Functional Characterization of a Novel Human Protein Methyltransferase Family

A typical cell contains about 10 000 proteins, referred to as the proteome. 20 different amino acids constitute the building blocks of proteins. After the proteins have been made (translated) their constituent amino acids can be further modified by the attachment of chemical groups, such as methyl and phosphate moieties, thereby further increasing the complexity of the proteome. These modifications are introduced by the help of specific enzymes, and they are important for regulating or improving the performance of the proteins. Lysine, one of the amino acids in proteins, can be further modified by methylation, i. e. the attachment of up to three methyl groups, and this process is catalyzed by a group of enzymes called lysine specific methyltransferases. Protein lysine methylation has been studied most intensely for the so-called histone proteins, which, together with DNA is the main constituent of chromosomes, the genetic material. Here, methylation of the lysines are important regulators of the gene activity, and the pattern of such methylations may also be inherited from one cell to another, and therefore contribute to so-called epigenetic inheritage; where traits are inherited independent of the DNA-sequence. It has recently become clear that, in addition to the histone proteins, many other cellular proteins are subject to lysine methylations that are important for their function. Recently, we have discovered a new family of human lysine specific methyltransferases that primarily target non-histone proteins. In the current project, we have revealed the function of yet uncharacterized members of this family, and shown that these methyltrasferases play important roles in cellular protein synthesis and in mitochondrial metabolism. Because such methyltransferases have been implicated in a wide range of human diseases, including cancer, it is likely that the knowledge obtained through this project will have important biomedical applications.

Through this project we have obtained new knowledge on a highly important group of human enzymes - so-called methyltransferases. This has lead to several publications in prestigious scientific journals, and some results are planned followed up regarding the medical significance of these MTases, e.g. in cancer.

Methylation of lysine residues in proteins represents a common post-translational modification which is introduced by dedicated protein lysine methyltransferases (MTases). Lysine methylation has been subjective to intensive studies in the case of histone proteins, where site-specific lysine methylations represent important epigenetic marks regulating chromatin state and transcriptional activity. However, non-histone proteins are also frequently methylated, and there is an increasing appreciation that also non-histone methylations play important functional and regulatory roles. The human genome encodes ~200 MTases, the majority of which remains uncharacterized. Recently, we found that a family of such enzymes, MTase Family 16 (MTF16), encompasses ten characterized and putative lysine specific methyltransferases in humans. We have in detail characterized two of these enzymes, which both methylate essential ATP-dependent molecular chaperones. We found that METTL21D methylates a single lysine residue in VCP (Valosin Containing Protein), whereas METTL21A targets a conserved lysine residue found in a number of human Hsp70 (HSPA) proteins. In the present project we will investigate MTF16 members that have not yet been characterized, and we will structurally characterize the interaction between MTF16 members and their substrates. As some human MTF16 members are implicated in protein translation and cell metabolism, we wish to study the effect of lysine methylation on these processes. Also, we wish to investigate the dynamics and reversibility of the lysine methylations introduced by the MTF16 enzymes, and whether the effects of such methylation are exerted through the binding of specific partner proteins. The project is expected to unravel novel MTases involved in lysine methylation, and to increase the understanding of the functional significance of such methylation.