Unlike the catalytic alpha-subunit, the beta-subunit of heterodimeric (alphabeta)2 phenylalanyl-tRNA synthetase (PheRS) has no invariant functional amino acids directly involved in the aminoacylation process as it is evident from the crystal structure of the T. thermophilus enzyme complexed with tRNAPhe. Having no catalytic function, the prokaryotic beta-subunit comprises OB-, RNP-, SH3-, and DNA-binding-like domains involved in a variety of biological functions in other proteins. It was shown that the mRNA of the human alpha-subunit overexpressed in the tumorigenic versus the nontumorigenic variant of the same acute-phase chronic myeloid leukemia cell line (CML). We cloned, sequenced, and expressed human PheRS. The layout of the human sequence indicates that the general tRNA binding mode and anticodon recognition differ between prokaryotes and eukaryotes for the phenylalanine system. Northern blot hybridization analysis from malignant and normal human tissues enabled us to assess the relative expression levels of the alpha- and beta-subunits independently, in view of the additional cellular role proposed for the beta-subunit in tumorigenic events. The levels of mRNA corresponding to the alpha- and beta-subunits were remarkably similar in all cell types and tissues examined, thus indicating the implication of the entire (alphabeta)2 heterodimer in tumorigenic events.
Copyright 1999 Academic Press.