N-TERMINAL ARGINYLATION OF PROTEINS IN EXPLANTS OF INJURED SCIATIC-NERVES AND EMBRYONIC BRAINS OF RATS

Posttranslational modification of proteins by arginine and lysine has been demonstrated in crude extracts of vertebrate nerves and brain but not in intact cells. In the present experiments we have exploited the fact that Arg is added posttranslationally only at the N-terminus of target proteins, to demonstrate these reactions in intact cells of sci atic nerves and embryonic brains of rats. Sciatic nerves were crushed in anaesthesized rats and 2 hrs later segments of nerve, including the site of the crush, were removed and incubated in media containing [H- 3]Arg. Incorporation of [H-3]Arg into total proteins was analyzed by a cid precipitation and the presence of label at the N-terminus was dete rmined by a modification of the Edman degradation procedure. Approxima tely 25% of protein bound [H-3]Arg was released from the N-terminus by the Edman reaction indicating that it was added posttranslationally r ather than through protein synthesis. N-terminal labeling was not dete ctable in nerves not crushed prior to explant and incubation. Slices o f embryonic day 20 visual cortex, when incubated under similar conditi ons as injured sciatic nerves, also showed approximately 25% of the pr otein incorporated [H-3]Arg at the N-terminus, while arginylation was not detectable in adult rat brain slices. Since Lys is not added postt ranslationally to the N-terminus, we have attempted to observe lysylat ion of proteins in intact cells by using cycloheximide (Cx) to block p rotein synthesis without interfering with protein modification. The po sttranslational incorporation of Arg/Lys into proteins was found to be insensitive to up to 2.0 mM Cx in tissue extracts (in vitro). However , in intact cells, doses as low as 10 uM Cx completely inhibited the i ncorporation of [H-3]Arg/Lys into proteins. One uM Cx allowed for some incorporation of [H-3]Arg/Lys into protein and approximately 40% of t he Cx insensitive Arg was incorporated into the N-terminal. These resu lts show that in vivo but not in vitro, Cx can block protein modificat ion, suggesting that either in intact cells protein modification requi res protein synthesis, or that Cx has effects other than as an inhibit or of protein synthesis on cells in culture, effects that it does not have on the partially purified components of the reaction.