Quantitative study of yeast Alg1 beta-1, 4 mannosyltransferase activity, a key enzyme involved in protein N-glycosylation
作者:Sheng-Tao Li、Ning Wang、Sha Xu、Jian Yin、Hideki Nakanishi、Neta Dean、Xiao-Dong Gao
DOI:10.1016/j.bbagen.2016.09.023
日期:2017.1
Background: Asparagine (N)-linked glycosylation begins with a stepwise synthesis of the dolichol-linked oligosaccharide (DLO) precursor, Glc3Man9GlcNAc2-PP-Dol, which is catalyzed by a series of endoplasmic reticulum membrane-associated glycosyltransferases. Yeast ALG1 (asparagine-linked glycosylation 1) encodes a beta-1, 4 mannosyltransferase that adds the first mannose onto GlcNAc2-PP-Dol to produce a core trisaccharide Man1GlcNAc2-PP-Dol.ALG1 is essential for yeast viability, and in humans mutations in the ALG1 cause congenital disorders of glycosylation known as ALG1-CDG. Alg1 is difficult to purify because of its low expression level and as a consequence, has not been well studied biochemically. Here we report a new method to purify recombinant Alg1 in high yield, and a mass spectral approach for accurately measuring its beta-1, 4 mannosyltransferase activity.Methods: N-terminally truncated yeast His-tagged Alg1 protein was expressed in Escherichia coli and purified by HisTrap HP affinity chromatography. In combination with LC-MS technology, we established a novel assay to accurately measure Alg1 enzyme activity. In this assay, a chemically synthesized dolichol-linked oligosaccharide analogue, phytanyl-pyrophosphoryl-alpha-N,N'-diacetylchitobioside (PPGn2), was used as the acceptor for the beta-1, 4 mannosyl transfer reaction.Results: Using purified Alg1, its biochemical characteristics were investigated, including the apparent K-m and V-max values for acceptor, optimal conditions of activity, and the specificity of its nucleotide sugar donor. Furthermore, the effect of ALG1-CDG mutations on enzyme activity was also measured.General significance: This work provides an efficient method for production of Alg1 and a new MS-based quantitative assay of its activity. (C) 2016 Elsevier B.V. All rights reserved.