Insect cells contain an unusual, membrane-bound β-N-acetylglucosaminidase probably involved in the processing of protein N-glycans

Abstract

The β-N-acetylglucosaminidase activity in the lepidopteran insect cell line Sf21 has been studied using pyridylaminated oligosaccharides and chromogenic synthetic glycosides as substrates. Ultracentrifugation experiments indicated that the insect cell β-N-acetylglucosaminidase exists in a soluble and a membrane-bound form. This latter form accounted for two-thirds of the total activity and was associated with vesicles of the same density as those containing GlcNAc-transferase I. Partial membrane association of the enzyme was observed with all substrates tested, i.e. 4-nitrophenyl β-N-acetylglucosaminide, tri-N-acetylchitotriose, and an N-linked biantennary agalactooligosaccharide. Inhibition studies indicated a single enzyme to be responsible for the hydrolysis of all these substrates. With the biantennary substrate, the β-N-acetylglucosaminidase exclusively removed β-N-acetylglucosamine from the α1,3-antenna. GlcNAcMan5GlcNAc2, the primary product of GlcNAc-transferase I, was not perceptibly hydrolyzed. β-N-Acetylglucosaminidases with the same branch specificity were also found in the lepidopteran cell lines Bm-N and Mb-0503. In contrast, β-N-acetylglucosaminidase activities from rat or frog (Xenopus laevis) liver and from mung bean seedlings were not membrane-bound, and they did not exhibit a strict branch specificity. An involvement of this unusual β-N-acetylglucosaminidase in the processing of asparagine-linked oligosaccharides in insects is suggested.