FIG. 4.

Infection assays in the absence of viral receptors. Results of virus-cell fusion assays on CHO target cells, which do not express the mCAT1 and PiT2 receptors (black bars) and were engineered to express receptors for the trans-acting RBD only (grey bars) or both the mCAT1 and PiT2 receptors (white bars). That the endogenous CHO alleles of these receptors were not able to interact with the viral particles was demonstrated by the absence of detectable binding with tagged ecotropic or amphotropic RBD polypeptides and also with the indicated panel of chimeric Env glycoproteins in binding assays performed as described previously (33). Infectivity is expressed as the number of LacZ infectious units per milliliter of viral supernatant. For each type of glycoprotein, infections were performed in the presence of ecotropic RBD polypeptides except for the MO, MOdelH, BDPROMO, and BDPROMOdelH glycoproteins, for which amphotropic RBD polypeptides were used. Infection performed with delH-mutated RBD polypeptides did not enhance the infectivity of virions (data not shown). The values show the means ± standard deviations of up to four independent experiments.