CD38 catalyzes the synthesis of cyclic ADP-ribose (cADPR), a Ca 2؉ messenger responsible for regulating a wide range of physiological functions. It is generally regarded as an ectoenzyme, but its intracellular localization has also been well documented. It is not known if internal CD38 is enzymatically active and contributes to the Ca 2؉ signaling function. In this study, we engineered a novel soluble form of CD38 that can be efficiently expressed in the cytosol and use cytosolic NAD as a substrate to produce cADPR intracellularly. The activity of the engineered CD38 could be decreased by mutating the catalytic residue Glu-226 and increased by the double mutation E146A/T221F, which increased its cADPR synthesis activity by >11-fold. Remarkably, the engineered CD38 exhibited the ability to form the critical disulfide linkages required for its enzymatic activity. This was verified by using a monoclonal antibody generated against a critical disulfide, Cys-254 -Cys-275. The specificity of the antibody was established by x-ray crystallography and site-directed mutagenesis. The engineered CD38 is thus a novel example challenging the general belief that cytosolic proteins do not possess disulfides. As a further refinement of this approach, the engineered CD38 was placed under the control of tetracycline using an autoregulated construct. This study has set the stage for in vivo manipulation of cADPR metabolism.
Mobilization of Ca2ϩ from intracellular stores is of fundamental importance in virtually all aspects of cellular activity. The major intracellular Ca 2ϩ stores are in the endoplasmic reticulum (ER) 2 and are mobilized by specific messenger molecules, inositol trisphosphate and cyclic ADP-ribose (cADPR). The latter is a novel cyclic nucleotide derived from NAD. It was first described in sea urchin eggs (1, 2) but has since been established as a second messenger molecule responsible for regulating a wide range of physiological functions as diverse as abscisic acid signaling in plants (3) and sponges (4) and social behavior in mice (Ref. 5; reviewed in Refs. 6 and 7). It targets the ryanodine receptor of the endoplasmic Ca 2ϩ stores. The synthesis and hydrolysis of cADPR in mammalian cells are catalyzed by CD38 (8), a transmembrane protein ubiquitously expressed in virtually all tissues (reviewed in Ref. 9). Gene knock-out studies have established that CD38 plays a critical role in a wide range of physiological functions, including insulin secretion (10), susceptibility to bacterial infection (11), and social behavior of mice through modulating neuronal oxytocin secretion (5).CD38 is a membrane protein with a short N-terminal tail, a single transmembrane segment, and a large C-terminal domain containing all the enzymatic activities (8, 12, 13). The crystal structure of the catalytic domain of CD38 has been solved, and the mechanism of how it catalyzes the multiple reactions responsible for metabolizing cADPR has also been elucidated to atomic resolution by x-ray crystallography (14 -16). It is established that the s...