Recycling cell surface glycoproteins undergo limited oligosaccharide reprocessing in LEC1 mutant Chinese hamster ovary cells

The ability of particular cell surface glycoproteins to recycle and become exposed to individual Golgi enzymes has been demonstrated. This study was d esigned to determine whether endocytic trafficking includes significant ree ntry into the overall oligosaccharide processing pathway. The Led mutant of Chinese hamster ovary (CHO) cells lack N-acetylglucosaminyl-transferase I( GlcNAc-TI) activity resulting in surface expression of incompletely process ed Man(5)GlcNAc(2) N-linked oligosaccharides. An oligosaccharide tracer was created by exoglycosylation of cell surface glycoproteins with purified po rcine GlcNAc-TI and UDP-[H-3]GlcNAc. Upon reculturing, all cell surface gly coproteins that acquired [H-3]GlcNAc were acted upon by intracellular manno sidase II, the next enzyme in the Golgi processing pathway of complex N-lin ked oligosaccharides (t(1/2) = 3-4 h), That all radiolabeled cell surface g lycoproteins were included in this endocytic pathway indicates a common int racellular compartment into which endocytosed cell surface glycoproteins re turn. Significantly, no evidence was found for continued oligosaccharide pr ocessing consistent with transit through the latter cisternae of the Golgi apparatus. These data indicate that, although recycling plasma membrane gly coproteins can be reexposed to individual Golgi-derived enzymes, significan t reentry into the overall contiguous processing pathway is not evident.