N-Glycan synthesis in the apical and basolateral secretory pathway of epithelial MDCK cells and the influence of a glycosaminoglycan domain

Different classes of glycans are implicated as mediators of apical protein sorting in the secretory pathway of epithelial cells, but recent research indicates that sorting to the apical and basolateral surfaces may occur before completion of glycan synthesis. We have previously shown that a proteoglycan (PG) core protein can obtain different glycosaminoglycan (GAG) structures in the apical and basolateral secretory routes (Tveit H, Dick G, Skibeli V, Prydz K. 2005. A proteoglycan undergoes different modifications en route to the apical and basolateral surfaces of Madin-Darby canine kidney cells. J Biol Chem. 280:29596-29603) of epithelial Madin-Darby canine kidney (MDCK) cells. We have now also determined the detailed N-glycan structures acquired by a single glycoprotein species in the same apical and basolateral secretory pathways. For this purpose, rat growth hormone (rGH) with two N-glycan sites (rGH-2N) inserted into the rGH portion (NAS and NFT) was fused to green fluorescent protein (GFP) and expressed in MDCK cells. Immunoisolated rGH variants were analyzed for site occupancy and N-glycan structure by mass spectrometry. The extent of NAS and NFT site occupancy was different, but comparable for rGH-2N secreted apically and basolaterally. Microheterogeneity existed for the glycans attached to each N-glycan site, but no major differences were observed in the apical and basolateral pathways. Transfer of the GAG modification domain from the PG serglycin to the fusion site of rGH-2N and GFP allowed polymerization of GAG chains onto the novel protein variant and influenced the microheterogeneity of the N-glycans toward more acidic glycans, but did not alter the relative site occupancy. In conclusion, no major differences were observed for N-glycan structures obtained by the expressed model proteins in the apical and basolateral secretory pathways of epithelial MDCK cells, but insertion of a GAG attachment domain shifted the N-glycans to more acidic structures.