N-Linked glycosylation and sialylation of the acid-labile subunit. Role in complex formation with insulin-like growth factor (IGF)-binding protein-3 and 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) and the 84-86-kDa acid-labile subunit (ALS), a glycoprotein containing 20 kDa of carbohydrate. The ternary complexes regulate IGF availability to the tissues. Since interactions of glycoproteins can be influenced by their glycan moieties, this study aimed to determine the role of ALS glycosylation in ternary complex formation. Complete deglycosylation abolished the ability of ALS to associate with IGFBP-3. To examine this further, seven recombinant ALS mutants each lacking one of the seven glycan attachment sites were expressed in CHO cells. All the mutants bound IGFBP-3, demonstrating that this interaction is not dependent on any single glycan chain. Enzymatic desialylation of ALS caused a shift in isoelectric point from 4.5 toward 7, demonstrating a substantial contribution of anionic charge by sialic acid. Ionic interactions are known to be involved in the association between ALS and IGFBP-3. Desialylation reduced the affinity of ALS for IGFBP-3. IGF complexes by 50-80%. Since serum protein glycosylation is often modified in disease states, the dependence of IGF ternary complex formation on the glycosylation state of ALS suggests a novel mechanism for regulation of IGF bioavailability.