Intramolecular backfolding of the carboxyl-terminal end of MxA protein is a prerequisite for its oligomerization

Mx proteins are large GTPases, which play a pivotal role in the interferon type I-mediated response against viral infections. The human MxA inhibits t he replication of several RNA viruses and is organized in oligomeric struct ures. Using two different experimental approaches, the mammalian two-hybrid system and an interaction dependent nuclear translocation approach, three domains in the carboxyl-terminal moiety were identified that are involved i n the oligomerization of MxA. The first consists of a carboxyl-terminal amp hipathic helix (LZ1), which binds to a more proximal part of the same molec ule, This intramolecular backfolding is a prerequisite for the formation of an intermolecular complex, This intermolecular interaction is mediated by two domains, a poorly defined region generated by the intramolecular intera ction and a domain located between amino acids 363 and 415. Go-expression o f wild-type MxA with various mutant fragments thereof revealed that the pre sence of the carboxyl-terminal region comprising the amphipathic helices LZ 1 and LZ2 is necessary and sufficient to exert a dominant negative effect. This finding suggests that the functional interference of the carboxyl-term inal region is due to competition for binding of an as yet unidentified cel lular or viral target molecules.