Are the tryptophanyl‐tRNA synthetase and the peptide‐chain‐release factor from higher eukaryotes one and the same protein?

Recently, cDNA clones encoding the bovine (b) [M. Garret, B. Pajot, V. Trézéguet, J. Labouesse, M. Merle, J.‐C. Gandar, J.‐C. Gandar, J.‐P. Benedetto, M.‐L. Sallafranque, J. Alterio, M. Gueguen, C. Sarger, B. Labouesses and J. Bonnet (1991) Biochemistry 30 , 7809–7817] and human (h) [L. Yu. Frolova, M. A. Sudomoina, A. Yu. Grigorieva, O. L. Zinovieva and L. L. Kisselev (1991) Gene 109 , 291–296] tryptophanyl‐tRNA synthetases (TrpRS) were sequnced; the deduced amino acid sequences exhibit typical structural features of class I aminoacyl‐tRNA synthetases [G. Eriani, M. Delarue, O. Poch, J. Gangloff and D. Moras (1990) Nature 237 , 203–206] and limited, although significant, similarity with bacterial TrpRS. Independently, it was shown that a major protein whose synthesis is stimulated in human cell cultures by interferon γ [J. Fleckner, H. H. Rasmussen and J. Justesen (1991) Proc. Natl Acad. Sci. USA 88 , 11 520–11 524], and interferons γ or α [B. Y. Rubin, S. L. Anderson, L. Xing, R. J. Powell and W. P. Tate (1991) J. Biol. Chem. 226 , 24 245–24 248], exhibits TrpRS activity and an amino acid sequence identical to that of hTrpRS. The amino acid sequences of bTrpRS and hTrpRS are highly similar and are surprisingly very similar to the amino acid sequence deduced from a cloned and sequenced cDNA reported to encode rabbit (r) peptide‐chain‐release factor (RF) [C. C. Lee, W. J. Craigen, D. M. Muzny, E. Harlow and C. T. Caskey (1990) Proc. Natl Acad. Sci. USA 87 , 3508–3512]. This close similarity between mammalian TrpRS and cloned RF is unexpected given the distinct functional properties of these proteins. Consequently, the question arises as to whether the mammalian TrpRS and RF activities reside on identical or very similar polypeptides. Alternatively, one may assume that the cloned rabbit cDNA encodes a protein other than rRF. Several properties (immunochemical, biochemical and physico‐chemical) of mammalian TrpRS and RF have been compared. rTrpRS and rRF have distinct thermostabiliy behaviours, and dissimilar chromatographic profiles on phosphocellulose. Both the anti‐bTrpRS polyclonal antibodies and the monoclonal antibody Am2 strongly inhibit the bTrpRS and hTrpRS aminoacylation activities, but not the rRF activity. In addition, neither bTrpRS nor hTrpRS exhibit RF activity. It is concluded that (a) the functional centers responsible for the TrpRS and RF activities are structurally and functionally dissimilar and could hardly belong to the same protein domain; (b) RF and TrpRS probably reside on different polypeptide chains encoded by different genes, although it is still not excluded that the RF‐ and TrpRS‐specific domains are situated on one and the same polypeptide chain; (c) the rabbit cDNA believed to encode RF, presumably encodes rTrpRS; this would explain why its deduced amino acid sequence has no similarity with bacterial RFs but is very similar to mammalian TrpRS.