An unstructured MET-2/SETDB1 cofactor ensures H3K9me2, focus formation and perinuclear anchoring [ChIP-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: Chromatin was isolated from early embryos (before bean stage) grown after L1 synchronization (60–65 h depending on each strain) in three independent biological replicates and Lem-2 immunoprecipitation was performed using a rabbit anti Lem-2 antibody (Novus Biologicals 48540002). Libraries were prepared from chromatin IP and input samples using the TruSeq ChIP Library Preparation Kit (Illumina IP-202-1012), according to the manufacturer's recommendations. No size selection was performed during sample preparation and the libraries were indexed and amplified using 12 PCR cycles, using the recommended conditions. After a final cleanup with Agencourt AmPure XP beads (Beckman # A63881), the library size distribution and concentrations were determined using a BioAnalyzer 2100 (Agilent technologies) and Qubit (Invitrogen) instrument, respectively. The final pools were prepared by mixing equimolar amounts of all individual indexed libraries and then sequenced on a HiSeq 2500 (Illumina) in Rapid mode (Paired-End 50).