Anaphase-Promoting Complex/Cyclosome–Dependent Proteolysis of Human Cyclin a Starts at the Beginning of Mitosis and Is Not Subject to the Spindle Assembly Checkpoint

Abstract

Cyclin A is a stable protein in S and G2 phases, but is destabilized when cells enter mitosis and is almost completely degraded before the metaphase to anaphase transition. Microinjection of antibodies against subunits of the anaphase-promoting complex/cyclosome (APC/C) or against human Cdc20 (fizzy) arrested cells at metaphase and stabilized both cyclins A and B1. Cyclin A was efficiently polyubiquitylated by Cdc20 or Cdh1-activated APC/C in vitro, but in contrast to cyclin B1, the proteolysis of cyclin A was not delayed by the spindle assembly checkpoint. The degradation of cyclin B1 was accelerated by inhibition of the spindle assembly checkpoint. These data suggest that the APC/C is activated as cells enter mitosis and immediately targets cyclin A for degradation, whereas the spindle assembly checkpoint delays the degradation of cyclin B1 until the metaphase to anaphase transition. The “destruction box” (D-box) of cyclin A is 10–20 residues longer than that of cyclin B. Overexpression of wild-type cyclin A delayed the metaphase to anaphase transition, whereas expression of cyclin A mutants lacking a D-box arrested cells in anaphase.

Aberrant expression of cell cycle regulators in Hodgkin and Reed–Sternberg cells of classical Hodgkin's lymphoma

“…In normal, as well as transformed, human cells, cyclin A becomes detectable at the onset of the S-phase, accumulates throughout S-and G 2 -phase, and then rapidly declines in early mitosis (reviewed in Gillett and Barnes, 13 den Elzen and Pines 15 and Geley et al 16 ). Cyclin A can, therefore, be regarded as a marker of proliferation, since only cells in S-and G 2 -phase or early mitosis stain positively with anticyclin A antibodies, while cells in G 1 -phase remain negative.…”

“…Cyclin A levels and associated cyclin-dependent kinase 2 activity steadily increase during S-phase until prometaphase, when cyclin A becomes an unstable protein. 15,16 Cyclin A can, therefore, be regarded as a marker for cells in S-or G 2 -phase of the cell cycle. Similarly, PCNA is extensively used (together with Ki-67) as a marker for cells in S-or G 2 -phase of the cell division cycle.…”

The Spindle Assembly Checkpoint: Clock or Domino?

“…It is important to note that another nonspindle assembly checkpoint-related protein has been suggested as a putative mitotic timer: Cyclin A (Geley et al 2001). Indeed, Cyclin A degradation is independent of the spindle checkpoint and required for anaphase onset (Geley et al 2001).…”

“…Indeed, Cyclin A degradation is independent of the spindle checkpoint and required for anaphase onset (Geley et al 2001). Cyclin A degradation by the APC/C already starts in prometaphase and is a relative long process, providing cells with enough time to attach all their chromosomes and form proper metaphase plates (den Elzen and Pines 2001; Geley et al 2001).…”

“…Indeed, Cyclin A degradation is independent of the spindle checkpoint and required for anaphase onset (Geley et al 2001). Cyclin A degradation by the APC/C already starts in prometaphase and is a relative long process, providing cells with enough time to attach all their chromosomes and form proper metaphase plates (den Elzen and Pines 2001; Geley et al 2001). Thus it seems clear that time is such a crucial parameter for mitosis in many organisms, that cells might have developed different, independent pathways that cooperate to ensure each cell has enough time to attach and align chromosomes before proceeding to anaphase.…”