Identification of the N-glycosylation sites on recombinant bovine CD38 expressed in Pichia pastoris: Their impact on enzyme stability and catalytic activity

Bovine CD38, a type 11 glycoprotein, contains two potential N-glycosylation sites (Asn-201 and Asn-268) in its extracellular domain. This contrasts with the other mammalian members of the ADP-ribosyl cyclase family, such as human CD38 and BST-1/CD157, in which four such sites are present. Our study was designed to determine the occupancy of these sites in a recombinant form of this ecto-enzyme and to evaluate its impact on the protein stability and catalytic functions. To that end we have successfully expressed the hydrosoluble ecto-domain of bovine CD38 (bCD38; residues 32-278), and corresponding glycosylation mutants, in the methylotrophic yeast Pichia pastoris. The secreted proteins were purified to homogeneity by affinity chromatography on immobilized Cibacron blue. We found by site-directed mutagenesis and mass spectrometry that bCD38 was a monoglycosylated protein at Asn-201. The expression yield of the deglycosylated mutants was not significantly affected, indicating that glycosylation at Asn-201 was not required for a proper processing and secretion of this protein by A pastoris. Significant alterations in the kinetic parameters of NAD(+) were observed for the deglycosylated mutants. The thermo-stability of the recombinant enzyme was also influenced by mutation at position 201. Interestingly both parameters were dependent on the nature of the mutant and a stable deglycosylated mutant N201D of bCD38 could be produced that can be further used for structural studies. (C) 2009 Elsevier Inc. All rights reserved.