ME31B globally represses maternal mRNAs by two distinct mechanisms during the Drosophila maternal-to-zygotic transition

Abstract

In animal embryos, control of development is passed from exclusively maternal gene products to those encoded by the embryonic genome in a process referred to as the maternal-to-zygotic transition (MZT). We show that the RNA-binding protein, ME31B, binds to and represses the expression of thousands of maternal mRNAs during the Drosophila MZT. However, ME31B carries out repression in different ways during different phases of the MZT. Early, it represses translation while, later, its binding leads to mRNA destruction, most likely as a consequence of translational repression in the context of robust mRNA decay. In a process dependent on the PNG kinase, levels of ME31B and its partners, Cup and Trailer Hitch (TRAL), decrease by over 10-fold during the MZT, leading to a change in the composition of mRNA–protein complexes. We propose that ME31B is a global repressor whose regulatory impact changes based on its biological context.

Egg activation triggers clearance of maternally deposited RNA binding proteins

“…To ask if the CTLH complex might be involved in the degradation of ME31B, we immunoprecipitated ME31B-GFP and Cup-GFP from pre-MZT embryos (in conditions that maintain complex formation) and determined the proteins bound by mass spectrometry (Figure 4B, 4C). Consistent with our previous work (Wang et al, 2017), both immunoprecipitations readily pulled-down other members of the Cup-TRAL-ME31B complex. This analysis identified ubiquitin, components of the SCF complex (both SkpA and Slmb), Gus, and both EloB and EloC, suggesting that these factors directly or indirectly interact with ME31B and Cup.…”

“…Although the MZT has typically been considered from the perspective of RNA, a role for maternal protein clearance is becoming more important. Over the past few years, the list of proteins degraded during the Drosophila MZT has grown and now includes GNU, Matrimony, Cort, Smaug, ME31B, TRAL, and Cup (Benoit et al, 2009;Hara et al, 2017;Pesin and Orr-Weaver, 2007;Wang et al, 2017;Whitfield et al, 2013). Unbiased mass spectrometry experiments also suggest that Wispy and Dhd are also robustly degraded (Sysoev et al, 2016).…”

“…We previously demonstrated through western blotting that ME31B, TRAL, and Cup were degraded 2-3 hours after egg laying (Wang et al, 2017). To understand the mechanisms underlying degradation of these RNA binding proteins, we decided to establish a fluorescence-based assay so that we could follow ME31B degradation in living embryos.…”

“…ME31B, TRAL, and Cup form a complex that blocks translation initiation (Kinkelin et al, 2012;Nakamura et al, 2004;Nelson et al, 2004;Wilhelm et al, 2003). All three proteins are required for oogenesis, and they appear to bind and repress thousands of deposited maternal mRNAs (Keyes and Spradling, 1997;Nakamura et al, 2001;Tritschler et al, 2008;Wang et al, 2017;Wilhelm et al, 2003). Interestingly, following the rapid destruction of ME31B, TRAL, and Cup, the remaining ME31B (at 10-to 20-fold lower abundance) appears to mediate mRNA destruction, rather than translational repression (Wang et al, 2017).…”

“…All three proteins are required for oogenesis, and they appear to bind and repress thousands of deposited maternal mRNAs (Keyes and Spradling, 1997;Nakamura et al, 2001;Tritschler et al, 2008;Wang et al, 2017;Wilhelm et al, 2003). Interestingly, following the rapid destruction of ME31B, TRAL, and Cup, the remaining ME31B (at 10-to 20-fold lower abundance) appears to mediate mRNA destruction, rather than translational repression (Wang et al, 2017). The degradation of ME31B, TRAL, and Cup tantalizingly coincides with many of the hallmarks of the MZT, but explorations into this issue have been hindered by a lack of understanding of how their destruction is controlled.…”