Protein methylation typically takes place on arginine or lysine amino acid residues in the protein sequence. Arginine can be methylated once (monomethylated arginine) or twice, with either both methyl groups on one terminal nitrogen (asymmetric dimethylated arginine) or one on both nitrogens (symmetric dimethylated arginine) by peptidylarginine methyltransferases (PRMTs). Lysine can be methylated once, twice or three times by lysine methyltransferases. Protein methylation has been most studied in the histones. The transfer of methyl groups from S-adenosyl methionine to histones is catalyzed by enzymes known as histone methyltransferases. Histones that are methylated on certain residues can act epigenetically to repress or activate gene expression. Protein methylation is one type of post-translational modification.

Reference
Wiki: Methylation

    protein lysine methylation, is a PTM involving the transfer of one, two or three methyl groups to the epsilon - amine of a lysine side chain. Lysine methylation represents a complex and often elusive PTM that has nonetheless the potential to alter the function of the modified protein. lysine methylation has been observed in both nuclear and cytoplasmic proteins and is now considered a prevalent modification in eukaryotes, prokaryotes and archaea. Two groups of enzymes, both using S - adenosyl - L - methionine (SAM) as a methyl donor, catalyze the addition of a methyl group to the epsilon - amine group of a lysine side chain. The first type of protein lysine methyltransferase regroups the enzymes containing a catalytic SET domain and the second class of PKMTs, the seven beta - strand methyltransferases (class I methyltransferases), belongs to an extended superfamily of methyltransferases found throughout eukaryotes, prokaryotes and archaea.

Reference
Pubmed: Lanouette S, Mongeon V, Figeys D, Couture JF. The functional diversity of protein lysine methylation. Mol Syst Biol. 2014 Apr 8;10:724. doi: 10.1002/msb.134974.

    Methylation on leucine was first observed in 1999, the carboxyl methyltransferase, which is claimed to exclusively methylate the carboxyl group of the C-terminal leucine residue of the catalytic subunit of protein phosphatase 2A (Leu(309)), was purified from porcine brain. The cDNA encoding the human homologue was also cloned. The cDNA of this gene encodes for a protein of 334 amino acids with a calculated M(r) of 38 305 and a predicted pI of 5.72. Database screening reveals the presence of this protein in diverse phyla. Recent studies revealed that PP2A methylation is associated with "PP2A" dysfunction which has been linked to tau hyperphosphorylation, amyloidogenesis and synaptic deficits that are pathological hallmarks of this neurodegenerative disorder.

Reference
Pubmed: Sontag JM, Sontag E. Protein phosphatase 2A dysfunction in Alzheimer's disease Front Mol Neurosci. 2014 Mar 11;7:16. doi: 10.3389/fnmol.2014.00016. eCollection 2014.

    Demethylation is the chemical process resulting in the removal of a methyl group (CH3) from a molecule. A common way of demethylation is the replacement of a methyl group by a hydrogen atom, resulting in a net loss of one carbon and two hydrogen atoms.In biochemical systems, the process of demethylation is catalyzed by demethylases. These enzymes oxidize N-methyl groups, which occur in histones and some forms of DNA. R2N-CH3 + O ? R2N-H + CH2O One such oxidative enzyme family is the cytochrome P450 Alpha-ketoglutarate-dependent nonheme enzymes are active for demethylation of DNA, operating by similar pathway.

Reference
Wiki: Demethylation

    Arginine methylation is a prevalent post-translational modification found on both nuclear and cytoplasmic proteins. The methylation of arginine residues is catalyzed by the protein arginine Nmethyltransferase (PRMT) family of enzymes. Proteins that are arginine methylated are involved in a number of different cellular processes, including transcriptional regulation, RNA metabolism and DNA damage repair (Bedford and Richard, 2005). Most PRMTs methylate glycine- and arginine-rich patches (GAR motifs) within their substrates.

Reference
Pubmed: Bedford MT. Arginine methylation at a glance. J Cell Sci. 2007 Dec 15;120(Pt 24):4243-6.