Aminoacyl-transferases and the N-end rule pathway of prokaryotic/eukaryotic specificity in a human pathogen

摘要

The N-end rule relates the in vivo half-life of a protein to the identity of its N-terminal residue. Primary destabilizing N-terminal residues (Ndp) are recognized directly by the targeting machinery. The recognition of secondary destabilizing N-terminal residues (Nds) is preceded by conjugation of an Ndp residue to Nds of a polypeptide substrate. In eukaryotes, ATE1-encoded arginyl-transferases (RD,E,C*-transferases) conjugate Arg (R), an Ndp residue, to Nds residues Asp (D), Glu (E), or oxidized Cys residue (C*). Ubiquitin ligases recognize the N-terminal Arg of a substrate and target the (ubiquitylated) substrate to the proteasome. In prokaryotes such as Escherichia coli, Ndp residues Leu (L) or Phe (F) are conjugated, by the aat-encoded Leu/Phe-transferase (L/FK,R-transferase), to N-terminal Arg or Lys, which are Nds in prokaryotes but Ndp in eukaryotes. In prokaryotes, substrates bearing the Ndp residues Leu, Phe, Trp, or Tyr are degraded by the proteasome-like ClpAP protease. Despite enzymological similarities between eukaryotic RD,E,C*-transferases and prokaryotic L/FK,R-transferases, there is no significant sequelogy (sequence similarity) between them. We identified an aminoacyl-transferase, termed Bpt, in the human pathogen Vibrio vulnificus. Although it is a sequelog of eukaryotic RD,E,C*-transferases, this prokaryotic transferase exhibits a "hybrid" specificity, conjugating Ndp Leu to Nds Asp or Glu. Another aminoacyl-transferase, termed ATEL1, of the eukaryotic pathogen Plasmodium falciparum, is a sequelog of prokaryotic L/FK,R-transferases (Aat), but has the specificity of eukaryotic RD,E,C*-transferases (ATE1). Phylogenetic analysis suggests that the substrate specificity of R-transferases arose by two distinct routes during the evolution of eukaryotes.