyotic cells. 

Table 36-7. Cyclins and cyclin-dependent kinases involved in cell cycle progression.

Cell cycle is one of the physiological processes in which the role of uhiquitin—proteasome-mediated proteolysis is well established. With the advent of yeast mutants that interfered with various phases of the cell cycle, cyclin-dependent kinases (Cdks) were found to have a critical role in regulating the cell cycle. Typically, Cdks activated hy regulatory proteins are known as cyclins. Different Cdk—cyclin complexes are formed at specific stages of the cell cycle such as the S-phase (in which DNA synthesis occurs) and the metaphase. The transition from metaphase to anaphase depends on degradation of cyclins. " " Systematic biochemical studies showed that cyclins were substrates for the uhiquitin—proteasome pathway (Table 5). 

The periodic recurrence of cell division suggests that globally the cell cycle functions like an autonomous oscillator. An extended model incorporating the sequential activation of the various cyclin-dependent kinases, followed by their inactivation, shows that even in the absence of control by cell mass, this sequence of biochemical events can operate as a limit cycle oscillator [145]. This supports the union of the two views of the cell cycle as dominoes and clock [146]. Because of the existence of checkpoints, however, the cell cycle stops at the end of certain phases before engaging in the next one. Thus the cell cycle looks more like an oscillator that slows down and makes occasional stops. A metaphor for such behavior is provided by the movement of the round plate on the table in a Chinese restaurant, which would rotate continuously under the movement imparted by the participants, were it not for frequent stops. 

The different cyclins preferentially form complexes with certain types of CDKs, which is why specific combinations of cyclins and CDKs are observed in the cell cycle (see Table 13.1). 

The equivalent cyclins in yeast have a different classification and are shown in Table 13.1. 

In the center of the duplex shifts upfield by 0.15 ppm (Table VI, Figure 25) and the thymidine CH3-5 which Is directed towards the major groove shifts upfield by 0.1 ppm (Figure 29). It should be noted that such an upfield shift of the thymidine CH3-5 group was not observed in the intercalation complexes of ethidium (11), proflavine (25), terpyridylplatinum II (11) and nitroaniline dication with poly(dA-dT). The results require that at least one thymidine CH3-5 group project onto the periphery of the anthra-cycline ring system at the intercalation site. 

Table 1 Selected cdk/cyclin kinase inhibitory agent derivatives and their corresponding analytical characterization data. H NMR data supplied by author...

The method of Kitagawa (1) was used to determine cyclin D2-CDK4 activity. The synthetic peptide Arg-Pro-Pro-Thr-Leu-Ser-Pro-Ile-Pro-His-Ile-Pro-Arg that corresponds to the amino acid sequence No. 775-787 of RB protein was used as a substrate. IC50 testing results are provided in Table 2. 

In the absence of cyclin-A, the C helix is twisted and the conserved Glu-51 residue on its surface faces the solvent and is unable to coordinate with Lys-33, which instead coordinates with Asp-145. The torsion angles of Phe-146 and Asp-145 in the DFG motif are typical for an inactive kinase [15] (Table 7.2) and show that the orientation of Asp-145 is unfit for catalysis. 

Since the main route of metabolism and ehmination of anthracyclines is via the bile, dosage reduction is recommended if there is hepatic impairment. This was first suggested after a report of increased toxicity in patients with hver metastases who received fuU-dose anthra-cycline treatment, followed by a second report that suggested that the clearance of anthracychnes is reduced in patients with hepatic metastases (90,91). These reports led to the current recommendations for anthracycline doses, based on serum bihrubin concentration or sulfo-bromophthalein clearance. However, the question of whether liver dysfunction significantly affects anthracycline clearance is unclear, and the dosage modifications suggested (see Table 2) have never been validated. Indeed, there is evidence that anthracycline kinetics are altered in patients with raised serum transaminases alone. 

Cannabinoids have been shown to induce cell cycle arrest in breast carcinoma (De Petrocellis et al. 1998), prostate carcinoma (Melck et al. 2000) and thyroid epithelioma cells (Bifulco et al. 2001). In breast carcinoma cells this has been ascribed to the inhibition of adenylyl cyclase and the cAMP/protein kinase A (PKA) pathway (Table 1). PKA phosphorylates and inhibits Raf-1, so cannabinoids prevent the inhibition of Raf-1 and induce prolonged activation of the Raf-l/MEK/ERK signalling cascade (Melck et al 1999). Cannabinoid-induced inhibition of thyroid epithelioma cell proliferation has been attributed to the induction of the cyclin-dependent kinase inhibitor p27 P (Portella et al. 2003). 

The role of the cyclins is to convert CDKs into an active state. This process confers specificity to CDK activation, because a specific cyclin preferentially binds and activates only a certain CDK (see Table 13.1). The concentration of the cyclins is an important factor in the control of CDK activity. Various mechanisms exist to control the level of cyclins available for CDK binding (see below). 

Peptide motifs. Proteins with certain homologous sequences are rapidly degraded. For example, proteins that have extended sequences containing proline, glutamate, serine, and threonine have half-lives of less than 2 hours. (PEST sequences are named for the one-letter abbreviations for these amino acids. See Table 5.1.) The cyclin destruction box is a set of homologous nine-residue sequences near the N-terminus of cyclins that ensures rapid ubiquination. 

In the following sections we describe critical experiments that led to the current model of eukaryotic cell-cycle regulation summarized in Figure 21-2 and present further details of the various regulatory events. As we will see, results obtained with different experimental systems and approaches have provided insights about each of the key transition points in the cell cycle. For historical reasons, the names of various cyclins and cyclln-dependent kinases from yeasts and vertebrates differ. Table 21-1 lists the names of those that we discuss in this chapter and indicates when in the cell cycle they are active. 

Table 21-1, presented early In this chapter, summarizes the various cyclins and CDKs that we have mentioned and the portions of the cell cycle In which they are active. The cyclins fall Into two major groups, Gi cyclins and B-type cy-... 

Several substrates of cyclin-dq>endent kinase have been identified from in vitro studies, including nuclear lamins, vimentin, caldesmon, histone Hi and RNA polymerase 11 (Table 11.1) and these are the most likely physiological substrates. The onset of mitosis is marked by at least four morphological changes ... 

---