Both allelic forms of the rat T cell differentiation marker RT6 display nicotinamide adenine dinucleotide (NAD)-glycohydrolase activity, yet only RT6.2 is capable of automodification upon incubation with NAD.

The finding that recently cloned mono-ADP-ribosyltransferases show sequence similarity to the rat T cell differentiation marker RT6 has led us to investigate the enzymatic activity of this alloantigenic system. To search for ADP-ribosylation of cell surface proteins, T cell populations from RT6.1- and RT6.2-expressing rat strains, as well as RT6.1+ and RT6.2+ T-T hybridoma cell lines, were incubated with [32P]nicotinamide adenine dinucleotide (NAD). All RT6.2+, but no RT6.1+ or RT6- cells, show incorporation of radioactivity into a single protein which could be identified as RT6.2 by immunoprecipitation with monoclonal antibodies. This automodification of RT6.2 is covalent, requires intact NAD as substrate, and displays characteristics typical for linkage of ADP-ribose to arginine. The alloantigens RT6.1 and RT6.2 differ in ten amino acids, RT6.2 having two arginine residues not present in RT6.1. Both alloantigens were found to display potent NAD-glycohydrolase activity.