Shabnam Moghareh

Many proteins required for embryogenesis are expressed in multiple tissues and across developmental time, often with tissue- and stage-specific roles. To resolve where and when proteins function, we developed a tissue-targeted auxin-inducible degradation toolkit for C. elegans embryogenesis. We generated a plasmid set compatible with single-copy insertion or high-copy extrachromosomal array generation that employs tissue-specific promoters (active in the germline, intestine, muscle, pharynx, neuroblasts, neurons, sensory neurons, epidermis, and pan soma) to drive an operon expressing an optimized auxin-dependent plant F-box factor TIR1 and, in most cases, a fluorescent histone. Using a ubiquitous nuclear envelope marker as a test substrate, we identified single-copy insertions and arrays that exhibit efficient auxin-dependent, tissue-specific target degradation. We also establish a workflow that ensures all functionally important isoforms of a target are tagged and that degradation reproduces the loss-of-function phenotype prior to tissue-specific analysis. Applying the toolkit to the broadly expressed Rho kinase LET-502, we find that degradation in the epidermis, but not in other embryonic tissues, disrupts elongation. Unexpectedly, temporal analysis revealed that LET-502 is required for late as well as early elongation. SUMMARY STATEMENT: A toolkit for auxin-mediated tissue- and stage-specific degradation of essential targets during C. elegans embryogenesis reveals that the contractility regulator LET-502 is required in the epidermis for late as well as early elongation.

During early embryogenesis, gene expression relies on maternally loaded mRNAs whose translation is controlled by RNA binding proteins. Here, we identify a critical role for the cyclin B3-CDK1 complex, known for its function in mitosis, in driving early embryonic gene expression in C. elegans . The cyclin B3-CDK1 complex works by marking the RNA binding OMA proteins (OMA-1 and OMA-2) for degradation, which ensures the de-repression and translation of their target mRNAs. OMA protein degradation relies on cyclin B3’s conserved phosphate-binding pocket, which promotes multi-site OMA phosphorylation and the generation of phospho-degrons. Notably, the phosphate-binding pocket of cyclin B3 does not substantially contribute to its mitotic roles, indicating that the mitotic and translational functions of the cyclin B3-CDK1 complex are separable. These findings establish that embryonic activation of the cyclin B3-CDK1 complex drives both mitotic divisions and mRNA de-repression, which ensures that cell division is coupled to the early gene expression program in development.

Meiotic progression requires the activity of the cyclin B–CDK1 complex. In this issue, Yang et al. (https://doi.org/10.1083/jcb.202502087) demonstrate that the phospho-adaptor protein CKS-1 functions as a critical component of this complex to ensure proper chromosome segregation during oocyte meiosis.

Mitosis in early embryos often proceeds at a rapid pace, but how this pace is achieved is not understood. Here, we show that cyclin B3 is the dominant driver of rapid embryonic mitoses in the C. elegans embryo. Cyclins B1 and B2 support slow mitosis (NEBD to anaphase ∼600 s), but the presence of cyclin B3 dominantly drives the approximately threefold faster mitosis observed in wildtype. Multiple mitotic events are slowed down in cyclin B1 and B2–driven mitosis, and cyclin B3–associated Cdk1 H1 kinase activity is ∼25-fold more active than cyclin B1–associated Cdk1. Addition of cyclin B1 to fast cyclin B3–only mitosis introduces an ∼60-s delay between completion of chromosome alignment and anaphase onset; this delay, which is important for segregation fidelity, is dependent on inhibitory phosphorylation of the anaphase activator Cdc20. Thus, cyclin B3 dominance, coupled to a cyclin B1–dependent delay that acts via Cdc20 phosphorylation, sets the rapid pace and ensures mitotic fidelity in the early C. elegans embryo.