Structure of the EMAPII domain of human aminoacyl-tRNA synthetase complex reveals evolutionary dimer mimicry

The EMAPII (endothelial monocyte-activating polypeptide II) domain is a tRN A-binding domain associated with several aminoacyl-tRNA synthetases, which becomes an independent domain with inflammatory cytokine activity upon apop totic cleavage from the p43 component of the multisynthetase complex, It co mprises a domain that is highly homologous to bacterial tRNA-binding protei ns (Trbp), followed by an extra domain without homology to known proteins, Trbps, which may represent ancient tRNA chaperones, form dimers and bind on e tRNA per dimer, In contrast, EMAPII domains are monomers, Here we report the crystal structure at 1.14 Angstrom of human EMAPII, The structure revea ls that the Trbp-like domain, which forms an oligonucleotide-binding (OB) f old, is related by degenerate 2-fold symmetry to the extra-domain. The pseu do-axis coincides with the dyad axis of bacterial TtCsaA, a Trbp whose stru cture was solved recently. The interdomain interface in EMAPII mimics the i ntersubunit interface in TtCsaA, and may thus generate a novel OB-fold-base d tRNA-binding site. The low sequence homology between the extra domain of EMAPII and either its own OB fold or that of Trbps suggests that dimer mimi cry originated from convergent evolution rather than gene duplication.