The Golgi apparatus is also where O-glycans are built. In mammals, their synthesis starts by the addition of a GalNAc residue, and is extended step by step by the addition of other monosaccharides (1, Fig. 1). Other types of O-glycans are attached in the ER, for instance O-fucose and O-glucose residues are transferred to consensus cysteines in certain proteins, a modification which is essential for protein interaction and signal transduction (2).
Glycosylation is not limited to the secretory pathway, nuclear and cytosolic proteins can be modified with O-GlcNAc at serine or threonine residues. This is a highly dynamic modification with an important role in signaling: the single sugar is attached and removed in cycles, modulating protein function much like phosphorylation (3) (Fig. 2). In fact in some cases O-GlcNAc competes directly with phosphate for occupancy of serine and threonine residues on the protein.