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) (PubMed:34774131, PubMed:35017528). N(3)-methylcytidine methylation modification regulates mitochondrial translation efficiency and is required for activity of the respiratory chain (PubMed:34774131, PubMed:35017528). N(3)-methylcytidine methylation of mitochondrial tRNA(Ser)(UCN) requires the formation of N(6)-dimethylallyladenosine(37) (i6A37) by TRIT1 as prerequisite (PubMed:34774131, PubMed:35017528). 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 (PubMed:34774131, PubMed:33313824). 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: 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. Belongs to the methyltransferase superfamily. METL family. The existence of N(3)-methylcytidine on mRNA is unclear (PubMed:34774131, PubMed:33313824). A report was unable to detect N(3)-methylcytidine formation in mammalian mRNAs using a nucleotide-resolution sequencing approach (PubMed:33313824). According to another publication, METTL8 has no activity on mRNAs and specifically methylates mitochondrial tRNAs (PubMed:34774131). A publication reported some nucleolar localization (PubMed:32199293). However, the protein used in this publication used a N-terminal tag, which possibly interferes with the mitochondrial targeting (PubMed:34774131).