**4. SET-domain protein implication in ageing**

SET-domain is a short name for a highly conserved 130 to 140 amino acid motif characterising a group of proteins known to methylate histones on lysine. The function of SET-domain proteins is to transfer a methyl group from S-adenosyl-Lmethionine (AdoMet) to the amino group of a lysine residue on the histone or other protein [24].

Initially, the SET-domain proteins were associated exclusively with the regulation of developmental genes in metazoan. However, the finding of SET-domain genes in the unicellular yeasts *Saccharomyces cerevisiae* and *Schizosaccharomyces pombe* suggested that SET-domain proteins regulate a much broader variety of biological programmes [25]. By performing a targeted RNAi screen in fertile worms with selected genes that encode known worm methyltransferases, proteins containing the enzymatic domain of methyltransferases (SET-domain), or orthologues of regulators of histone methylation, the results depicted that *set-9* and *set-15* knock-down extended lifespan and also *set-2* and *set-4* knock-down extended fertile worm lifespan as previously reported by many investigators [8].

The authors also investigated whether the known histone lysine methyltransferase regulates lifespan and screened genes encoding the SET domain in *C. elegans* by RT-qPCR, and the results revealed that the mRNA levels of *set-2* and *set-15* increased significantly on day 11; both genes had been reported to promote ageing. In addition to these two genes, they found that the mRNA levels of *set-10*, *set-18*, *set-32* and F54F7.7 were also upregulated in nematodes, and these genes have not yet been reported to be involved in lifespan regulation [11].

Focusing on *set-18*, results revealed that this gene expression increases according to animal age and is highly expressed in muscle. The mutation on this gene revealed a lifespan extension and oxidative stress resistance compared to the wild-type worms. It is important to note that, when there is increased exposure to reactive oxygen species (ROS), the cell enters the state of chronic oxidative stress. The more a cell is growing, the more oxidative stress damages are increasing; thus, this has an influence on longevity [3].

Studies have shown that there is a close relationship between the survivals of the mitochondrial and that of the cells due to the central role of mitochondria in programmed cell death (apoptosis) as well as the important involvement of ROS produced at 90% in mitochondria. High levels of ROS and calcium, acting together, can trigger the mechanism of cell death *via* apoptosis or necrosis. For several years, plenty of researches aimed to understand the adverse effects of ageing and were conducted out on a wide range of model organisms, and nine general hallmarks of ageing in living organisms have been identified. These hallmarks affect the organism at different scales. Some occur at the molecular level within cells, while others impact tissues (muscles) and even beyond at the level of an organ or the entire organism [3]. These mechanisms have been revealed to influence longevity partially dependent on *daf-16*, a prominent longevity gene that encodes the worm orthologue of the highly conserved forkhead transcription factor forkhead box O (FOXO).

Sequence analysis showed that *set-18* has high homology to the mammalian histone methyltransferase SMYD family, which contains a SET-domain split into two segments by a myeloid, nervy, and DEAF-1 (MYND) domain [11]. *set-18* contains a conserved SET-domain that encodes proteins homologous to human SMYD1, SMYD2, and SMYD3 [26, 27]. The human homologue SMYD1, SMYD2, and SMYD3 of *set-18* have been reported to have the activity of histone methyltransferase. The SMYD family encloses five discrete proteins, of which SMYD1-5, with reported functions in both normal and pathologic conditions (ageing diseases). The key feature of all SMYD family members has been established to be the methylation of H3K4. For many SMYD family members, the SET-domain contains two sections: 1) the S-sequence, which may work as a cofactor binder as well as for protein–protein interactions and 2) the core SET-domain, which functions as the primary catalytic location [28, 29]. In close relation to the SET domain are two other domains: 1) the post-SET and 2) the SET-I, which assist in cofactor binding, substrate binding, and protein stabilization [30, 31].
