Recent studies have shown that paramyxovirus might adopt a similar molecular mechanism of virus entry and fusion in which the attachment glycoprotein binds receptor/s and triggers the conformational changes of the fusion protein. There are two conserved regions of heptad repeat (HR1 and HR2) in the fusion protein and they were shown with fusion-inhibition effects in many paramyxoviruses, including measles virus. They also appear to show characteristic structure in the fusion core: the HR1/HR2 forms stable six-helix coiled-coil centered by HR1 and is surrounded by HR2 (trimer of HR1/HR2), which represents the post-fusion conformational structure. In this study, we expressed the HR1 and HR2 of measles virus fusion protein as a single chain (named 2-Helix) and subsequently tested its formation of trimer. Indeed, the results do show that the HR1 and HR2 interact with each other and form stable six-helix coiled-coil bundle. This is the first member in genus Morbillivirus of family Paramyxoviridae to be confirmed with this characteristic structure and provides the basis for the HR2-inhibition effects on virus fusion/entry for measles virus. (C) 2002 Elsevier Science (USA). All rights reserved.