FIG. 4.

Infection via K234D- and K234E-expressing receptors is comparable when their cell surface expression levels are similar. Replacing lysine 234 with alanine (K234A) does not reduce infection. There is synergism between a lysine 234-to-aspartic acid change (K234D) and a tyrosine 235-to-alanine change (Y235A) but not between K234D and a glutamic acid 237-to-alanine change (E237A). New HEK 293 cells stably expressing HA-tagged single K234D and K234E mutant receptors at comparable levels were generated, as well as cell lines stably expressing receptors with a new single mutation, K234A, and two new double mutations, K234D plus Y235A and K234D plus E237A. (A) Infectious titers were determined from the end point dilution (n = 4) of cells exposed to serial 10-fold dilutions of ecotropic MLV-pseudotyped BAG virus stock. All receptors contained the HA1 epitope tag. IFU, infection-forming units. (B) Immunoblot analysis of the surface expression of mutant receptors was performed as described in the legend to Fig. 2.