| Insulin like growth factor ( IGF ) binding protein 3 ( IGFBP 3 ) is known to be secreted as a phosphoprotein , constitutively phosphorylated at casein kinase 2 ( CK 2 ) sites . ^^^ To examine the effect of phosphorylation by CK 2 on the properties of glycosylated human IGFBP 3 , we phosphorylated plasma derived IGFBP 3 , containing less than 1 mol / mol phosphoserine , in vitro . ^^^ Relative resistance to proteolysis and poor binding to cells suggest that CK 2 phospho IGFBP 3 may be a significant inhibitor of IGF activity in the extracellular environment . . ^^^ |