When produced by Pichia pastoris, three of the five Asn-Xaa-Ser/Thr sequences (corresponding to Asn-24, Asn-73 and Asn-87) in the carbohydrate-binding module CBM2a of xylanase 10A from Cellulomonas fimi are glycosylated. The glycans are of the high-mannose type, ranging in size from GlcNAc2Man8 to GlcNAc2Man14. The N-linked glycans block the binding of CBM2a to cellulose. Analysis of mutants of CBM2a shows that glycans on Asn-24 decrease the association constant (Ka) for the binding of CBM2a to bacterial microcrystalline cellulose approx. 10-fold, whereas glycans on Asn-87 destroy binding. The Ka of a mutant of CBM2a lacking all three N-linked glycosylation sites is the same when the polypeptide is produced by either Escherichia coli or P. pastoris and is approx. half that of wild-type CBM2a produced by E. coli.
Abbreviations used: CBM, carbohydrate-binding module; ConA, concanavalin A from Canavalia ensiformis; EndoF1, endoglycosidase F1 from Chryseobacterium meningosepticum; Ka, association constant; MALDI–TOF-MS, matrix-assisted laser desorption ionization–time-of-flight MS; TFA, trifluoroacetic acid.