HP1γ self-assembles and cooperates with KAP1 in repression of long noncoding RNA AI662270 in ESCs

HP1s are involved in the assembly of heterochromatin and transcriptional regulation. Here, we report the molecular mechanisms underlying binding of the chromoshadow domain of HP1γ (HP1γ_CSD) to the transcriptional co-repressor KAP1 and HP1γ self-assembly. Using crystallography, NMR, and mass photometry, we show that HP1γ_CSD recognizes the HP1 box of KAP1 (KAP1_Hbox) and forms a relatively stable dimer of dimers, assembled in an antiparallel manner, in contrast to the corresponding HP1α_CSD complex, which shows concentration-dependent oligomerization and arrangement of HP1α_CSD protomers in a parallel manner. The β-sheet interface between HP1γ_CSD dimers is stabilized through electrostatic interactions, unlike the hydrophobic β-sheet interface of HP1α_CSD. In vivo rescue experiments using KAP1- and HP1-knockout mouse embryonic stem cells reveal a unique cooperative action of KAP1 and HP1γ, but not other HP1s, in the repression of the long noncoding RNA AI662270, underscoring the notion that cellular functions of HP1 proteins are not redundant.