Semliki Forest virus (SFV) membrane fusion is mediated by the viral E1 protein at acidic pH and regulated by the dimeric interaction of E1 with the E2 membrane protein. During low pH-triggered fusion, the E2/E1 heterodimer dissociates, freeing E1 to drive membrane fusion. E2 is synthesized as a precursor, p62, which is processed to mature E2 by the cellular protease furin. Both the dissociation of the p62/E1 dimer and the fusion reaction of p62 virus have a more acidic pH threshold than that of the mature E2 virus. We have previously isolated SFV mutations that allow virus growth in furin-deficient cells. Here we have used such pci mutations to compare the interactions of the p62/E1 and E2/E1 dimers. Our data suggest that there is an important p62/E1 dimer interaction site identified by an E2 R250G mutation and that this interaction is maintained after processing to the mature E2 protein.