C-Terminal Peptide Appendix in a Class I tRNA Synthetase Needed for Acceptor-Helix Contacts and Microhelix Aminoacylation

The 10 class I tRNA synthetases have an N-terminal nucleotide-binding fold which contains the catalytic center. Insertions into the nucleotide-binding fold provide contacts for acceptor-helix interactions, which stabilize the amino acid acceptor end of the tRNA substrate in the active site. A separate and largely nonconserved C-terminal domain provides contacts with distal parts of the tRNA, including the anticodon. For Escherichia coli methionyl tRNA synthetase, whose structure is known, the C-terminal domain is predominantly α-helical and forms a loop which interacts with the anticodon trinucleotide located about 76 Å from the amino acid attachment site. Fused to the end of this helical domain is a peptide which curls back into the N-terminal nucleotide-binding fold and region of the active site. We show here that mutations in this peptide appendix disrupt aminoacylation and binding of a 7 base pair microhelix substrate based on the acceptor stem of tRNA^fMet, without affecting interactions with ATP or methionine or with the tRNA^fMet anticodon. The impairment of acceptor-helix interactions by mutation of the C-terminal peptide can offset favorable anticodon interactions and severely reduce aminoacylation of tRNA^fMet. Thus, in addition to, or as an alternative to, acceptor-helix-binding insertions into the N-terminal nucleotide-binding fold, C-terminal peptide epitopes in some class I enzymes may provide a mechanism for facilitating RNA microhelix interactions with the catalytic site.