The present invention relates to a method of preparing a trimeric protein comprising culturing a host cell transformed or transfected with an expression vector encoding a fusion protein comprising a ZymoZipper (ZZ) domain and a heterologous protein. In one embodiment, the heterologous protein is a membrane protein, the portion of the heterologous protein that included in the fusion protein is the extracellular domain of that protein, and the resulting fusion protein is soluble. In another embodiment of the present invention, the ZZ domain is derived from the transmembrane (TM) subunit of a virus envelope protein or another heptad repeat containing gene of a virus genome. The method can be used to produced homo- and hetero-trimeric proteins. The present invention also encompasses DNA molecules, expression vectors, and host cells used in the present method and fusion proteins produced by the present method.