The dynamic envelope of a fusion class II virus. Prefusion stages of semliki forest virus revealed by electron cryomicroscopy.
Semliki Forest virus is among the prototypes for Class II virus fusion and targets the endosomal membrane. Fusion protein E1 and its envelope companion E2 are both anchored in the viral membrane and form an external shell with protruding spikes. In acid environments, mimicking the early endosomal milieu, surface epitopes in the virus rearrange along with exposure of the fusion loop. To visualize this transformation into a fusogenic stage, we determined the structure of the virus at gradually lower pH values. The results show that while the fusion loop is available for external interaction and the shell and stalk domains of the spike begin to deteriorate, the E1 and E2 remain in close contact in the spike head. This unexpected observation points to E1 and E2 cooperation beyond the fusion loop exposure stage and implies a more prominent role for E2 in guiding membrane close encounter than has been earlier anticipated.
Wu
Shang-Rung
SR
Department of Biosciences and Nutrition, Karolinska Institutet, S-141 57 Huddinge, Sweden. suw@biosci.ki.se
Haag
Lars
L
Hammar
Lena
L
Wu
Bomu
B
Garoff
Henrik
H
Xing
Li
L
Murata
Kazuyoshi
K
Cheng
R Holland
RH
D020285:Cryoelectron Microscopy; D006863:Hydrogen-Ion Concentration; D008561:Membrane Fusion; D015394:Molecular Structure; D044367:Phase Transition; D011487:Protein Conformation; D012672:Semliki forest virus / Q000737:chemistry* / Q000472:pathogenicity*; D014759:Viral Envelope Proteins / Q000737:chemistry* / Q000502:physiology; D014760:Viral Fusion Proteins / Q000737:chemistry* / Q000502:physiology
D014759:Viral Envelope Proteins; D014760:Viral Fusion Proteins