P a s t S t a rt 22 , , For a cell colony to thrive in the wild it is not enough simply to pass Start: it is essential to pass it

14 18 P a s t S t a rt 22 , , For a cell colony to thrive in the wild it is not enough simply to pass Start: it is essential to pass it

at the right moment. For budding yeast at least three conditions are crucial - cell size, the availability of nutrients, and the demands of sex. If the cell is too small, the control system pauses to give time for growth. If the cell is starved, the control system also pauses, delaying the cell's at the right moment. For budding yeast at least three conditions are crucial - cell size, the availability of nutrients, and the demands of sex. If the cell is too small, the control system pauses to give time for growth. If the cell is starved, the control system also pauses, delaying the cell's

haploid partner (see Figure 17-22). Thus size, food, and sex together govern a three-way choice that the cell faces as it approaches Start (Figure 17-28). Mutations affecting the response of budding yeast cells to these influences have identified the cell-cycle control components that

govern progress from G 1 to S.

The search for cdc genes in budding yeast revealed several that are necessary for a cell to pass Start. A mutant with a defect in any of these genes comes to a halt in G 1 despite being large

enough to pass Start. One of the cdc genes identified in this way, named CDC28 in budding yeast, turned out to be homologous to the fission yeast cdc2: the two genes have similar sequences and are functionally interchangeable. This discovery exposed a remarkable link between Start (the predominant checkpoint in budding yeast) and mitotic entry (the predominant checkpoint in fission yeast). We now know that the cell cycles of both types of yeast include both types of checkpoint, and in both types of yeast the product of the same gene serves both to drive the cell into mitosis and to drive it past Start so as to initiate the replication of DNA. In this chapter, for clarity and to emphasize its universal role, we call this gene cdc2 regardless of species.

The Cdc2 protein has distinct activities at the two different checkpoints and is associated with different cyclins. In G 2 , as we have seen, it associates with mitotic cyclin to form MPF; in G 1 it

associates with G 1 cyclin to form a complex that we shall refer to as Start kinase. Start kinase and MPF presumably phosphorylate different sets of target proteins, or phosphorylate them differently,

or both. The specificity of Cdc2 action, therefore, appears to depend on the type of cyclin that is associated with it (Figure 17-29).

The G 1 cyclin class of proteins was discovered in budding yeast through studies of mutant cells that passed Start either prematurely or under conditions where nonmutant cells would not. Three

genes identified by these mutations turned out to code for proteins distantly related to mitotic cyclins. This earned the proteins the name of "G1 cyclins" and immediately suggested that the gene products might play an activating role at Start analogous to the role of mitotic cyclins at the

G 2 checkpoint (Figure17-30). The mutant phenotypes could be shown to result from an excess of G 1 -cyclin activity. Surprisingly,

however, deletion of any single one of the G 1 -cyclin genes had practically no effect; only when all three of the identified genes were simultaneously deleted did the cells become arrested in G 1 , unable to pass Start. It seems therefore that there are at least three G 1 cyclins in the normal yeast cell, that their function is required for the cell to get past Start, and that they are, to some extent at

least, functionally interchangeable so that the cell cycle can still continue if one or two of the three are missing.

In a normal cycle the three G 1 cyclins are thought to collaborate to ensure that cells pass Start briskly, decisively, and irreversibly by an explosive activation of Start kinase analogous to the

explosive activation of MPF at the onset of mitosis. Although many uncertainties remain, the mechanism is again thought to depend on positive feedback, although by a different pathway: the

Cdc2 kinase when bound to one of the G 1 cyclins is believed to form an active complex that induces transcription of the genes encoding the other two G 1 cyclins, which bind to Cdc2 in turn Cdc2 kinase when bound to one of the G 1 cyclins is believed to form an active complex that induces transcription of the genes encoding the other two G 1 cyclins, which bind to Cdc2 in turn

cell, and, so far as is known, they then play no further part until the G 1 phase of the next cycle.

Th e G 1 Cy c lin s Me d ia t e Mu lt ip le Co n t ro ls Th a t Op e ra t e a t