The ORB2 RNA-binding protein negatively regulates its target transcripts during the Drosophila maternal-to-zygotic transition via its functionally conserved Zinc-binding “ZZ” domain

Posttranscriptional regulation is particularly prominent during the maternal-to-zygotic transition (MZT), a developmental phase during which a large proportion of maternally provided mRNAs is repressed and cleared from metazoan embryos. RNA-binding proteins (RBPs) are key components of the posttranscriptional regulatory machinery. We show that the ORB2 RBP, the Drosophila ortholog of the human cytoplasmic polyadenylation element binding protein (hCPEB) 2-4 protein subfamily, binds to hundreds of maternally provided, rare-codon-enriched mRNAs in early embryos and that ORB2 targets are translationally repressed and unstable during the MZT. We identify a U-rich motif enriched in ORB2 targets' 3'UTRs and show that this motif confers ORB2 binding and repression to a luciferase reporter mRNA in S2 tissue culture cells. When tethered to a luciferase reporter, ORB2 and hCPEB2 (but not ORB and hCPEB1) repress translation and the C-terminal Zinc-binding ("ZZ") domain of ORB2 is necessary and sufficient for repression. ORB2 interacts with a suite of posttranscriptional regulators in early embryos; a subset of these interactions is lost upon deletion of the ZZ domain, notably with the Cup repressive complex. ORB2 targets significantly overlap with those previously identified for the repressive RBP, Smaug (SMG). Analysis of the early embryo's translatome in the presence or absence of the endogenous ZZ domain shows that mRNAs bound by ORB2 but not by SMG move onto polysomes upon ZZ domain deletion, whereas cobound transcripts do not, consistent with coregulation of the latter set of transcripts by both RBPs. Our results assign a function to the ZZ domain and position ORB2 in the posttranscriptional network that regulates maternal transcripts during the Drosophila MZT.