An unstructured MET-2/SETDB1 cofactor ensures H3K9me2, focus formation and perinuclear anchoring [RNA-seq]

The segregation of the genome into accessible euchromatin and histone H3 lysine 9 methylated (H3K9me) heterochromatin is essential for the repression of repetitive elements and tissue-specific genes. In C. elegans, the SETDB1 homolog MET-2 catalyzes H3K9me1 and me2. In worms as in vertebrates, the regulation of this crucial enzyme remains enigmatic. Contrary to the localization of overexpressed MET-2, we find endogenous MET-2 to be nuclear throughout development, enriched in perinuclear foci in a cell cycle-dependent manner. We show by mass spectrometry that MET-2 associates with a highly unstructured protein, LIN-65, and a conserved GTPase effector ARLE-14. All three colocalize at heterochromatic foci. Ablation of lin-65, but not arle-14, mislocalizes and destabilizes MET-2, resulting in decreased H3K9me2, derepression of MET-2 targets, and loss of fertility. Importantly, mutation of met-2 or lin-65 is sufficient to disrupt the perinuclear clustering of heterochromatin genome-wide. Thus, LIN-65 is an essential cofactor of MET-2 required for H3K9me2 deposition, heterochromatin clustering, perinuclear anchoring, and transcriptional repression. Overall design: Total RNAseq of RNA isolated from early C. elegans embryos (~1-300 cell stage) in N2 (Bristol), met-2(n4256) III, lin-65(delta) I and lin-65(delta) I;met-2(n4256) III mutants. Extraction of RNA was performed according to the WormBook protocol (Stiernagle, 2006). Total RNA was purified using RNeasy kit (QIAGEN 74104) including DNase treatment. Depletion of ribosomal RNA was done for 5 lg of total RNA with Ribo-Zero? Margnetic Gold Kit (Epicentre MRGZG12324) and further concentrated with RNA Clean & Concentrator? kit (Zymo Research R1015) according to corresponding manufacturer?s instructions.