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

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Abstract
[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. Labeled putrescine and spermidine were 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 and .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.