THE TRANSMEMBRANE GLYCOPROTEIN CD38 IS A CATALYTICALLY ACTIVE TRANSPORTER RESPONSIBLE FOR GENERATION AND INFLUX OF THE 2ND-MESSENGER CYCLICADP-RIBOSE ACROSS MEMBRANES

CD38 is a type II transmembrane glycoprotein expressed in many vertebr ate cells. It is a bifunctional ectoenzyme that catalyzes both the syn thesis of Cyclic ADP-ribose (cADPR) from NAD(+) and the degradation of cADPR to ADP-ribose by means of its ADP-ribosyl cyclase and cADPR-hyd rolase activities, respectively. The cyclase also converts NGD(+) to c yclic GDP-ribose (cGDPR), which is refractory to cADPR-hydrolase, cADP R, but not cGDPR, is a potent calcium mobilizer from intracellular sto res. It has been demonstrated to be a new second messenger involved in the regulation of calcium homeostasis in many cell types, from plants to mammals. The number of physiological processes shown to be regulat ed by cADPR is steadily increasing. A topological paradox exists becau se ectocellularly generated cADPR acts intracellularly. Here we demons trate that the catalytic functioning of CD38 is accompanied by a cADPR (cGDPR) -transporting activity across natural and artificial membrane s. In resealed membranes from CD38(+) human erythrocytes, transport of catalytically generated cADPR or cGDPR was saturation dependent and o ccurred against a concentration gradient. Likewise, CD38-reconstituted proteoliposomes were active in concentrating NAD(+) (NGD(+))-derived cADPR (cGDPR) inside the vesicle compartment. Moreover, the cADpR-tran sporting activity in CD38 proteoliposomes prevented the hydrolase-cata lyzed degradation to ADPR that occurs conversely with detergent-solubi lized CD38, resulting in selective influx of cADPR. In the CD38 proteo liposomes, catalytically active CD38 exhibited monomeric, dimeric, and tetrameric structures. In CD38 sense- but not in antisense-transfecte d HeLa cells, externally added NAD+ resulted in significant, transient increases in cytosolic calcium. These data suggest that transmembrane juxtaposition of two or four CD38 monomers can generate a catalytical ly active channel for selective formation and influx of cADPR (cGDPR) to reach cADPR-responsive intracellular calcium stores.