Phosphorylation of proteoglycans. Identification of phosphorylation sites in chondroitin sulfate‐rich region of core protein

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Abstract
Serine residues, which are the sites of phosphorylation in proteoglycans, were demonstrated to be located on chondroitin sulfate–containing peptides. These peptides appeared to be derived primarily from the chondroitin sulfate–rich region of the proteoglycan core protein. The localization of phosphate moieties in the chondroitin sulfate–containing peptides was observed in all experiments. The phosphate moieties were retained on chondroitin sulfate–containing peptides after the protein core was treated with either papain or trypsin. Two phosphopeptide preparations, derived from chondroitin sulfate–containing peptides of proteoglycan subunits, were extensively purified. These 2 phosphopeptide preparations were shown by carbohydrate analysis to be free of keratan sulfate–containing peptides or peptides from the hyaluronic acid binding region of the core protein. One of the phosphopeptide preparations had a phosphate: serine molar ratio of 0.40. This indicated that nearly one‐half of the serine residues were phosphorylated rather than glycosylated. Peptides derived from the core protein that contained keratan sulfate had no phosphate moieties.