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

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

Puerto Varas

Congreso; CHILEAN SOCIETY FOR CELL BIOLOGY - XXIX ANNUAL MEETING; 2015

Sociedad de Biología Celular de Chile

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). Whereas Asn92 is only conserved in mammalian species, Asn211 is highly conserved in mammals, birds and fish. We studied the occupancy and relevance for intracellular trafficking and enzyme activity of these potential N-glycans in human ST3Gal-II. By confocal microscopy, pharmacological and biochemical experiments and site-directed mutagenesis we found that ST3Gal-II distributes along the Golgi complex, mainly in proximal compartments, being N-glycosylated mainly at Asn211. This co-translational modification is critical for its exiting from the endoplasmic reticulum and proper Golgi localization. The individual N-glycosylation sites had different effects on ST3Gal-II enzymatic activity. Whereas 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 Asn92 had only a moderate effect. Lastly, we demonstrated that the N-terminal ST3Gal-II domain containing the cytosolic, transmembrane and stem region (aminoacids 1-51) is able to drive a protein reporter out of the endoplasmic reticulum and to retain it in 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.