Cellular topography of nucleocytoplasmic glycosylation. (A) Monoglycosylation (m) in the cytoplasm (e.g., O-GlcNAc and O-Fuc). (B) Complex (c) glycosylation in the cytoplasm by O-GlcNAc transferases (GTs) that extend the first sugar. (C) Cytoplasmic monoglycosylation mediated by bacterial toxins introduced from the outside. (D) The responsible GTs and glycoproteins may enter the nucleus via nuclear pores. (E) Potential mechanism by which nucleocytoplasmic proteins acquire secretory pathway–type glycans, involving transient entry into the rough endoplasmic reticulum (rER) or Golgi. (F) Potential mechanism by which nucleocytoplasmic proteins acquire secretory pathway–type glycans, involving movement of GTs out of the secretory pathway, shown into the nucleus, by an unknown mechanism. (G) Conventional glycosylation of soluble and membrane bound proteins (N- or O-) and lipids in the secretory pathway. Glycoproteins and glycolipids are typically fated for secretion, or transport to the plasma membrane– or organelle-like lysosomes (not shown). (H) Glycoproteins generated in the secretory pathway might become nucleocytoplasmic by endoplasmic reticulum–associated degradation (ERAD)-associated retrograde transport, but diversion from subsequent degradation. (I) A potential source of external or cell surface glycoproteins that may accumulate in the nucleus, by an unknown mechanism that might involve endocytosis and retrograde transport via the secretory pathway. (J) A pathway by which mitochondrial or chloroplast proteins receive secretory pathway type glycans, via ill-defined trafficking with the rER. (K) Glycosylation of mitochondrial or chloroplast proteins by intrinsic glycosyltransferases (topology relative to membranes not specified). (L) Example of a virally encoded protein modified by virally encoded cytoplasmic GTs, but fated to the exterior following cell lysis. (M) The synthesis of certain secretory pathway–type glycans (e.g., LLO for N-glycosylation) is initiated by cytoplasmic GTs on specific membrane lipids before they are flipped inside. (N) Polysaccharides (e.g., cellulose, hyaluronic acid) are synthesized by cytoplasmically oriented, membrane embedded GTs but coordinately translocated to the exterior. (O) In prokaryotes, many proteins are glycosylated before translocation across the membrane, rather than after as is usually the case in eukaryotes.