THE BOVINE MANNOSE 6-PHOSPHATE INSULIN-LIKE GROWTH FACTOR-II RECEPTOR- LOCALIZATION OF THE INSULIN-LIKE GROWTH FACTOR-II BINDING-SITE TO DOMAINS 5-11

The mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IG F-II receptor) binds two distinct ligands, mannose 6-phosphate (Man-6- P) and insulin-like growth factor II (IGF-II). The extracytoplasmic re gion of the receptor is composed of 15 homologous repeating domains an d domains 1-3 and 7-9 have been shown to contain the two Man-6-P bindi ng sites. To determine the location of the single IGF-II binding site, truncated forms of the M6P/IGF-II receptor were expressed transiently in COS-1 cells and assayed for their ability to bind iodinated human recombinant IGF-II. The binding of [I-125]IGF-II to the receptors in t he presence or absence of excess unlabeled IGF-II, IGF-I, or insulin w as determined by incubation with homobifunctional cross-linking agents followed by SDS-polyacrylamide gel electrophoresis. These binding stu dies demonstrated that a construct encoding domains 5-11 bound 0.9 mol of IGF-II/mol of receptor, whereas a construct encoding domains 5-10 exhibited no detectable binding to IGF-II. These results indicate that the IGF-II binding site of the M6P/IGF-II receptor is contained withi n domains 5-11 and that residues in domain 11 play an important role i n IGF-II binding.