where protein cores made in the rough endoplasmic reticulum are posttranslationally modified with O-linked glycosylations by glycosyltransferases forming proteoglycans. Keratan sulfate may modify core proteins through N-linked glycosylation or O-linked glycosylation of the proteoglycan. The fourth class of GAG, hyaluronic acid, is not synthesized by the Golgi, but rather by integral membrane synthases which immediately secrete the dynamically elongated disaccharide chain.