Alternative Splicing Results in Differential Expression, Activity, and Localization of the Two Forms of Arginyl-tRNA-Protein Transferase, a Component of the N-End Rule Pathway

Abstract

ABSTRACT The N-end rule relates the in vivo half-life of a protein to the identity of its N-terminal residue. The underlying ubiquitin-dependent proteolytic system, called the N-end rule pathway, is organized hierarchically: N-terminal aspartate and glutamate (and also cysteine in metazoans) are secondary destabilizing residues, in that they function through their conjugation, by arginyl-tRNA-protein transferase (R-transferase), to arginine, a primary destabilizing residue. We isolated cDNA encoding the 516-residue mouse R-transferase, ATE1p, and found two species, termed Ate1-1 and Ate1-2 . The Ate1 mRNAs are produced through a most unusual alternative splicing that retains one or the other of the two homologous 129-bp exons, which are adjacent in the mouse Ate1 gene. Human ATE1 also contains the alternative 129-bp exons, whereas the plant ( Arabidopsis thaliana ) and fly ( Drosophila melanogaster ) Ate1 genes encode a single form of ATE1p. A fusion of ATE1-1p with green fluorescent protein (GFP) is present in both the nucleus and the cytosol, whereas ATE1-2p–GFP is exclusively cytosolic. Mouse ATE1-1p and ATE1-2p were examined by expressing them in ate1Δ Saccharomyces cerevisiae in the presence of test substrates that included Asp-βgal (β-galactosidase) and Cys-βgal. Both forms of the mouse R-transferase conferred instability on Asp-βgal (but not on Cys-βgal) through the arginylation of its N-terminal Asp, the ATE1-1p enzyme being more active than ATE1-2p. The ratio of Ate1-1 to Ate1-2 mRNA varies greatly among the mouse tissues; it is ∼0.1 in the skeletal muscle, ∼0.25 in the spleen, ∼3.3 in the liver and brain, and ∼10 in the testis, suggesting that the two R-transferases are functionally distinct.