Wu, K.; Chang, R.; Garcia, A.; Fang, J.; Song, J.; Ninova, M.

Heterochromatin and its core effectors from the Heterochromatin Protein 1 (HP1) family are essential for epigenetic silencing of repetitive regions and genome integrity from yeast to humans. However, how HP1s and associated factors regulate heterochromatin properties in vivo to sustain silencing across development, aging, and environmental factors remains incompletely understood. Here, we identify Clump/CG30403 - a previously uncharacterized MADF-BESS domain protein - as a novel heterochromatin factor required for robust transposon silencing during Drosophila oogenesis and sustained fertility with age and temperature stress. Clump/CG30403 interacts with the main HP1-family silencing effector Su(var)205/HP1a through a noncanonical binding motif within its large intrinsically disordered region. Notably, in the absence of Clump/CG30403, HP1a mobility and silencing capacity are compromised despite largely unperturbed genomic distribution, showing that HP1a presence alone is insufficient for repression. We also show that Clump/CG30403 uniquely accumulates at its own promoter to self-repress and prevent ectopic aggregation, revealing a feedback mechanism to constrain protein dosage and phase behavior. Overall, we propose that Clump/CG30403 is a HP1 corepressor that acts as a tightly calibrated safeguard of the heterochromatin environment properties to ensure stable silencing, genome integrity, and persistent reproductive function.