Pattern- and growth-linked cell cycles in Drosophila development.

During Drosophila development the cell cycle is subject to diverse regulatory inputs. In embryos, cells divide in stereotypic patterns that correspond to the cell fate map. There is little cell growth during this period, and cell proliferation is regulated at G2/M transitions by patterned transcription of the Cdk1-activator, Cdc25/String. The string locus senses pattern information via a > 40 kb cis-regulatory region composed of many cell-type specific transcriptional enhancers. Later, in differentiated larval tissues, the cell cycle responds to nutrition via mechanisms that sense cellular growth. These larval cell cycles lack mitoses altogether, and are regulated at G/S transitions. Cells in developing imaginal discs exhibit a cycle that is regulated at both G1/S and G2/M transitions. G2/M progression in disc cells is regulated, as in the embryo, by string transcription and is thus influenced by the many transcription factors that interact with string's 'pattern-sensing' control region. G1/S progression in disc cells is controlled, at least in part, by factors that regulate cell growth such as Myc, Ras and phosphatidylinositol-3-kinase. Thus G1/S progression appears to be growth-coupled, much as in the larval endocycles. The dual control mechanism used by imaginal disc cells allows integration of diverse inputs which operate in both cell specification and cell metabolism.

The puc1 cyclin regulates the G1 phase of the fission yeast cell cycle in response to cell size.

Eukaryotic cells coordinate cell size with cell division by regulating the length of the G1 and G2 phases of the cell cycle. In fission yeast, the length of the G1 phase depends on a precise balance between levels of positive (cig1, cig2, puc1, and cdc13 cyclins) and negative (rum1 and ste9-APC) regulators of cdc2. Early in G1, cyclin proteolysis and rum1 inhibition keep the cdc2/cyclin complexes inactive. At the end of G1, the balance is reversed and cdc2/cyclin activity down-regulates both rum1 and the cyclin-degrading activity of the APC. Here we present data showing that the puc1 cyclin, a close relative of the Cln cyclins in budding yeast, plays an important role in regulating the length of G1. Fission yeast cells lacking cig1 and cig2 have a cell cycle distribution similar to that of wild-type cells, with a short G1 and a long G2. However, when the puc1(+) gene is deleted in this genetic background, the length of G1 is extended and these cells undergo S phase with a greater cell size than wild-type cells. This G1 delay is completely abolished in cells lacking rum1. Cdc2/puc1 function may be important to down-regulate the rum1 Cdk inhibitor at the end of G1.

Regulation of the G1 phase of the cell cycle by periodic stabilization and degradation of the p25rum1 CDK inhibitor.

In fission yeast, the cyclin-dependent kinase (CDK) inhibitor p25(rum1) is a key regulator of progression through the G1 phase of the cell cycle. We show here that p25(rum1) protein levels are sharply periodic. p25(rum1) begins to accumulate at anaphase, persists in G1 and is destroyed during S phase. p25(rum1 )is stabilized and polyubiquitinated in a mutant defective in the 26S proteasome, suggesting that its degradation normally occurs through the ubiquitin-dependent 26S proteasome pathway. Phosphorylation of p25(rum1 )by cdc2-cyclin complexes at residues T58 and T62 is important to target the protein for degradation. Mutation of one or both of these residues to alanine causes stabilization of p25(rum1) and induces a cell cycle delay in G1 and polyploidization due to occasional re-initiation of DNA replication before mitosis. The CDK-cyclin complex cdc2-cig1, which is insensitive to p25(rum1 )inhibition, seems to be the main kinase that phosphorylates p25(rum1). Phosphorylation of p25(rum1) in S phase and G2 serves as the trigger for p25(rum1) proteolysis. Thus, periodic accumulation and degradation of the CDK inhibitor p25(rum1 )in G1 plays a role in setting a threshold of cyclin levels important in determining the length of the pre-Start G1 phase and in ensuring the correct order of cell cycle events.

Recent advances on cyclins, CDKs and CDK inhibitors.

In eukaryotes, cell division is controlled by cyclin-dependent kinases (CDKs). Here we summarize a few new developments on the regulation of the cell cycle by CDK-cyclin complexes. We have focused on three aspects in which there has been recent progress: the structural analysis of these complexes, the phenotypes of mice carrying knockouts of CDK inhibitors and the role of proteolysis in the regulation of the cell cycle.

B-type cyclins regulate G1 progression in fission yeast in opposition to the p25rum1 cdk inhibitor.

The onset of S phase in fission yeast is regulated at Start, the point of commitment to the mitotic cell cycle. The p34cdc2 kinase is essential for G1 progression past Start, but until now its regulation has been poorly understood. Here we show that the cig2/cyc17 B-type cyclin has an important role in G1 progression, and demonstrate that p34cdc2 kinase activity is periodically associated with cig2 in G1. Cells lacking cig2 are defective in G1 progression, and this is particularly clear in small cells that must regulate Start with respect to cell size. We also find that the cig1 B-type cyclin can promote G1 progression. Whilst p25rum1 can inhibit cig2/cdc2 activity in vitro, and may transiently inhibit this complex in vivo, cig1 is regulated independently of p25rum1. Since cig1/cdc2 kinase activity peaks in mitotic cells, and decreases after mitosis with similar kinetics to cdc13-associated kinase activity, we suggest that cig2 is likely to be the principal fission yeast G1 cyclin. cig2 protein levels accumulate in G1 cells, and we propose that p25rum1 may transiently inhibit cig2-associated p34cdc2 activity until the critical cell size required for Start is reached.

Regulation of G1 progression in fission yeast by the rum1+ gene product.

Recently it has been found that B-type cyclins in fission yeast regulate the activation of the cdc2 kinase to promote the onset of both DNA replication and mitosis. cig2 is the major G1 cyclin while cdc13 is the principal mitotic cyclin. cdc13 also has an additional function in G2 phase, preventing more than one round of DNA replication per cell cycle. In opposition to these cyclins the rum1 inhibitor, a protein present exclusively in G1, prevents premature activation of the cdc2/cig2 and the cdc2/cdc13 complexes until cells have reached the critical cell size required to pass Start and initiate a new cell cycle.