Substrate-binding subunit of tRNA (adenine-N(1)-)- methyltransferase, which catalyzes the formation of N(1)-methyladenine at position 58 (m1A58) in initiator methionyl-tRNA (PubMed:16043508). Together with the TRMT61A catalytic subunit, part of a mRNA N(1)- methyltransferase complex that mediates methylation of adenosine residues at the N(1) position of a small subset of mRNAs: N(1) methylation takes place in tRNA T-loop-like structures of mRNAs and is only present at low stoichiometries (PubMed:29072297, PubMed:29107537). {ECO:0000269|PubMed:16043508, ECO:0000269|PubMed:29072297, ECO:0000269|PubMed:29107537}. This is the function of TRMT6 (tRNA methyltransferase 6 non-catalytic subunit, Ensembl gene identifier ENSG00000089195).