Site-directed removal of N-glycosylation sites in BST-1/CD157: effects on molecular and functional heterogeneity

Abstract

Cyclic ADP ribose (cADPR) is a novel second messenger that releases calcium from intracellular calcium stores, but works independently of inositol 1,4,5-trisphosphate. In mammals ADP-ribosyl cyclase function is found in two membrane proteins, CD38 and bone marrow stromal cell antigen 1 (BST-1)/CD157. These enzymes are exposed extracellularly and also possess cADPR hydrolase activity, but an intracellular soluble ADP-ribosyl cyclase has been reported in human T-cells. Previously, a soluble form of BST-1/CD157 (sBST-1), which lacked the glycosylphosphatidylinositol-anchored portion, was expressed by a baculovirus–insect-cell system. In this study, we have purified the sBST-1, and it migrated as two major bands by SDS/PAGE, suggesting that it is post-translationally modified. BST-1 contains four putative N-glycosylation sites. Tunicamycin treatment reduced sBST-1 expression in the culture medium, indicating that N-glycosylation is essential for secretion. Site-directed mutagenesis was performed to generate sBST-1 mutants (N1–N4), each preserving a single N-glycosylation site. N1, N3 and N4 were well secreted into the medium, and were each detected as a single band. Although N3 and N4 retained the ADP-ribosyl cyclase activity, the cADPR-hydrolase activity was retained only in N4. We conclude that N-glycosylation of sBST-1 facilitates the folding of the nascent polypeptide chain into a conformation that is conductive for intracellular transport and enzymic activity. Furthermore a crystal has been obtained using the N4 mutant, but not the wild-type sBST-1. Thus the artificial engineering of N-glycosylation sites could be an effective method to generate homogeneous material for structural studies.