The single ligand-binding repeat of Tva, a low density lipoprotein receptor-related protein, contains two ligand-binding surfaces

The receptor for avian sarcoma/leukosis virus subtype A (ASLV-A), Tva, is the simplest member of the low density lipoprotein receptor family containing a single ligand-binding repeat (LBR). Most LBRs contain a central Trp (Trp(33) in Tva) that is important for ligand binding and, for the low density lipoprotein receptor, is associated with familial hypercholesterolemia. The Tva ligand-binding module contains a second Trp (Trp(48)) that is part of a DEW motif present in a subset of LBRs. Trp(48) is important for ASLV-A infectivity. A soluble Tva (sTva) ligand-binding module is sufficient for ASLV-A infectivity. Tva interacts with the viral glycoprotein, and a soluble receptor-binding domain (SUA) binds sTva with picomolar affinity. We investigated whether Tva, a retroviral receptor, could behave as a classic LBR by assessing sTva interactions with the universal receptor-associated protein (RAP) and comparing these interactions with those between sTva and its viral ligand (SUA). To address the role of the two Trp residues in Tva function, we prepared sTva harboring mutations of Trp(33), Trp(48), or both and determined the binding kinetics with RAP and SUA. We found that sTva behaved as a "normal" receptor toward RAP, requiring both calcium and Trp(33) for binding. However, sTva binding to SUA required neither calcium nor Trp(33). Furthermore, sTva could bind both RAP and SUA simultaneously. These results show that the single LBR of Tva has two ligand-binding sites, raising the possibility that other LBRs may also.