Multiple kinases inhibit origin licensing and helicase activation to ensure reductive cell division during meiosis.

David V Phizicky,Luke E Berchowitz,Stephen P Bell

doi:10.7554/elife.33309

Abstract

Meiotic cells undergo a single round of DNA replication followed by two rounds of chromosome segregation (the meiotic divisions) to produce haploid gametes. Both DNA replication and chromosome segregation are similarly regulated by CDK oscillations in mitotic cells. Yet how these two events are uncoupled between the meiotic divisions is unclear. Using Saccharomyces cerevisiae, we show that meiotic cells inhibit both helicase loading and helicase activation to prevent DNA replication between the meiotic divisions. CDK and the meiosis-specific kinase Ime2 cooperatively inhibit helicase loading, and their simultaneous inhibition allows inappropriate helicase reloading. Further analysis uncovered two previously unknown mechanisms by which Ime2 inhibits helicase loading. Finally, we show that CDK and the polo-like kinase Cdc5 trigger degradation of Sld2, an essential helicase-activation protein. Together, our data demonstrate that multiple kinases inhibit both helicase loading and activation between the meiotic divisions, thereby ensuring reductive cell division.

Highlights

The production of haploid gametes is required for sexual reproduction. These gametes are produced by meiosis, a specialized cell division program during which a single round of DNA replication is followed by two rounds of chromosome segregation, Meiosis I (MI) and Meiosis II (MII)
The lack of DNA replication between MI and MII is essential for the reduction in ploidy inherent to meiosis, but it is unclear how the meiotic program differs from mitosis to allow for two sequential chromosome segregation events without an intervening S phase
We address a fundamental question concerning the regulation of meiosis; how do meiotic cells undergo two sequential rounds of chromosome segregation without an intervening S– phase? We found that meiotic cells prevent DNA replication between the meiotic divisions using cyclin-dependent kinase (CDK), Ime2, and Cdc5 to inhibit both helicase loading and activation

Summary

Introduction

The production of haploid gametes is required for sexual reproduction. These gametes are produced by meiosis, a specialized cell division program during which a single round of DNA replication is followed by two rounds of chromosome segregation (the meiotic divisions), Meiosis I (MI) and Meiosis II (MII). The lack of DNA replication between MI and MII is essential for the reduction in ploidy inherent to meiosis, but it is unclear how the meiotic program differs from mitosis to allow for two sequential chromosome segregation events without an intervening S phase In mitotic cells, both DNA replication and chromosome segregation require cyclin-dependent kinase (CDK) activity to oscillate during the cell cycle. S–CDK phosphorylates two essential proteins, Sld and Sld, that subsequently promote helicase activation, replisome assembly, and chromosome duplication (Masumoto et al, 2002; Tanaka et al, 2007; Zegerman and Diffley, 2007) Both S–CDK and M-CDK (CDK bound to mitotic cyclins Clb1-4) inhibit new helicase loading during S, G2, and M phases. The alternative– kinase model suggests that a second kinase inhibits Mcm loading during the MI–MII transition, allowing the oscillation of CDK activity to reset the chromosome segregation program without resetting the DNA replication program. These data show that meiotic cells use multiple kinases to inhibit both Mcm loading and activation, ensuring that MI and MII occur sequentially without an intervening S–phase

Results

Discussion

Materials and methods