Juvenile hormone (JH) prevents immature insects from entering metamorphosis prematurely by inducing the zinc-finger transcription factor Krüppel homolog 1 (Kr-h1), which directly represses the adult-specifier gene E93. Kayukawa and colleagues (2017) showed that this repression requires a conserved C-terminal region of Kr-h1 (the CtC domain, containing the LPPRKR and SVIQFA motifs) and leaves open which corepressor(s) Kr-h1 recruits, how it represses transcription even while the ecdysone receptor EcR/USP occupies the same promoter, and whether the mechanism is conserved in vivo across holometabolous insects. Later work partially resolved the picture: phosphorylated Kr-h1 was shown to recruit the corepressor CtBP (and the coactivator CBP in adults), and a SUMOylation switch with the histone methyltransferase SmydA-8, plus enhancer chromatin accessibility, explains Kr-h1's dual repressor/activator behavior on Broad-Complex in Drosophila. However, no study has identified proteins that physically interact with the CtC domain itself, reconciled this with the phosphorylation- and SUMOylation-dependent recruitment routes, demonstrated the chromatin-level execution of repression at E93 in vivo, or functionally validated the cofactor mechanism in vivo across holometabolous orders. The open core is the physical cofactor-interaction basis and chromatin execution mechanism of Kr-h1-mediated E93 repression, together with the in vivo functional conservation of this cofactor-dependent mechanism across holometabolous insects.

Background

Holometabolous insects (those with complete metamorphosis: larva, pupa, adult) coordinate development through two hormones: 20-hydroxyecdysone (20E), a steroid that triggers each molt and ultimately metamorphosis, and juvenile hormone (JH), a sesquiterpenoid whose presence during larval molts maintains the juvenile state — the so-called "status quo" action of JH. 20E acts through a nuclear receptor heterodimer, EcR/USP (ecdysone receptor and its partner Ultraspiracle), which binds ecdysone response elements (EcREs) to activate target genes. JH acts through its own receptor, Methoprene-tolerant (Met), which induces the primary-response gene Krüppel homolog 1 (Kr-h1), a C2H2 zinc-finger transcription factor. Two 20E-regulated "stage-specifier" transcription factors define the pupal and adult programs: Broad-Complex (BR-C), the pupal specifier, and Ecdysone-induced protein 93F (E93), the adult specifier that triggers pupal-to-adult remodeling. If JH is present at a molting event, Kr-h1 represses E93 (and, in larvae, BR-C), so the animal cannot skip the pupal stage and proceed directly to adult development; precocious E93 expression causes precocious metamorphosis.

Kayukawa, Jouraku, Ito, and Shinoda (PNAS 2017) established the molecular link in the silkworm Bombyx mori: they defined a consensus Kr-h1 binding sequence (KBS, TGACCTNNNNYAAC) by genome-wide ChIP-seq, matched by the functional 14-bp KBS in the E93 promoter (−2221 to −2208), and showed that JH-inducible BmKr-h1 binds the KBS directly and represses E93 transcription in a cell-autonomous manner. Two of their findings framed the open mechanistic questions. First, electrophoretic mobility-shift assays showed that Kr-h1 and the EcR/USP complex bind the E93 promoter simultaneously rather than competing, so repression must operate despite concurrent 20E-receptor occupancy. Second, they discovered a conserved C-terminal region of Kr-h1 — the C-terminal conserved (CtC) domain, spanned by two conserved putative protein-interaction motifs, LPPRKR and SVIQFA — and showed that deleting it (BmKr-h1_ΔCtC) abolishes repression in reporter assays, while the canonical docking motifs for the known insect corepressors Groucho (WRPW) and C-terminal binding protein/CtBP (PXDLS) are absent from Kr-h1. They therefore concluded that the CtC motif likely recruits an unknown corepressor, and asked how Kr-h1 chooses between repressive and activating modes on targets such as BR-C (which Kr-h1 represses in larvae but helps activate at the prepupal stage, as first shown by Kayukawa et al. 2016), how the repressive complex shuts down 20E-driven transcription despite EcR/USP binding, and whether the KBS/CtC-dependent mechanism operates in vivo across holometabolous insects.

Subsequent literature reshaped these questions. Wu, Yang, Li, and Zhou (BMC Biology 2021), working mainly in the hemimetabolous migratory locust, showed that PKCα phosphorylates Kr-h1 at a serine in its third zinc finger (Ser154 in the locust; corresponding residues in Bombyx, Tribolium, and Drosophila), and that phosphorylated Kr-h1 recruits CtBP as a corepressor to repress E93 and block metamorphosis, whereas in adults phosphorylated Kr-h1 recruits the coactivator CBP to activate reproductive genes — a cofactor-switch explanation for Kr-h1's dual action, demonstrated across four insect species in Drosophila S2-cell assays (notably answering, at the functional level, the corepressor-identity question that Kayukawa's PXDLS-based exclusion of CtBP had left open). He, Chen, Chen, and Gao (Journal of Advanced Research 2025) showed in Drosophila that Kr-h1 is SUMOylated (attachment of the small ubiquitin-like modifier SUMO) at lysine 113 in larvae, that SUMOylated Kr-h1 recruits the arthropod-specific histone methyltransferase SmydA-8 to repress BR-C, and that 20E-induced deSUMOylation at prepupation permits Kr-h1's activating function. Long et al. (PNAS 2025) showed in Drosophila that 20E raises H3K27 acetylation (an activating histone mark) and opens chromatin at BR-C enhancers containing positive Kr-h1 binding sites, so the same transcription factor activates or represses the same target gene depending on enhancer accessibility. Ojani et al. (2018) had documented the promoter-specific repressor/activator duality of Kr-h1 in mosquitoes without a mechanism, and Zhang et al. (2018) extended Kr-h1's direct-repression targets to ecdysone-biosynthesis genes. None of these studies, however, tested binding to the CtC domain: the phosphorylation and SUMOylation sites lie in zinc fingers and the N-terminal region, not in the CtC domain, leaving the Bombyx ΔCtC loss-of-repression phenotype unexplained at the protein-interaction level, and the chromatin events at E93 (as opposed to BR-C) uncharacterized in vivo.

Problem Statement

The open problem is to complete the mechanistic account of how Krüppel homolog 1 executes juvenile-hormone-dependent transcriptional repression that prevents precocious larval–adult metamorphosis in holometabolous insects, under one overarching objective: a physically grounded, causally tested model of the Kr-h1 repressor complex at the E93 promoter. Concretely, an answer must determine: (1) which protein(s) physically interact with the conserved CtC domain of Kr-h1 (including its LPPRKR and SVIQFA motifs) in larval tissues of holometabolous insects, and how this CtC-dependent interaction explains the published finding that deleting the CtC domain abolishes repression in Bombyx cells, given the phosphorylation- and SUMOylation-dependent recruitment of CtBP and SmydA-8 through sites outside the CtC domain; (2) the chromatin-level and biochemical mechanism by which the assembled Kr-h1 complex suppresses 20E/EcR/USP-driven E93 transcription in vivo despite simultaneous EcR/USP occupancy of the same promoter (for example, which histone modifications or remodeling activities are recruited to the KBS/EcRE region of E93, and with what causal consequences for E93 expression and developmental timing); and (3) whether this cofactor-dependent repression mechanism is functionally conserved in vivo across holometabolous insects, beyond the cell-culture conservation evidence currently available. The “in vivo” qualifiers above specify the biological setting the model must account for.

References

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