Posttranslational Processing of Human α2‐HS Glycoprotein (Human Fetuin)

α2‐HS glycoprotein (α2‐HS) is a major protein occuring in human blood and calciferous tissues. Due to extensive sequence identity, α2‐HS has been grouped with the fetuins, a family of proteins that occur in fetal plasma in high concentrations. Native α2‐HS undergoes a series of posttranslational modifications including proteolytic processing, multiple N‐glycosylations and O‐glycosylations, and sulfation of the carbohydrate side chains. Various two‐chain forms of α2‐HS have been prepared from human plasma, however, the single‐chain precursor has not yet been isolated. Here, we have studied the biosynthesis of α2‐HS by a human hepatoma cell line, HepG2. We demonstrate that a single‐chain form and the two‐chain form of α2‐HS are secreted by this cell line. The α2‐HS forms are further modified by phosporylation on multiple serine residues. Mapping studies indicate that the connecting peptide region releasable from the heavy chain of α2‐HS contains at least one such Phosphorylation site. Our results identify proteolytic trimming and/or phosphorylation as modifications possibly regulating the biological effects of α2‐HS and the homologous fetuins.