Cyclin A triggers Mitosis either via the Greatwall kinase pathway or Cyclin B

Abstract

Two mitotic cyclin types, cyclin A and B, exist in higher eukaryotes, but their specialised functions in mitosis are incompletely understood. Using degron tags for rapid inducible protein removal, we analyse how acute depletion of these proteins affects mitosis. Loss of cyclin A in G2-phase prevents mitotic entry. Cells lacking cyclin B can enter mitosis and phosphorylate most mitotic proteins, because of parallel PP2A:B55 phosphatase inactivation by Greatwall kinase. The final barrier to mitotic establishment corresponds to nuclear envelope breakdown, which requires a decisive shift in the balance of cyclin-dependent kinase Cdk1 and PP2A:B55 activity. Beyond this point, cyclin B/Cdk1 is essential for phosphorylation of a distinct subset of mitotic Cdk1 substrates that are essential to complete cell division. Our results identify how cyclin A, cyclin B and Greatwall kinase coordinate mitotic progression by increasing levels of Cdk1-dependent substrate phosphorylation.

Release from cell cycle arrest with Cdk4/6 inhibitors generates highly synchronized cell cycle progression in human cell culture

“…Finally, because many cell lines will bypass the requirement for Cdk4/6 by exploiting Cdk2 to drive cells into cycle, partial inhibition of Cdk2 in these lines [13,[86][87][88] should sensitize cells to Cdk4 inhibition. One challenge with this approach is the accompanying risk that strong Cdk2 inhibition will impact not only upon DNA replication in the preceding cycle, but upon the regulation of mitotic commitment because Cdk2-Cyclin A has been tied directly to regulation of Wee1 and the G2/M transition [13,[89][90][91]. Given the variety of means by which resistance to Cdk4/6 inhibitors arises [48], it is perhaps easiest to empirically test whether a line will be amenable to Cdk4/6i induction synchrony.…”

Ordered dephosphorylation initiated by the selective proteolysis of cyclin B drives mitotic exit

“…Here, we have primarily focused on CCNB1 in the context of driving mitotic exit, however both CCNA2 and CCNB2 play important roles during mitosis. CCNA2 is sufficient to trigger mitotic entry in the absence of CCNB1, and can form active CDK1-CCNA2 complexes ( Desai et al, 1995 ; Furuno et al, 1999 ; Gong and Ferrell, 2010 ; Hégarat et al, 2020 ; Swenson et al, 1986 ). Pro-metaphase destruction of CCNA2 is required for successful segregation of chromosomes and timely mitotic exit ( Kabeche and Compton, 2013 ).…”

“…The role of CCNB2 is less clear, with CCNB2 deficient mice developing normally into fertile adults (Brandeis et al, 1998). Some studies in cells have proposed that CCNB2 can compensate, at least partially, for CCNB1 (Bellanger et al, 2007;Gong and Ferrell, 2010;Hégarat et al, 2020). One possible explanation for these observations is that, in the absence of CCNB1, CCNB2 in combination with the declining levels of CCNA2 can provide the necessary CDK1 activity to complete mitosis.…”

The G2-to-M transition from a phosphatase perspective: a new vision of the meiotic division

“…Released from the activity of its counteracting enzyme, PP2A-B55δ, Cdk1 can phosphorylate Myt1, Cdc25 and its other M-phase substrates. Importantly, Gwl-Arpp19 activation and the subsequent PP2A-B55δ inhibition require Cdk1 activity, either achieved by Cdk1-Cyclin B in Xenopus oocytes [37] and Cdk1-Cyclin A in human cells [9]. Therefore, Cdk1 not only phosphorylates its own activation module, Cdc25-Myt1, but it further indirectly inactivates its antagonizing protein phosphatase, PP2A-B55δ, through Gwl-Arpp19 (Fig.…”

“…At the end of M-phase, MPF activates the Cyclin proteolytic degradation system, thus inducing its own inactivation entry into mitosis. Cyclin A would be instrumental as a triggering factor, promoting Wee1 phosphorylation and priming mitotic entry through Cyclin B [6][7][8][9][10]. Activated Cdk1-Cyclin B phosphorylates many mitotic substrates, including Wee1/Myt1 and Cdc25.…”