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Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000.

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An Introduction to Genetic Analysis. 7th edition.

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Cell proliferation machinery

Cell cycle

There are four main parts to the cell cycle: M phasemitosis—and the three parts that are components of interphase; G1, the gap period between the end of mitosis and the start of DNA replication; S, the period during which DNA synthesis occurs; and G2, the gap period following DNA replication and preceding the initiation of the mitotic prophase. In mammals, where the cell cycle is particularly well studied, differences in the rate of cell division are largely due to differences in the length of time between entering and exiting G1. This variation is due to an optional G0 resting phase into which G1-phase cells can shunt and remain for variable lengths of time, depending on the cell type and on environmental conditions. Conversely, S, G2, and M phases are normally quite fixed in duration. In this section, we consider the molecules that drive the cell cycle. In a later section, we shall consider how these molecules are integrated into the overall biology of the cell.

Cyclins and cyclin-dependent protein kinases

The engines that drive progression from one step of the cell cycle to the next are a series of protein complexes composed of two subunits: a cyclin and a cyclin-dependent protein kinase (abbreviated CDK). In every eukaryote, there is a family of structurally and functionally related cyclin proteins. Cyclins are so named because each is found only during one or another segment of the cell cycle. The onset of the appearance of a specific cyclin is due to cell-cycle-controlled transcription, in which the previously active cyclin–CDK complex leads to the activation of a transcription factor that activates the transcription of this new cyclin. The disappearance of a cyclin depends on three events: rapid inactivation of the activator of transcription of this cyclin’s gene (so that no new mRNA is produced), a high degree of instability of the cyclin mRNA (so that the existing pool of mRNA is eliminated), and a high level of instability of the cyclin itself (so that the pool of cyclin protein is destroyed).

Cyclin-dependent protein kinases also constitute a family of structurally and functionally related proteins. Kinases are enzymes that add phosphate groups to target substrates; for protein kinases such as CDKs, the substrates are proteins. CDKs are so named because their activities are regulated by cyclins and because they catalyze the phosphorylation of specific serine and threonine residues of specific target proteins.

The target proteins for CDK phosphorylation are determined by the associated cyclin. In other words, the cyclin tethers the target protein so that the CDK can phosphorylate it (Figure 22-1), thereby changing the activity of each target protein. Because different cyclins are present at different phases of the cell cycle (Figure 22-2), different phases of the cell cycle are characterized by the phosphorylation of different target proteins. The phosphorylation events are transient and reversible. When the cyclin–CDK complex disappears, the phosphorylated substrate proteins are rapidly dephosphorylated by protein phosphatases.

Figure 22-1. The steps in phosphorylation of target proteins by the cyclin–CDK complex.

Figure 22-1

The steps in phosphorylation of target proteins by the cyclin–CDK complex. First, a cyclin and a CDK subunit bind to form an active cyclin–CDK complex. Then, the target protein binds to the cyclin part of the complex, placing the target (more...)

Figure 22-2. A current view of the variations in cyclin–CDK activities throughout the cell cycle of a mammalian cell.

Figure 22-2

A current view of the variations in cyclin–CDK activities throughout the cell cycle of a mammalian cell. The widths of the bands indicate the relative kinase activities of the various cyclin–CDK complexes. Note that several different cyclins (more...)

CDK targets

How does the phosphorylation of some target proteins control the cell cycle? Phosphorylation initiates a chain of events that culminates in the activation of certain transcription factors. These transcription factors promote the transcription of certain genes whose products are required for the next stage of the cell cycle. Much of our knowledge of the cell cycle comes from both genetic studies in yeast (see next section) and from biochemical studies of cultured mammalian cells. A well-understood example is the Rb–E2F pathway in mammalian cells. Rb is the target protein of a CDKcyclin complex called Cdk2–cyclin A, and E2F is the transcription factor that Rb regulates (Figure 22-3). From late M phase through the middle of G1, the Rb and E2F proteins are combined in a protein complex that is inactive in promoting transcription. In late G1, the Cdk2–cyclin A complex is produced and phosphorylates the Rb protein. This phosphorylation produces a change in the shape of Rb such that it can no longer bind to the E2F protein. The free E2F protein is then able to promote transcription of certain genes that encode enzymes vital for DNA synthesis. This allows the next phase of the cell cycle—S phase—to proceed.

