Proteins are important for the regulation of biochemical processes and are crucial for the structure and function of cells. Proteins are post-translationally often chemically modified, which regulates their function and enzymatic activities. The work presented in this thesis shows that the mono-methyltransferase SET7/9 modifies the linker histone H1.4 only at the C-terminal domain. Several lysine residues (K121, K129, K159, K171, K177 and K192) were identified to be methylated. SET7/9 only modified lysine residues at the terminal position of a given KAK motif. Also, the addition of DNA to the methylation reaction inhibited the modification of H1.4 by SET7/9. Expression analysis of GFP-tagged full length H1.4 or its C-terminal or N- terminal fragments in U2OS cells suggested that all fusion proteins localize to either hetero- or euchromatin in the nucleus. Furthermore, the ADP-ribosyltransferase ARTD1 modified several amino acid residues in the C-terminal domain of H1.4, a process that inhibited SET7/9-dependent methylation. This mutual exclusion of the two modifications suggests that post- translational modification of structurally close amino acids can influence each other and thereby regulate the functionality of proteins.

Keywords: H1.4, SET7/9, ARTD1