Removal of domain D2 or D3 of the human urokinase receptor does not affect ligand affinity

The main ligand-binding determinant of the human urokinase receptor (uPAR) is located in the amino terminal domain D1, but when isolated this domain presents a 1500 fold lower affinity than the intact three-domain uPAR (D1D2D3). uPAR mutants missing either domain 2 (D1HD3) or domain 3 (D1D2) were expressed in murine LB6 cells and showed to be properly GPI-anchored to the cell surface. Binding assays with [125I]ATF demonstrated that these mutants possessed a normal (D1D2) or slightly reduced (D1HD3) affinity, indicating that a high ligand-affinity may be achieved by a combination of D1 with domain D2 or D3.