Precise Temporal Regulation of Ribonucleoprotein Complexes during the Drosophila Maternal-to-Zygotic Transition

In early animal embryos, maternally loaded mRNAs and proteins regulate development prior to the handover of control to zygotically expressed gene products in a process known as the maternal-to-zygotic transition (MZT). RNA-binding proteins (RBPs) tightly regulate the translation, stability, and localization of maternal mRNAs through post-transcriptional regulatory mechanisms. For example, in the Drosophila melanogaster embryo, the Smaug (SMG) RBP forms ribonucleoprotein complexes with corepressors, Trailer hitch (TRAL), Maternal expression at 31B (ME31B) and Cup, as well as AGO1 and the CCR4-NOT deadenylase to regulate the translation and stability of hundreds of maternal mRNAs, and this regulation is required for proper development through the MZT. However, regulation of maternal proteins, including these RBPs is not well-studied. In this thesis, I show that the maternal proteome represents over half of the protein coding capacity of the Drosophila genome and that 2% of this proteome is rapidly degraded during the MZT. Cleared proteins include the post-transcriptional repressors Cup, TRAL, ME31B and SMG. While the ubiquitin-proteasome system is necessary for clearance of these repressors, distinct E3 ligase complexes target their degradation at distinct times: the C-terminal to Lis1 Homology (CTLH) complex targets Cup, TRAL and ME31B for degradation early in the MZT; the Skp/Cullin/F-box-containing (SCF) complex targets SMG at the end of the MZT. Deleting the C-terminal 233 amino acids of SMG abrogates interactions with F-box proteins and results in failure of the protein to degrade during the MZT. I map the regulatory elements of SMG protein degradation to conserved motifs within the C-terminus. Furthermore, I found that many of maternal mRNAs targeted for degradation by SMG are subsequently re-expressed zygotically. Unlike Cup, TRAL and ME31B, the deadenylase complex remains stably expressed in the embryo. Consequently, after zygotic genome activation, persistent SMG continues to function in complex with the CCR4-NOT deadenylase and downregulates zygotic re-expression of these mRNAs. Thus, timely clearance of SMG during the MZT permits an orderly gene expression program in the early embryo.

Department dc:contributor.department

Molecular and Medical Genetics

Year dc:date.issued

2020

Author dc:creator

• Cao, Wen Xi

Advisor dc:contributor.advisor

• Lipshitz, Howard D

• CTLH complex
• RNA-binding protein
• SCF complex
• ubiquitin ligase

Handle dc:identifier.uri

http://hdl.handle.net/1807/126031

OAI identifier oai:identifier

oai:utoronto.scholaris.ca:1807/126031