Binding of urokinase-type plasminogen activator receptor (uPAR) to the mannose 6-phosphate/insulin-like growth factor II receptor - Contrasting interactions of full-length and soluble forms of uPAR

Urokinase-type plasminogen activator receptor ( uPAR) binding by the mannose 6-phosphate/insulinlike growth factor II receptor ( Man-6-P/IGF2R) is considered important to Man-6-P/IGF2R tumor suppressor function via regulation of cell surface proteolytic activity. Our goal was to map the uPAR binding site of the Man-6-P/IGF2R by analyzing the uPAR binding characteristics of a panel of minireceptors containing different regions of the Man-6-P/IGF2R extracytoplasmic domain. Coimmunoprecipitation assays revealed that soluble recombinant uPAR ( suPAR) bound the Man-6-P/IGF2R at two distinct sites, one localized to the amino-terminal end of the Man-6-P/IGF2R extracytoplasmic domain ( repeats 1 - 3) and the other to the more carboxyl-terminal end ( repeats 7 - 9). These sites correspond with the positions of the two Man-6-P binding domains of Man-6-P/ IGF2R. Indeed, the suPAR-Man-6-P/IGF2R interaction was inhibited by Man-6-P, and binding-competent suPAR species represented a minor percentage ( 8 - 30%) of the suPAR present. In contrast, Man-6-P/ IGF2R binding of endogenous, full-length uPAR solubilized from plasma membranes of the prostate cancer cell line, PC-3, was not inhibited by Man-6-P. Further studies showed that very little (< 5%) endogenous uPAR was Man-6-P/ IGF2R binding-competent. We conclude that, contrary to previous reports, the interaction between uPAR and Man-6-P/ IGF2R is a low percentage binding event and that suPAR and full-length uPAR bind the Man-6-P/ IGF2R by different mechanisms.