CD38-MEDIATED RIBOSYLATION OF PROTEINS

The lymphocyte cell-surface Ag CD38 catabolizes NAD to adenosine 5' di phosphoribose (ADPR) and cyclic ADPR (cADPR). We show here that the so luble extracellular domain of CD38 (sCD38) mediates ADP ribosylation o f several proteins. This was demonstrated by mass spectrometric analys es which revealed the addition of mass in units of 541.1 Da to these p roteins, presumably corresponding to the covalent attachment of one or more ADPR moieties. Separate experiments showed that the same protein s became specifically radiolabeled following incubation with [(32)p]NA D plus sCD38. Additionally, it is shown that sCD38 can autoribosylate. Moreover, sCD38-mediated protein ribosylation was found to occur spec ifically at cysteine residues, since it was effectively blocked by add ition of L-cysteine but not by other amino acids, and CD38-mediated pr otein ribosylation could be reversed by the addition of HgCl2, which s pecifically cleaves thiol-glycosidic bonds. ADPR purified from the rea ction of sCD38 with NAD could itself be covalently transferred to targ et proteins at rates similar to the sCD38-mediated reaction, indicatin g that the ribosylation proceeds via the generation of this reactive i ntermediate. In vitro mutagenesis of a catalytic Glu residue that is c onserved in numerous ADP-ribosyl transferases revealed that this amino acid is also important for catalysis in CD38. These data suggest that CD38 has the potential to cause ribosylation of experimental proteins , and raises the possibility that its specific ribosylation of a curre ntly unidentified lymphocyte protein may contribute to its array of im munoregulatory activities.