A mitochondrial SET domain protein is essential in Trypanosoma brucei and required for mitochondrial morphology and function

Protein lysine methylation is a ubiquitous post-translational modification. First identified in histone tails, many non-histone proteins are methylated at lysine residues. SET domain proteins catalyze the transfer of a methyl group from a donor such as S-adenosyl-L-methionine (SAM) to lysine residues. In this work, we identify a novel SET domain protein, TbSETD, in the African trypanosome, Trypanosoma brucei, and demonstrate that it is essential in both insect stage procyclic form and bloodstream form parasites. Epitope-tagged TbSETD localizes to the mitochondrion and TbSETD-deficient parasites exhibit growth defects, altered mitochondrial morphology, increased ROS levels, and increased sensitivity to apoptotic triggers. TbSETD binding proteins identified in immunoprecipitation experiments were enriched in mitoribosomal proteins. We also demonstrate that TbSETD immunoprecipitated from parasites has methyltransferase activity in vitro and methyllysine western blots reveal that TbSETD-deficient parasites have reduced levels of a 37 kDa mitochondrial protein. OrthoMCL DB indicates TbSETD is conserved only in kinetoplastids, suggesting a unique role in the biology of these parasites, and highlighting its potential as a drug target. SignificanceKinetoplastid parasites include several medically important organisms for which current treatments are insufficient. SET domain protein lysine methyltransferases (PKMTs) are highly druggable and have predominately been studied in the context of chromatin remodeling and gene expression. This work provides the first example of a SET domain PKMT that localizes to the mitochondria in any protozoan parasite and the first implication that protein lysine methylation may be important to mitoribosomal biogenesis.