Drosophila E2F1 is Degraded During S Phase in a PCNA-, Cul4-, and Cdt2-Dependent Manner

During the development of multicellular organisms, cell proliferation is tightly regulated by intrinsic and extrinsic cues, generating a spatiotemporal cell cycle pattern. For instance, cell cycles are very rapid during early embryogenesis, resulting in a sufficient number of cells for tissue formation. In contrast, cells that are going to differentiate usually arrest the cell cycle in G1 phase and subsequently enter the quiescent state (G0). Failure to maintain active cell cycles during early embryogenesis and to arrest the cell cycle before differentiation will cause destructive effects on tissue development and homeostasis. Since cells usually arrest in G1, an important decision step in the cell cycle is whether the cell stays in G1 phase or enters S phase. When G1 cells enter S phase, positive cell cycle regulators such as Cyclin E, RnrS, PCNA, and DNA polymerase are coordinately induced by the family of the E2F transcription factors. In Drosophila, these genes are regulated by a single E2F (E2F1). During the early embryogenesis of Drosophila, E2F1-target genes are expressed ubiquitously, facilitating the rapid cell cycles. Later in embryogenesis, E2F1-target genes are downregulated before cells arrest in G1. This implies that during embryogenesis developmentally-regulated E2F1 activity causes this characteristic cell cycle pattern. In this thesis, we show that the initial downregulation of E2F1-target genes is preceded by the developmentally-regulated onset of E2F1 destruction. Furthermore, we discovered that DNA replication induces E2F1 destruction in a PCNA-, Cul4-, and Cdt2-dependent manner. Expression of a stabilized form of E2F1 in the larval wing disc caused apoptosis and disrupted adult wing morphology, while expression in the larval salivary gland arrested the endocycle, a variant G1-S cell cycle that lacks mitosis and results in polyploidy. Taken together, our data suggests the existence of a robust negative feedback mechanism where E2F1 induces DNA replication, which in turn downregulates E2F1 by proteolysis, and this negative feedback loop is required for normal development of Drosophila

Rbf1-independent termination of E2f1-target gene expression during early Drosophila embryogenesis

The initiation and maintenance of G1 cell cycle arrest is a key feature of animal development. In th

Intrinsic Negative Cell Cycle Regulation Provided by PIP Box- and Cul4Cdt2-Mediated Destruction of E2f1 during S Phase

E2F transcription factors are key regulators of cell proliferation that are inhibited by pRb family tumor suppressors. pRb-independent modes of E2F inhibition have also been described, but their contribution to animal development and tumor suppression is unclear. Here we show that S phase-specific destruction of Drosophila E2f1 provides a novel mechanism for cell cycle regulation. E2f1 destruction is mediated by a PCNA-interacting-protein (PIP) motif in E2f1 and the Cul4Cdt2 E3 ubiquitin ligase, and requires the Dp dimerization partner but not direct Cdk phosphorylation or Rbf1 binding. E2f1 lacking a functional PIP motif accumulates inappropriately during S phase and is more potent than wild type E2f1 at accelerating cell cycle progression and inducing apoptosis. Thus, S phase-coupled destruction is a key negative regulator of E2f1 activity. We propose that pRb-independent inhibition of E2F during S phase is an evolutionarily conserved feature of the metazoan cell cycle that is necessary for development

Autophagy and autophagy-related proteins in the immune system

Autophagy is an intracellular bulk degradation system that is highly conserved in eukaryotes. The discovery of autophagy-related ('ATG') proteins in the 1990s greatly advanced the mechanistic understanding of autophagy and clarified the fact that autophagy serves important roles in various biological processes. In addition, studies have revealed other roles for the autophagic machinery beyond autophagy. In this Review, we introduce advances in the knowledge of the roles of autophagy and its components in immunity, including innate immunity, inflammatory responses and adaptive immunity