The cell surface receptor (ecoR) for ecotropic host-range murine retroviruses is a Na+-independent transporter for cationic amino acids that is distantly related to yeast transporters for arginine, histidine, and choline. All members of this transporter family contain a conserved glutamic acid that occurs in a hydrophobic presumptive transmembrane region of ecoR at position 107. To address the function of this site, we conservatively mutated ecoR by substituting aspartic acid at this position. The mutant protein was processed to cell surfaces where it bound the ecotropic virus envelope glycoprotein and functioned as a viral receptor, but it lacked amino acid transport activity. Functional transporter cycling is not required for viral reception by ecoR.