Critical role of evolutionarily conserved glycosylation at Asn211 in the intracellular trafficking and activity of sialyltransferase ST3Gal-II.

FERNANDO M. RUGGIERO; A. ALEJANDRO VILCAES; RAMIRO IGLESIAS-BARTOLOME; JOSE L. DANIOTTI

BIOCHEMICAL JOURNAL

PORTLAND PRESS LTD

Lugar: Londres; Año: 2015

0264-6021

ST3Gal-II, a type II transmembrane protein, is the main mammalian sialyltransferase responsible for GD1a and GT1b ganglioside biosynthesis in brain. It contains two putative N-glycosylation sites (Asn92 and Asn211). While Asn92 is only conserved in mammalian species, Asn211 is highly conserved in mammals, birds and fish. The current study explores the occupancy and relevance for intracellular trafficking and enzyme activity of these potential N-glycosylations in human ST3Gal-II. We found that ST3Gal-II distributes along the Golgi complex, mainly in proximal compartments. By pharmacological, biochemical and site-directed mutagenesis, we observed that ST3Gal-II is mostly N-glycosylated in Asn211 and that this co-translational modification is critical for its exit from the endoplasmic reticulum and proper Golgi localization. The individual N-glycosylation sites had different effects on ST3Gal-II enzymatic activity. While the N-glycan at position Asn211 seems to negatively influence the activity of the enzyme using both glycolipid and glycoprotein as acceptor substrates, the single N-glycan mutant at position Asn92 had only a moderated effect. Lastly, we demonstrated that the N-terminal ST3Gal-II domain containing the cytosolic, transmembrane and stem region (aa 1-51) is able to drive a protein reporter out of the endoplasmic reticulum and to retain it into the Golgi complex. This suggests that the C-terminal domain of ST3Gal-II depends on N-glycosylation to attain an optimum conformation for proper exit from the endoplasmic reticulum, but it does not represent an absolute requirement for Golgi complex retention of the enzyme.