Egg activation triggers clearance of maternally deposited RNA binding proteins

The maternal-to-zygotic transition (MZT) is a conserved step in animal development, where control is passed from the maternal genome to the zygotic one. Although the MZT is typically considered from its impact on the transcriptome, we previously found that three maternally deposited Drosophila RNA binding proteins (ME31B, Trailer Hitch [TRAL], and Cup) are also cleared during the MZT by unknown mechanisms. Here, we show that these proteins are degraded by the ubiquitin-proteasome system. Kondo, an E2 conjugating enzyme, and the E3 CTLH ligase are required for the destruction of ME31B, TRAL, and Cup. Importantly, despite occurring hours earlier, egg activation establishes the timer for clearance of these proteins by activating the Pan Gu kinase, which in turn stimulates translation of Kondo mRNA. In other words, egg activation triggers a series of regulatory events that culminate in the degradation of maternally deposited RNA binding proteins several hours later. Clearance of the maternal protein dowry thus appears to be a coordinated, but as-yet underappreciated, aspect of the MZT.