Localization of the insulin-like growth factor II binding site to amino acids 1508-1566 in repeat 11 of the mannose 6-phosphate/insulin-like growth factor II receptor.

The mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF-II receptor) binds insulin-like growth factor II (IGF-II) with high affinity. To localize the IGF-II binding site within the 15 repeating units that form the extracytoplasmic domain of the receptor, purified human M6P/IGF-II receptor was digested with thermolysin, and the fragments were analyzed for their ability to bind 125I-IGF-II in a cross-linking assay. Two IGF-II-binding receptor fragments of 23 and 37 kDa were purified. Sequence analysis revealed that the fragments consist of disulfide connected peptides comprising amino acids 1331-1566 and 1331-1697 of the receptor repeats 9-12. In a second approach we expressed truncated forms of the M6P/IGF-II receptor fused to the C terminus of the extracytoplasmic domain of the 46-kDa mannose 6-phosphate receptor. Fusion proteins containing M6P/IGF-II receptor repeats 10-15, 10-11, or 11-15 bound IGF-II, whereas a fusion protein containing the single repeat 10 failed to bind. This result indicates that repeat 11 (amino acids 1508-1650) is sufficient for binding of IGF-II. Residues 1508-1566, which are shared by the 23-kDa IGF-II-binding fragment and repeat 11, are proposed to form the IGF-II binding site of the M6P/IGF-II receptor.