Bistable, Biphasic Regulation of PP2A-B55 Accounts for the Dynamics of Mitotic Substrate Phosphorylation

• Published in: Current biology 2021-02, Vol.31 (4), p.794-808.e6
• Main Authors: Kamenz, Julia, Gelens, Lendert, Ferrell, James E.
• Format: Article
• Language: English
• Subjects: Animals; bistability; CDC2 Protein Kinase - metabolism; Cdk1; Cell Extracts; Enzyme Activation; Feedback, Physiological; incoherent feedforward regulation; Interphase; Mitosis; mitotic entry and exit; Ovum - enzymology; Phosphorylation; PP2A-B55; Protein Phosphatase 2 - genetics; Protein Phosphatase 2 - metabolism; Substrate Specificity; Xenopus
• ISSN: 0960-9822; 1879-0445
• DOI: 10.1016/j.cub.2020.11.058

The phosphorylation of mitotic proteins is bistable, which contributes to the decisiveness of the transitions into and out of M phase. The bistability in substrate phosphorylation has been attributed to bistability in the activation of the cyclin-dependent kinase Cdk1. However, more recently it has been suggested that bistability also arises from positive feedback in the regulation of the Cdk1-counteracting phosphatase PP2A-B55. Here, we demonstrate biochemically using Xenopus laevis egg extracts that the Cdk1-counteracting phosphatase PP2A-B55 functions as a bistable switch, even when the bistability of Cdk1 activation is suppressed. In addition, Cdk1 regulates PP2A-B55 in a biphasic manner; low concentrations of Cdk1 activate PP2A-B55 and high concentrations inactivate it. As a consequence of this incoherent feedforward regulation, PP2A-B55 activity rises concurrently with Cdk1 activity during interphase and suppresses substrate phosphorylation. PP2A-B55 activity is then sharply downregulated at the onset of mitosis. During mitotic exit, Cdk1 activity initially falls with no obvious change in substrate phosphorylation; dephosphorylation then commences once PP2A-B55 spikes in activity. These findings suggest that changes in Cdk1 activity are permissive for mitotic entry and exit but that the changes in PP2A-B55 activity are the ultimate trigger. [Display omitted] •APC/C and PP2A-B55 activities are bistable, even when Cdk1 activity is not•Intermediate levels of Cdk1 activate PP2A-B55; high levels of Cdk1 inactivate it•PP2A-B55 activity peaks right before mitotic entry and spikes during mitotic exit•PP2A-B55 controls mitotic substrate phosphorylation more than Cdk1 does Mitotic transitions are accompanied by drastic changes in the phosphorylation state of proteins. Kamenz et al. demonstrate biochemically that the major mitotic phosphatase PP2A-B55 is regulated by incoherent feedforward and double-negative feedback loops to promote rapid and switch-like mitotic entry and exit.