Structural And Functional Characterizations Of An Endogenous Retroviral Envelope Protein In Zebrafish

Syncytin is an endogenous retroviral envelope protein. Syncytin genes were captured by different hosts about 10-85 million years ago. After a long period of evolution, it was integrated into the host genome and expressed as endogenous functional proteins in the hosts. These proteins played key roles in the development and the immune regulation of the host. In previous studies, several syncytin proteins (such as syncytin-1 and 2 in human beings, syncytin-A and B in mouse, syncytin-Oryl in rabbit, syncytin-Carl in dog and cat, and other syncytin proteins) have already been identified, which could induce cell-cell fusion in the formation of syncytiotrophoblast in placental development or the membrane fusion during retroviral infection.Recently it was found that an intact envelope protein, which we named syncytin-ZF (SynZF), from an endougenous retrovirus in zebrafish, was mainly expressed in thymus. Through sequence alignment, we fount it shared similar structural profiles with other identified syncytins, especially in two regions:N-and C-terminal heptad repeats (NHR and CHR) predicted by computer anaylsis. To characterize SynZF potential fuogenic core structure, we expressed the region from residue 459 to 567 in SynZF including predicted NHR, CHR and a natural linker (SynZF N+C). We found SynZF N+C could exist as a trimer under physiological conditions measured by size exclusion chromatography and chemical cross-linking. It showed rich a-helix structure with very high Tm (> 90?) determined by circular dichroism (CD). At low pH, SynZF N+C was denatured indicating that SynZF N+C could form stable secondary structure at natural pH without induction by low pH. By limited proteinase K digestion experiment, we got a stable SynZF N+C core sequence containing the two short peptides from NHR and CHR respectively. The region might be party of interaction region between NHR and CHR in SynZF N+C trimer. This result was consistent with the 3-D structure predicted by molecular modeling and docking.Accoding to the structural information on SynZF in zebrafish, we attempt to set up cell fusion model induced by SynZF in vitro to understand physiological function. We also expect to design peptides accoding to NHR and CHR for inhibition of membrane fusion and hopefully could establish screening system like T-20, which may give a new way to study the molecular mechism of viral invasion into host cell and control the diseases related to membrane fusion.