The C-terminal ZZ domain of the Drosophila ORB2 RNA-binding protein is required for spermatid individualization

Abstract

ORB2 is the Drosophila ortholog of the human CPEB2-4 family of RNA-binding proteins, which include a conserved C-terminal Zinc-binding ('ZZ') domain. We have recently shown that this domain interacts with several translational co-repressors in the early embryo, and that deletion of this domain from the endogenous orb2 gene results in derepression of its target mRNAs (Low et al. 2025). Here we assess the effect of deletion of the ZZ domain on spermatogenesis. We find that deletion of the ZZ domain does not affect spatial localization of the ORB2 protein; orb2ΔZZ flies are sterile and lack mature sperm; meiosis is mostly normal in orb2ΔZZ testes; individualization complexes are defective and spermatid individualization fails; proteins known to play a role in spermatid individualization - ORB, IMP, SOTI - are mislocalized; and the SOTI-dependent Cleaved Caspase-3 gradient no longer forms in orb2ΔZZ mutant testes.

Keywords: Cytoplasmic Polyadenylation Element Binding Protein (CPEB); IMP; ORB; ORB2; RNA-binding protein (RBP); SOTI; cleaved caspase 3 (CC3); meiosis; post-transcriptional regulation; spermatogenesis.