The Inhibition of Polo Kinase by Matrimony Maintains G2 Arrest in the Meiotic Cell Cycle

Open Access
Publisher Website
Google Scholar
Abstract
Many meiotic systems in female animals include a lengthy arrest in G2 that separates the end of pachytene from nuclear envelope breakdown (NEB). However, the mechanisms by which a meiotic cell can arrest for long periods of time (decades in human females) have remained a mystery. The Drosophila Matrimony (Mtrm) protein is expressed from the end of pachytene until the completion of meiosis I. Loss-of-function mtrm mutants result in precocious NEB. Coimmunoprecipitation experiments reveal that Mtrm physically interacts with Polo kinase (Polo) in vivo, and multidimensional protein identification technology mass spectrometry analysis reveals that Mtrm binds to Polo with an approximate stoichiometry of 1:1. Mutation of a Polo-Box Domain (PBD) binding site in Mtrm ablates the function of Mtrm and the physical interaction of Mtrm with Polo. The meiotic defects observed in mtrm/+ heterozygotes are fully suppressed by reducing the dose of polo+, demonstrating that Mtrm acts as an inhibitor of Polo. Mtrm acts as a negative regulator of Polo during the later stages of G2 arrest. Indeed, both the repression of Polo expression until stage 11 and the inactivation of newly synthesized Polo by Mtrm until stage 13 play critical roles in maintaining and properly terminating G2 arrest. Our data suggest a model in which the eventual activation of Cdc25 by an excess of Polo at stage 13 triggers NEB and entry into prometaphase. Many meiotic systems in females animals include a lengthy arrest period (spanning days in flies and to decades in humans) that separates the early and late stages of meiosis. Such an arrest raises the question: how can the quiescent meiotic cell cycle be precisely awakened or re-started? At least in principle, the answer to this phenomenon, which we refer to as “The Sleeping Beauty Kiss,” might have two molecular solutions: the controlled expression of a protein that re-starts the cell cycle, or the inactivation of an inhibitory protein that prevents such a re-start. We show here that the re-start of the meiotic cycle in Drosophila depends on both mechanisms: the controlled expression of an “activator” known as Polo kinase, and the presence of a regulatory protein called Matrimony (Mtrm), which binds to and physically inactivates Polo. Indeed, Mtrm is the first known protein inhibitor of Polo kinase. The excess of Mtrm prior to the time of normal meiotic re-start, keeps Polo inactive. However, either the production of an excess quantity of Polo, or the destruction of Mtrm, at the appropriate time, releases active Polo, permitting a properly controlled re-start of meiotic progression.