Post-translational modification of CD38 protein into a high molecular weight form alters its catalytic properties.

Human CD38 is a 45-kDa transmembrane protein that acts as a bifunctional ectoenzyme, catalyzing the synthesis of cyclic ADP-ribose (cADPR) from NAD+ and the hydrolysis of cADPR to ADP-ribose. All-trans-retinoic acid (RA) is a potent and specific inducer of CD38 in myeloid cells. In this report, we demonstrate that RA-induced CD38 protein from human myeloid (HL-60) leukemia cells coimmunoprecipitates with another protein of molecular mass approximately190 kDa (p190). The p190 protein is localized exclusively in the membranes and is a consequence of post-translational cross-linking of CD38 protein. This conclusion was based on the observations that purified CD38 effectively competes with p190, its accumulation is preceded by the accumulation of CD38, it immunoreacted with three different monospecific anti-CD38 antibodies on immunoblots, and its peptide map revealed several peptides in common with CD38. Furthermore, CD38 could serve as a suitable substrate for transglutaminase (TGase)-catalyzed cross-linking reactions in vitro, and the accumulation of p190 in RA-treated HL-60 cells is effectively blocked by the presence of TGase-specific inhibitor. The purified p190 showed at least three times more cyclase activity than CD38. Conversely, p190 was at least 2.5-fold less active than CD38 in hydrolyzing cADPR to ADPR. These results suggest that post-translational modification of CD38 may represent an important mechanism for regulating the two catalytic activities of this bifunctional enzyme.