The Drosophila cell cycle kinase PAN GU forms an active complex with PLUTONIUM and GNU to regulate embryonic divisions

Abstract

Early embryonic cell cycles in Drosophila consist of rapidly alternating S and M phases. Three genes, pan gu (png), plutonium (plu), and giant nuclei (gnu) coordinate these early S-M cycles by ensuring adequate Cyclin B protein levels. Mutations in any of these genes result in unregulated DNA replication and a lack of mitosis ("giant nuclei" phenotype). png encodes a serine/threonine protein kinase, and plu and gnu encode small, novel proteins. We show that PNG, PLU, and GNU constitute a novel protein kinase complex that specifically regulates S-M cell cycles. All three proteins are required for PNG kinase activity and are phosphorylated by PNG in vitro. Yeast two-hybrid screening revealed a direct interaction between PNG and PLU, and their co-expression is required for physical association and activation of PNG kinase. Artificial dimerization of PLU via fusion to either GST or FK506 binding protein (in the presence of dimerizing agent) abrogates the requirement for GNU to activate PNG kinase. We propose a model in which GNU normally regulates embryonic cell cycles by promoting transient dimerization of a core PNG/PLU complex, thereby stimulating PNG kinase activity. Protein kinases play crucial regulatory roles in the cell cycle. CDK/cyclin complexes control transitions throughout the cell cycle, and their proper regulation is essential for ensuring the orderly progression of DNA replication and mitosis (for review, see Murray and Hunt 1993). Association of a cyclin subunit with a CDK subunit is needed both for kinase activity and to confer substrate specificity. Thus, one mechanism for control of these kinase complexes involves the accumulation of threshold levels of cyclins via transcription and regulated degradation of cyclin proteins.Modified cell cycles are used to achieve particular developmental goals. Organisms that must undergo rapid embryogenesis, such as marine invertebrates, amphibians, and insects, utilize a streamlined cell cycle in which DNA replication (S phase) and mitosis (M phase) alternate without intervening gaps. The S-M cycles are driven by maternally provided stockpiles of protein and RNA, eliminating a need for gap phases for gene expression or growth (Foe et al. 1993). Zygotic transcription has not yet begun, so the S-M cell cycles differ from the archetypal cell cycle in that they are regulated solely by posttranscriptional mechanisms. In Drosophila embryos, an additional distinction is that the nuclei divide synchronously in a common cytoplasm (syncytium) during the S-M cycles.During the first seven cell cycles of Drosophila embryogenesis, levels of the mitotic Cyclins A and B as well as their partner kinase CDK1 are high due to maternal stockpiling, and no detectable fluctuations in their levels or CDK1 activity occur (Edgar et al. 1994). However, localized degradation of Cyclin B during these early cycles has been reported (Huang and Raff 1999), and injection of a stabilized form of Cyclin B into early embryos causes mitotic arrest (Su et al. 1998). Thus, localized oscillations in ...

Phospho-Regulation Pathways During Egg Activation in Drosophila melanogaster

“…GNU acts within the PNG kinase complex (Lee et al 2003), which is necessary for the translation of Smaug upon egg activation (Tadros et al 2007). It has been shown previously that Smaug is not translated in sra embryos (Cui et al 2008); however, no mechanism for how sra affects Smaug translation had been proposed.…”

“…GNU is a Drosophila-specific protein that is required for the assembly of the Pan Gu kinase complex (Lee et al 2003), which is essential for chromosome condensation and the coupling of S phase and mitosis during early embryo cell cycles (Renault et al 2003;Zhang et al 2004). The Pan Gu kinase complex is also required for translation of Smaug, a protein that regulates maternal mRNA degradation during egg activation (Tadros et al 2007).…”

Transitioning from egg to embryo: Triggers and mechanisms of egg activation

“…GNU is part of the PNG GU kinase complex, which is essential for chromosome condensation and mitotic progression in the early embryo (Lee et al, 2003;Renault et al, 2003). YA is a nuclear protein necessary for the first embryonic mitosis (Lin and Wolfner, 1991).…”

Translational regulation of the cell cycle: when, where, how and why?

“…The GLD-2 poly(A) polymerase promotes further lengthening of the poly(A) tail of these and other mRNAs at activation [101]. In addition, translation of the cyclin mRNAs is promoted by a protein kinase complex composed of the Pan Gu (PNG) kinase and two regulatory subunits (figure 2c) [41,43,104]. When PNG kinase is nonfunctional, meiosis is completed but mitosis does not occur in the embryo.…”