Figure 22-3. The contributions of the Rb and E2F proteins in the regulation of the transition from G1 to S phase in a mammalian cell.

Figure 22-3

The contributions of the Rb and E2F proteins in the regulation of the transition from G1 to S phase in a mammalian cell. (From H. Lodish, D. Baltimore, A. Berk, S. L. Zipursky, P. Matsudaira, and J. Darnell, Molecular Cell Biology, 3d ed. Copyright © (more...)

Rb and E2F are in fact representatives of two families of related proteins. In mammals, different cyclinCDK complexes (Figure 22-2) are thought to selectively phosphorylate different proteins of the Rb family, each of which in turn releases the specific E2F family member to which it is bound. The different E2F transcription factors then promote the transcription of different genes that execute different aspects of the cell cycle.

MESSAGE

Sequential activation of different CDKcyclin complexes ultimately controls progression of the cell cycle.

Yeasts: genetic models for the cell cycle

Genetic contributions to our understanding of the cell cycle have largely come from studies of two fungi: the budding yeast Saccharomyces cerevesiae (Figure 22-4) and the fission yeast Schizosaccharomyces pombe (Figure 22-5). In each of these species, cell cycle genetics has revealed a large array of genetic functions needed to maintain the proper cell cycle. These functions are identified as a special class of ts (temperature-sensitive) mutations called cdc (cell division cycle) mutations

Figure 22-4. The cell cycle of the budding yeast Saccharomyces cerevisiae.

Figure 22-4

The cell cycle of the budding yeast Saccharomyces cerevisiae. The scanning electron micrograph shows cells at different points in the cell cycle, as indicated by different bud sizes. The principal events in the cell cycle are shown. (Courtesy of E. Schachtbach (more...)

Figure 22-5. The cell cycle of the fission yeast Schizosaccharomyces pombe.

Figure 22-5

The cell cycle of the fission yeast Schizosaccharomyces pombe. The scanning electron micrograph shows cells dividing symmetrically and at different points in the cell cycle. The diagram indicates the timing of different major events in the S. pombe cell (more...)

When cultured at low temperature, yeasts with these cdc mutations grow normally. When shifted to higher, restrictive temperatures, these cdc mutant yeasts no longer grow. What makes these cdc mutations novel among the more general class of ts mutations is that a particular cdc mutant stops growing at a specific time in the cell cycle, and all the yeast cells look alike. Consider some examples in S. cerevesiae, a yeast that normally divides through “budding” (Figure 22-4), a process in which a mother cell develops a small outpocketing, a “bud.” The bud grows and mitosis occurs such that one spindle pole is in the mother cell and the other is in the bud. The bud continues to grow until it is as big as the mother cell. The mother cell and the bud then separate into two daughter cells. Any run-of-the-mill ts mutation in S. cerevesiae, when shifted to restrictive temperature, stops growth at variable times in the cycle of bud formation and cell division. In contrast, after a shift to restrictive temperature, one S. cerevesiae cdc mutation produces yeast cells that arrest with only tiny buds, whereas another produces yeast cells that arrest with larger buds, half the size of the mother cell. Such different Cdc phenotypes are indicative of different defects in the machinery required to execute specific events in the progression of the cell cycle. In a similar fashion, the fission yeast S. pombe, which divides in the more usual symmetrical (fission) fashion to produce two equivalent daughter cells, has been used to generate cdc mutation and characterize the cell cycle. Interestingly, the cdc genes identified in genetic screens in these two very different yeasts encode the same sets of proteins. In other words, the cell cycle machinery in these two species is essentially identical.

With the completion of the sequencing of the S. cerevesiae genome (Chapter 14), we are in the unprecedented position of being able to identify the entire array of proteins of the cyclin and CDK families (22 and 5 members, respectively). These genes are now being systematically mutated and genetically characterized to understand how each contributes to the cell cycle.

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2000, W. H. Freeman and Company.
Bookshelf ID: NBK21924

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