Abstract
The correct amino acid sequence of E. coli isoleucyl-tRNA synthetase (IleRS) was established by means of peptide mapping by MALDI mass spectrometry, using a set of four endoproteases (trypsin, LysC, AspN and GluC). Thereafter, the active site of IleRS was mapped by affinity labeling with reactive analogs of the substrates. For the ATP binding site, the affinity labeling reagent was pyridoxal 5'-diphospho-5'-adenosine (ADP-PL), whereas periodate-oxidized tRNAIle, the 2',3'-dialdehyde derivative of tRNAIle was used to label the binding site for the 3'-end of tRNA on the synthetase. Incubation of either reagent with IleRS resulted in a rapid loss of both the tRNAIle aminoacylation and isoleucinedependent isotopic ATP-PPi exchange activities. The stoichiometries of IleRS labeling by ADP-PL or tRNAIleox corresponded to 1 mol of reagent incorporated per mol of enzyme. Altogether, the oxidized 3'-end of tRNAIle and the pyridoxal moiety of the ATP analog ADP-PL react with the lysyl residues 601 and 604 of the consensus sequence 601KMSKS605. Identification of the binding site for L-isoleucine or for non cognate amino acids on E. coli IleRS was achieved by qualitative comparative labeling of the synthetase with bromomethyl ketone derivatives of L-isoleucine (IBMK) or of the non-cognate amino acids valine (VBMK), phenylalanine (FBMK) and norleucine (NleBMK). Labeling of the enzyme with IBMK resulted in a complete loss of isoleucine-dependent isotopic [32P]PPi-ATP exchange activity. VBMK, NleBMK and FBMK were also capable of abolishing the activity of IleRS, FBMK being the less efficient in inactivating the synthetase. Analysis by MALDI mass spectrometry designated cysteines-462 and -718 as the target residues of the substrate analog IBMK on E. coli IleRS, whereas VBMK, NleBMK and FBMK labeled in common His-394, His-478 and Cys-718. In addition, VBMK and NleBMK, which are chemically similar to IBMK, were found covalently bound to Cys-462, and VBMK was specifically attached to His-332 (or His-337) of the synthetase. The amino acid residues labeled by the substrate analogs are mainly distributed between three regions in the primary structure of E. coli IleRS: these are segments [325-394], [451-479] and [591-604]. In the 3-D structures of IleRS from T. thermophilus and S. aureus, the [325-394] stretch is part of the editing domain, while fragments [451-479] and [591-604] representing the isoleucine binding domain and the dinucleotide (or Rossmann) fold domain, respectively, are located in the catalytic core. His-332 of E. coli IleRS, that is strictly conserved among all the available IleRS sequences is located in the editing active site of the synthetase. It is proposed that His-332 of E. coli IleRS participates directly in hydrolysis, or helps to deprotonate the hydroxyl group of threonine at the hydrolytic site.
Highlights
Aminoacyl-tRNA synthetases catalyze at the expense of ATP the activation of specific amino acids, and their subsequent transfer onto the 3'-end of homologous isoacceptor tRNAs
Amino acid residues at the ATP-binding site of aminoacyl-tRNA synthetases were affinity labeled with ATP derivatives such as adenosine di- or triphosphopyridoxal or 5'-p-fluorosulfonyl-benzoyladenosine [7,8,9]
A previous MALDI-MS peptide mapping experiment demonstrated that none of these primary structures is correct
Summary
Aminoacyl-tRNA synthetases catalyze at the expense of ATP the activation of specific amino acids, and their subsequent transfer onto the 3'-end of homologous isoacceptor tRNAs. Lysyl residues at the binding site for the 3'-OH acceptor arm of tRNA were identified by affinity labeling with periodate-oxidized tRNA [1,2,3,4,5], leading to the discovery of the KMSKS consensus sequence [3, 4, 6] characteristic of class 1 aminoacyltRNA synthetases. Amino acid residues at the ATP-binding site of aminoacyl-tRNA synthetases were affinity labeled with ATP derivatives such as adenosine di- or triphosphopyridoxal or 5'-p-fluorosulfonyl-benzoyladenosine [7,8,9]. Nucleophilic amino acid residues at the binding site for L-valine or for non-cognate amino acids were identified by comparative labeling of valyl-tRNA synthetase (ValRS) with bro-
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