SUMOylation modulates the dual functions of Krüppel homolog 1 in transcriptional regulation of Broad-Complex expression

Introduction: Insects undergo intricate metamorphosis processes governed by hormonal regulatory mechanisms, exemplified by the antagonistic actions of 20-hydroxyecdysone (20E) and juvenile hormone (JH). Krüppel homolog 1 (Kr-h1) is a key transcription factor mediating JH's antagonistic effects on 20E. Previous studies have shown that Kr-h1 plays dual roles in regulating the 20E signaling gene Broad-Complex (Br-C), repressing it in larvae and activating it during pupation. However, the mechanisms underlying this dual functionality remain elusive.

Objectives: This study aimed to determine whether SUMOylation, a post-translational modification, functions as part of a molecular switch mechanism for Kr-h1's stage-specific transcriptional roles.

Methods: Using Drosophila melanogaster, we analyzed Kr-h1 SUMOylation dynamics via Western blot, immunoprecipitation combined with Drosophila genetics. Immunostaining, RT-qPCR, and luciferase reporter assays were performed to investigate the impact of SUMOylation on the bidirectional transcriptional regulation of Kr-h1 on Br-C. SUMOylation sites were identified through mutagenesis and bioinformatics. Coimmunoprecipitation and mass spectrometry explored Kr-h1 interactors. Hormonal effects on regulation of Kr-h1 SUMOylation were tested using JH mimic (methoprene) and 20E treatments.

Results: Kr-h1 undergoes stage-specific SUMOylation: it is highly SUMOylatedin early third-instar larvae, enabling interaction with the Histone Methyltransferase SmydA-8 to repress Br-C. Conversely, at the white prepupal stage, deSUMOylation disrupts the Kr-h1- SmydA-8 interaction, thereby activating Br-C expression. K113 was identified as the primary SUMOylation site in Kr-h1. JH stimulated Kr-h1 SUMOylation by enhancing its interaction with the SUMO-conjugating enzyme Ubc9. Conversely, 20E promotedKr-h1 deSUMOylation via increased association with the deSUMOylating enzyme ubiquitin-like protease 1 (Ulp1). Co-stimulation with JH and 20E synergistically amplified deSUMOylation.

Conclusion: SUMOylation of Kr-h1 serves as a molecular switch governing its dual regulatory functions in Br-C expression. JH and 20E antagonistically control this post-translational modification. Our study establishes SUMOylation as a critical regulator of Kr-h1's stage-specific activity, revealing how it orchestrates crosstalk between JH and 20E signaling during insect development. These findings provide a mechanistic framework for understanding how post-translational modifications confer functional plasticity to transcription factors at pivotal developmental transitions. Moreover, identifying SUMOylation as a central node in JH/20E crosstalk opens new avenues for targeting this pathway in insect growth regulators for pest management.

20-hydroxyecdysone; Broad-Complex; Juvenile hormone; Krüppel homolog 1; SUMOylation.