Quantitative study of yeast Alg1 beta-1, 4 mannosyltransferase activity, a key enzyme involved in protein N-glycosylation

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 β-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 β-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-α-N, N′-diacetylchitobioside (PPGn2), was used as the acceptor for the β-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.