ABSTRACT
UDP-GlcNAc:alpha3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I (GnTI; EC 2.4.1.101) is a medial-Golgi enzyme that is essential for the processing of oligomannose to hybrid and complex N-glycans. On the basis of highly conserved sequences obtained from previously cloned mammalian GnTI genes, cDNAs for two closely related GnTI isoenzymes were isolated from a Xenopus laevis ovary cDNA library. As typical for glycosyltransferases, both proteins exhibit a type II transmembrane protein topology with a short N-terminal cytoplasmic tail (4 amino acids); a transmembrane domain of 22 residues; a stem region with a length of 81 (isoenzyme A) and 77 (isoenzyme B) amino acids, respectively; and a catalytic domain consisting of 341 residues. The two proteins differ not only in length but also at 13 (stem) and 18 (catalytic domain) positions, respectively. The overall identity of the catalytic domains of the X. laevis GnTI isoenzymes with their mammalian and plant orthologues ranges from 30% (Nicotiana tabacum) to 67% (humans). Isoenzymes A and B are encoded by two separate genes that were both found to be expressed in all tissues examined, albeit in varying amounts and ratios. On expression of the cDNAs in the baculovirus/insect cell system, both isoenzymes were found to exhibit enzymatic activity. Isoenzyme B is less efficiently folded in vivo and thus appears of lower physiological relevance than isoenzyme A. However, substitution of threonine at position 223 with alanine was sufficient to confer isoenzyme B with properties similar to those observed for isoenzyme A.
