Identification of vitronectin as a novel insulin-like growth factor-II binding protein

We have previously reported the presence of a 70 kDa insulin-like growth fa ctor (IGF)-II-specific binding protein in chicken serum using Western ligan d blotting approaches. In order to ascertain the identity of this 70 kDa IG F-II binding species, the protein has been purified from chicken serum usin g a combination of ion-exchange and gel-permeation chromatography. Interest ingly, amino acid sequencing of the purified protein revealed that it has t he same N-terminal sequence as chicken vitronectin (VN). The protein has th e ability to specifically bind IGF-II and not IGF-I as determined by ligand blotting, cross-linking and competitive binding assay approaches In additi on, the protein binds I-125-des(1-6)-IGF-II, suggesting that the interactio n with IGF-II is different to those with other characterized IGF-binding pr oteins. Importantly, we have ascertained that both human and bovine VN also specifically bind IGF-II. These results are particularly relevant in the l ight of the recent report that the urokinase-type plasminogen activator rec eptor, a protein that also binds VN, has been shown to associate with the c ation-independent mannose-6-phosphate/IGF-II receptor and suggest a possibl e role for IGF-II in cell adhesion and invasion.