B-type cyclins

Bueno, A., and Russell, P. (1993). Two fission yeast B-type cyclins, cig2 and cdc 13, have different functions in mitosis. Mol. Cell. Biol. 13 2286-2297. 

Bueno, A Richardson, H., Reed, S. I., and Russell, P. (1991). A fission yeast B-type cyclin functioning early in the cell cycle. Cell 66 149-159. 

Chapman, D. L., and Wolgemuth, D. J. (1992). Identification of a mouse B-type cyclin which exhibits developmentally regulated expression in the germ line. Mol. Reprod. Dev. 33 259-269. 

Fitch, I., Dahmann, C., Surana, U., Amon, A., Nasmyth, K., Goetsch, L., Byers, B., and Futcher, B. (1992). Characterization of four B-type cyclin genes of the budding yeast Saccharomyces cerevisiae. Mol. Biol. Cell 3 805-818. 

Galaktionov, K., and Beach, D. (1991). Specific activation of cdc25 tyrosine phosphatases by B-type cyclins evidence for multiple roles of mitotic cyclins. Cell 67 1181-1194. 

Grandin, N., and Reed, S. I. (1993). Differential function and expression of Saccharomy-ces cerevisiae B-type cyclins in mitosis and meiosis. Mol. Cell. Biol. 13 2113— 2125. 

Whitfield, W. G. F., Gonzales, C., Maldonado-Codina, G., and Glover, D. M. (1990). The A- and B-type cyclins of Drosophila are accumulated and destroyed in temporally distinct events that define separable phases of the G2-M transission. EMBO J. 9 2563-2572. 

Hunt There is another complication here it turns out that the cyclin A in CSF-arrested frog eggs is extremely unstable. The APC is certainly active in that situation, and the checkpoint that prevents the degradation of B-type cyclins can somehow recognize and spare the cyclin B, but has no effect on cyclin A. 

Hunt I think it is very mysterious. The other very mysterious thing is since we know that MAPK is required for entry into meiosis I, how come there is this round of degradation of what is presumably the Pdsl equivalent and the B-type cyclins between meiosis I and meiosis II ... 

Irniger S, Piatti S, Michaelis C, Nasmyth K 1995 Genes involved in sister chromatid separation are needed for B-type cyclin proteolysis in budding yeast. Cell 81 269-278 (erratum 1998 Cell 93 487)... 

Hunt In frogs, if you remove the B type cyclins using antisense RNA, this allows you to go straight into a premature S phase. 

ScHWOB, E., Bohm, T., Mendenhall, M. D., and Nasmyth, K. The B-type cyclin kinase inhibitor p40SICl controls the Cl to S transition in S. cerevisiae. Cell 1994, 79, 233-44. 

The G]/S cyclins include the D and E type cyclins the M phase specific cyclins include the B type cyclins. Cyclins of type A are active in S, G2 and M phases. 

There are marked differences in the rate and duration of proteolysis of cyclins in different organisms. At the early stages of amphibian embryogenesis, cyclin destruction is only of brief duration and cyclin concentrations are kept high, as one would expect, because they are needed. To the contrary, in cultured fibroblasts, proteolysis continues for several hours and is only terminated at the onset of the S phase, when all B-type cyclins have been completely removed. 

Cdk s are cyclin dependent serine threonine protein kinases. There are several Cdk s. Cdkl is identical with cdc2, the kinase encoded by cdc2. Mammalian Cdkl is associated with both A and B type cyclins. 

The M-phase-specific cyclins include the B type cyclins. 

Most substrates for APC-mediated ubiquitinylation carry a particular sequence, the destruction box. For B-type cyclins, the consensus sequence of the destruction box is R-ALGVN/D/EI-N. Deletion of the destruction box in cyclin B causes its stabilization. 

Further studies revealed that vertebrate cells contain three proteins that can function like cyclin B to stimulate Xenopus oocyte maturation two closely related cyclin Bs and cyclin A. Collectively called B-type cyclins, these proteins exhibit regions of high sequence homology. (B-type cyclins are distinguished from Gi cyclins described in Section 21.5.) In intact cells, degradation of all the B-type cyclins begins after the onset of anaphase, the period of mitosis when sister chromatids are separated and pulled toward opposite spindle poles. 

B-type cyclin required to activate the catalytic subunit. By analogy, it seemed likely that 5. cerevisiae contains an S phase-promoting factor (SPF) that phosphorylates and regulates proteins required for DNA synthesis. Similar to MPF, SPF was proposed to be a heterodimer composed of the S. cerevisiae CDK and a cyclin, in this case one that acts in Gi (see Figure 21-2, steps [2]- 4]). 

As the S-phase cyclin-CDK heterodimers accumulate In late Gi, they are immediately inactivated by binding of an Inhibitor, called Sid, that is expressed late In mitosis and in early Gi. Because Sid specifically inhibits B-type cyclin-CDK complexes, but has no effect on the Gi cyclin-CDK complexes, it functions as an S-phase inhibitor. 

As discussed already, Slcl also inhibits the S-phase cyclln-CDKs as they are formed in mld-Gi (see Figure 21-24). This inhibitor of B-type cyclins thus serves a dual function in the cell cycle, contributing to the exit from mitosis and delaying entry into the S phase until the cell is ready. 

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