The centromere is the locus on the chromosome that acts as the essential connection point between the chromosome and the mitotic spindle. A histone H3 variant, CENP-A, defines the location of the centromere, but centromeric chromatin consists of a mixture of both CENP-A-containing and H3-containing nucleosomes. We report a surprisingly uniform pattern of primarily monomethylation on lysine 20 of histone H4 present in short polynucleosomes mixtures of CENP-A and H3 nucleosomes isolated from functional centromeres. Canonical H3 is not a component of CENP-A-containing nucleosomes at centromeres, so the H3 we copurify from these preparations comes exclusively from adjacent nucleosomes. We find that CENP-A-proximal H3 nucleosomes are not uniformly modified but contain a complex set of PTMs. Dually modified K9me2-K27me2 H3 nucleosomes are observed at the centromere. Side-chain acetylation of both histone H3 and histone H4 is low at the centromere. Prior to assembly at centromeres, newly expressed CENP-A is sequestered for a large portion of the cell cycle (late S-phase, G2, and most of mitosis) in a complex that contains its partner, H4, and its chaperone, HJURP. In contrast to chromatin associated centromeric histone H4, we show that prenucleosomal CENP-A-associated histone H4 lacks K20 methylation and contains side-chain and ␣-amino acetylation. We show HJURP displays a complex set of serine phosphorylation that may potentially regulate the deposition of CENP-A.

Summary Protein arginine methyltransferases (PRMTs) catalyze the post-translational monomethylation (Rme1), asymmetric (Rme2a), or symmetric (Rme2s) dimethylation of arginine. To determine the cellular consequences of type I (Rme2a) and II (Rme2s) PRMTs, we developed and integrated multiple approaches. First, we determined total cellular dimethylarginine levels, revealing that Rme2s was ∼3% of total Rme2 and that this percentage was dependent upon cell type and PRMT inhibition status. Second, we quantitatively characterized in vitro substrates of the major enzymes and expanded upon PRMT substrate recognition motifs. We also compiled our data with publicly available methylarginine-modified residues into a comprehensive database. Third, we inhibited type I and II PRMTs and performed proteomic and transcriptomic analyses to reveal their phenotypic consequences. These experiments revealed both overlapping and independent PRMT substrates and cellular functions. Overall, this study expands upon PRMT substrate diversity, the arginine methylome, and the complex interplay of type I and II PRMTs.

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Arginine methylation is essential for both cellular viability and development and is also dysregulated in cancer. PRMTs catalyze the post translational monomethylation (Rme1/MMA, catalyzed by Type I-III), asymmetric (Rme2a/ADMA, Type I enzymes)-, or symmetric (Rme2s/SDMA, Type II enzymes) dimethylation of arginine. Despite many studies, a thorough integration of PRMT enzyme substrate determination and proteomic and transcriptomic consequences of inhibiting Type I and II PRMTs is lacking. To characterize cellular substrates for Type I (Rme2a) and Type II (Rme2s) PRMTs, human A549 lung adenocarcinoma cells were treated with either Type I (MS023) or Type II (GSK591) inhibitors. Using total proteome hydrolysis, we developed a new mass spectrometry approach to analyze total arginine and lysine content. We showed that Rme1 was a minor population (~0.1% of total arginine), Rme2a was highly abundant (~1.1%), and Rme2s was intermediate (~0.4%). While Rme2s was mostly eliminated by GSK591 treatment, total Rme1 and Rme2a were more resistant to perturbation. To quantitatively characterize substrate preferences of the major enzymes PRMT1, PRMT4(CARM1), and PRMT5, we used oriented peptide array libraries (OPAL) in methyltransferase assays. We demonstrated that while PRMT5 tolerates aspartic acid residues in the substrate, PRMT1 does not. Importantly, PRMT4 methylated previously uncharacterized hydrophobic motifs. To integrate our studies, we performed PTMScan on PRMT-inhibited A549 cells and enriched for methylated arginine containing tryptic peptides. For detection of highly charged peptides, a method to analyze the samples using electron transfer dissociation was developed. Proteomic analysis revealed distinct methylated species enriched in nuclear function, RNA-binding, intrinsically disordered domains, and liquid-liquid phase separation. Parallel studies with proteomics and RNA-Seq revealed distinct, but ontologically overlapping, consequences to PRMT inhibition. Overall, we demonstrate a wider PRMT substrate diversity and methylarginine functional consequence than previously shown.