Hyperglycosylation of hen egg white lysozyme in yeast

The large molecular size of N-glycosylated lysozyme with a polymannose chain was predominantly expressed in the yeast carrying the lysozyme expression plasmid in 9-fold greater secretion compared with the wild type. Complementary DNA encoding hen egg white lysozyme was subjected to site-directed mutagenesis to obtain the Asn-X-Ser/Thr sequence that is the signal for asparagine-linked (N-linked) glycosylation. At positions 49, 67, 70, and 103, the signal for N-linked glycosylation was created. Only the mutant lysozyme whose glycine 49 was substituted with asparagine was expressed in the two types of glycosylated forms, a small oligomannose chain (Man18GlcNAc2)-linked form and a large polymannose chain (Man310GlcNAc2)-linked form, whereas other mutants were not glycosylated. The secreted amount of polymannosyl lysozyme was much higher than that of the oligomannosyl lysozyme. Both types of glycosylated lysozymes were susceptible to endo-beta-N-acetylglucosaminidase cleavage of their carbohydrate chains. The average molecular masses of oligomannosyl and polymannosyl lysozymes were 18 and 71 kDa, respectively. The length of the polymannose chain was found to be 200-350 residues/molecule of lysozyme according to the estimation of the molecular mass distribution by low angle laser light scattering measurements. The protein conformation estimated by CD analysis was completely conserved in these glycosylated lysozymes. The enzymatic activities of oligomannosyl and polymannosyl lysozymes were 100 and 91%, respectively, of wild-type protein when glycol chitin was used as a substrate. In addition, the polymannosyl lysozyme revealed remarkable heat stability in that no coagulation was observed under conditions in which the wild-type lysozyme coagulated. Thus, this novel glycoprotein can be used as a reporter in studies of the processing and sorting of glycoproteins and as a model of the expression of foreign genes in yeast for the construction of stable enzymes.