IMPORTANCE OF CYSTEINES IN THE LDLR-RELATED DOMAIN OF THE SUBGROUP-A AVIAN-LEUKOSIS AND SARCOMA-VIRUS RECEPTOR FOR VIRAL ENTRY

The extracellular domain of the subgroup A avian sarcoma and leukemia virus (ALSV-A) receptor contains a region that is related in sequence to the ligand-binding motifs of the tow-density lipoprotein receptor ( LDLR). This domain contains six cysteines that are highly conserved be tween different members of the LDLR protein superfamily, and these res idues are presumed to participate in intrachain disulfide bonds. To as sess the importance of each cysteine in the ALSV-A receptor, individua l or multiple cysteines were mutated to alanines and the altered recep tors were tested for the ability to confer susceptibility to viral inf ection. Receptors bearing single mutations allowed subgroup A viral en try, albeit at less than wild-type levels. Receptors containing two or three substitutions were completely inactive if one of the changed re sidues was Cys-35 or Cys-50. Of the altered receptors tested, the only exception to this rule was a functional receptor which lacked both Cy s-35 and Cys-50, an activity that was dependent on the presence of oth er cysteines in this protein. Most interestingly, a receptor containin g both Cys-35 and Cys-50 but lacking the other four cysteines was comp letely functional. These results demonstrate the importance of Cys-35 and Cys-50 for viral entry mediated by the ALSV-A receptor and show th at in the presence of these two residues, all of the other cysteines i n this protein can be removed without loss of this function.