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CD38 is a transmembrane glycoprotein that functions as an ectoenzyme and as a receptor. Based on the structural similarity between CD38 and ADP-ribosyl cyclase from Aplysia californica, it was hypothesized that CD38 is expressed as a homodimer on the surface of cells. Indeed, CD38 dimers have been reported, however, the structural requirements for their stabilization on the plasma membrane are unknown. We demonstrate that the majority of CD38 is assembled as noncovalently associated homodimers on the surface of B cells. Analysis of CD38 mutants, expressed in Ba/F3 cells, revealed that truncation of the cytoplasmic region or mutation of a single amino acid within the a1-helix of CD38 decreased the stability of the CD38 homodimers when solubilized in detergent. Cells expressing the unstable CD38 homodimers had diminished expression of CD38 on the plasma membrane and the half-lives of these CD38 mutant proteins on the plasma membrane were significantly reduced. Together, these results show that CD38 is expressed as noncovalently associated homodimers on the surface of murine B cells and suggest that appropriate assembly of CD38 homodimers may play an important role in stabilizing CD38 on the plasma membrane of B cells.Keywords: B lymphocytes; CD38; homodimer stability; NAD + glycohydrolase; protein structure.CD38 is a type II transmembrane ectoenzyme expressed by many cell types [1][2][3]. CD38 plays an important role in calcium signaling as it catalyzes the production of several calcium mobilizing metabolites including adenosine(5¢)- [4,5]. In addition to its role as an enzyme, CD38 can also serve as a receptor on the plasma membrane of leukocytes and lymphocytes. For example, incubation of B lymphocytes with agonistic antibodies to CD38 induces calcium mobilization, protein phosphorylation, proliferation, class switching, rescue from cell death and induction of apoptosis [1,[6][7][8][9][10]. In order to understand the dual receptor and enzyme properties of CD38, a number of structure/function studies have been performed. These studies have been guided by analyses of two CD38 homologues, the cytosolic Aplysia californica ADP-ribosyl cyclase [11,12] and the mammalian GPI-anchored NAD + glycohydrolase, CD157 [13,14].Crystallographic and X-ray diffraction analyses of these two proteins indicated that both proteins form noncovalently associated homodimers [15,16]. Thus, it has been proposed that CD38 is also likely to be expressed as a homodimer on the plasma membrane and, in agreement with this hypothesis, initial reports showed that high molecular mass aggregates of CD38 are formed after incubation of human erythrocytes with NAD + or 2-mercaptoethanol [17]. In addition, it was reported that CD38 formed dimers and oligomers on the membrane of CD38 transfected HeLa cells [18]. It is not clear, however, whether CD38 is always present in a dimeric form on the surface of cells as many groups have reported finding only the monomeric form of CD38 [19][20][21]. Furthermore, it remains to be determined whether CD38 dimers ar...

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