A complex degradation signal in Cyclin A required for G1 arrest, and a C-terminal region for mitosis

Abstract

The destruction box (D-box) consensus sequence has been defined as a motif mediating polyubiquitylation and proteolysis of B-type cyclins during mitosis. We show here that the regions with similarity to D-boxes are not required for mitotic degradation of Drosophila Cyclin A. Instead of a simple D-box, a complex N-terminal degradation signal is present in this cyclin. Mutations that impair or abolish mitotic Cyclin A destruction delay progression through metaphase, but only when overexpressed. Moreover, these mutations prevent epidermal cells from entering the first G1 phase of embryogenesis and lead to a complete extra division cycle instead of a timely cell proliferation arrest. Residual Cyclin A activity after mitosis, therefore, has S phase-promoting activity. In principle, an S phase defect could also explain why epidermal cells fail to enter mitosis 16 in mutants lacking zygotic Cyclin A function. However, we demonstrate that this failure of mitosis is not caused simply by DNA replication or damage checkpoints. Entry into mitosis requires a function of Cyclin A that does not depend on the presence of the N-terminal region.

Anaphase-promoting complex-dependent proteolysis of cell cycle regulators and genomic instability of cancer cells

“…Cyclin A2 is also a substrate of APC Cdh1 , keeping cyclin A2 activity low during G1 (Sorensen et al, 2000(Sorensen et al, , 2001Geley et al, 2001). Mutation of the destruction box alone does not stabilize cyclin A and the degradation signal is more complex (den Elzen and Pines, 2001;Geley et al, 2001;Jacobs et al, 2001). Overexpression of stable cyclin A2 in human cells can delay chromosome alignment and sister chromatid segregation (den Elzen and Pines, 2001) or arrest cells in anaphase (Geley et al, 2001).…”

“…Overexpression of stable cyclin A2 in human cells can delay chromosome alignment and sister chromatid segregation (den Elzen and Pines, 2001) or arrest cells in anaphase (Geley et al, 2001). Expression of stable cyclin A at endogenous expression levels had little effect on mitosis at least in flies, but is required for G1 arrest (Jacobs et al, 2001). The role of cyclin A2 degradation in human cells at endogenous expression is not clear to date and remains to be defined.…”

Triggering the all-or-nothing switch into mitosis

“…Further genetic analysis has led to the demonstration that there are three distinct cyclins that make overlapping contributions to mitosis 5,7,8 . Cyclin A is the only cyclin that is essential for mitosis in Drosophila: cyclin A mutants arrest in G2, indicating that this cyclin has a role in triggering entry into mitosis 5,9 . Cyclin B3 (which, despite its name, is not a subtle variant of cyclin B but a separate, more distantly related, cyclin type that is conserved from Caenorhabditis elegans to vertebrates 10,11 ) also has a mitotic role 7 .…”

Centrosome overduplication, increased ploidy and transformation in cells expressing endoplasmic reticulum-associated cyclin A2

“…Our data indicate that constitutive cytoplasmic cyclin A2-kinase activity impinges on cell cycle regulation, without arresting the cell-cycle. In contrast, stabilized versions of cyclin A have resulted in premature S phase entry (Sprenger et al, 1997) and metaphase delay (Jacobs et al, 2001;Parry and O'Farrell, 2001) when overexpressed in Drosophila. Similarly, in human Hela cells, the expression of cyclin A mutants lacking the degradation signal arrested cells in the anaphase (Geley et al, 2001).…”