Post-translational modification of neuronal proteins: evidence for transglutaminase activity in R2, the giant cholinergic neuron of Aplysia.

[3H]Putrescine injected into the cell body of the giant neuron R2 of Aplysia was readily converted to gamma-aminobutyric acid, acetylputrescine, spermidine, and spermine. In addition, labeled putrescine and spermidine were found covalently linked to protein through the action of an intracellular transglutaminase. This was shown by exhaustively treating the acid-insoluble fraction from injected cells with Pronase, aminopeptidase M, and carboxypeptidases A and B. High-performance liquid chromatography of the digest revealed labeled gamma-glutamylputrescine and gamma-glutamylspermidine, the products expected from the transglutaminase-catalyzed post-translational modification of intracellular proteins. In vitro assays of Aplysia nervous tissue showed the presence of transglutaminase as well as gamma-glutaminyl cyclotransferase, an enzyme that cleaves the gamma-glutamylpolyamine bond. Incorporation of polyamine into proteins in R2 is a specific process because only a few 3H-labeled polypeptides were found after injection.