Mitochondrial S-adenosyl-L-methionine-dependent methyltransferase that mediates N(3)-methylcytidine modification of residue 32 of the tRNA anticodon loop of mitochondrial tRNA(Ser)(UCN) and tRNA(Thr) (By similarity). N(3)-methylcytidine methylation modification regulates mitochondrial translation efficiency and is required for activity of the respiratory chain (By similarity). N(3)-methylcytidine methylation of mitochondrial tRNA(Ser)(UCN) requires the formation of N(6)-dimethylallyladenosine(37) (i6A37) by TRIT1 as prerequisite (By similarity). May also mediate N(3)-methylcytidine modification of mRNAs (PubMed:28655767). The existence of N(3)-methylcytidine modification on mRNAs is however unclear, and additional evidences are required to confirm the role of the N(3)-methylcytidine-specific mRNA methyltransferase activity of METTL8 in vivo (By similarity). Overexpression in lung progenitor cells stimulates smooth muscle-specific gene expression and suppresses adipogenic gene expression. Stimulates adipogenesis. Stimulates adipogenesis. cytidine(32) in tRNA(Ser) + S-adenosyl-L-methionine = H(+) + N(3)-methylcytidine(32) in tRNA(Ser) + S-adenosyl-L-homocysteine cytidine(32) in tRNA(Thr) + S-adenosyl-L-methionine = H(+) + N(3)-methylcytidine(32) in tRNA(Thr) + S-adenosyl-L-homocysteine a cytidine in mRNA + S-adenosyl-L-methionine = an N(3)-methylcytidine in mRNA + H(+) + S-adenosyl-L-homocysteine Interacts with EP300. Mitochondrial protein (By similarity). The cytoplasmic or nuclear localization observed by some groups is either the result of an incorrect localization caused by N-terminal tagging that interferes with mitochondrial targeting, or splice isoforms that lack the N-terminal mitochondrial transit sequence (By similarity). Isoform 5 localizes to the nucleus upon cell stretch in embryonic lung progenitor cells. Absent in embryonic lung but is induced in a fibroblast cell line by stretch. Expressed in undifferentiated progenitor cells, while its expression is inhibited by stretch. Absent in undifferentiated embryonic lung mesenchymal cells, but expression is induced by stretch. Expressed in mature adipose tissue. Induced in stretched embryonic lung mesenchymal cells. Mice were born with normal Mendelian ratio without developmental defects (PubMed:28655767). Cells show reduced N(3)-methylcytidine modification in mRNAs (PubMed:28655767). Belongs to the methyltransferase superfamily. METL family. According to some authors, contains a SANT domain but this domain is not detected by any prediction tool. Sequence differs from the displayed sequence in the N-terminal region.