N-linked glycosylation and sialylation of the acid-labile subunit - Role in complex formation with insulin-like growth factor (IGF)-binding protein-3and the IGFs

Over 75% of the circulating insulin-like growth factors (IGF-I and -II) are bound in 140-kDa ternary complexes with IGF-binding protein-3 (IGFBP-3) an d the 84-86-kDa acid-labile subunit (ALS), a glycoprotein containing 20 kDa of carbohydrate. The ternary complexes regulate IGF availability to the ti ssues. Since interactions of glycoproteins can be influenced by their glyca n moieties, this study aimed to determine the role of ALS glycosylation in ternary complex formation. Complete deglycosylation abolished the ability o f ALS to associate with IGFBP-3. To examine this further, seven recombinant ALS mutants each lacking one of the seven glycan attachment sites were exp ressed in CRO cells. All the mutants bound IGFBP-3, demonstrating that this interaction is not dependent on any single glycan chain. Enzymatic desialy lation of ALS caused a shift in isoelectric point from 4.5 toward 7, demons trating a substantial contribution of anionic charge by sialic acid. Ionic interactions are known to be involved in the association between ALS and IG FBP-3. Desialylation reduced the affinity of ALS for IGFBP-3 IGF complexes by 50-80%. Since serum protein glycosylation is often modified in disease s tates, the dependence of IGF ternary complex formation on the glycosylation state of ALS suggests a novel mechanism for regulation of IGF bioavailabil ity.