The 44-amino-acid E5 protein of bovine papillomavirus is a dimeric transmembrane protein that exists in a stable complex with the platelet-derived growth factor (PDGF) β receptor, causing receptor activation and cell transformation. The transmembrane domain of the PDGF β receptor is required for complex formation, but it is not known if the two proteins contact one another directly. Here, we studied a PDGF β receptor mutant containing a leucine-to-isoleucine substitution in its transmembrane domain, which prevents complex formation with the wild-type E5 protein in mouse BaF3 cells and inhibits receptor activation by the E5 protein. We selected E5 mutants containing either a small deletion or multiple substitution mutations that restored binding to the mutant PDGF β receptor, resulting in receptor activation and growth factor independence. These E5 mutants displayed lower activity with PDGF β receptor mutants containing other transmembrane substitutions in the vicinity of the original mutation, and one of them cooperated with a receptor mutant containing a distal mutation in the juxtamembrane domain. These results provide strong genetic evidence that the transmembrane domains of the E5 protein and the PDGF β receptor contact one another directly. They also demonstrate that different mutations in the E5 protein allow it to tolerate the same mutation in the PDGF β receptor transmembrane domain and that a mutation in the E5 protein can allow it to tolerate different mutations in the PDGF β receptor. Thus, the rules governing direct interactions between transmembrane helices are complex and not restricted to local interactions.