Diverse Enveloped Viruses Fail to Undergo Complete Fusion with the Cell Plasma Membrane

Abstract

Recent work from our laboratory (Miyauchi et al. 2009, Cell 137:433-444) provided functional evidence for HIV-1 entry via an endocytic pathway. In contrast, HIV-1 fusion with the plasma membrane (PM) did not proceed beyond the lipid mixing step. We have therefore assessed the ability of other enveloped viruses to fuse with the PM of permissive cells. Imaging of single amphotropic Murine Leukemia Virus particles revealed complete endoplasmic fusion with occasional lipid mixing at the PM. Next, we tested whether low pH-dependent viruses can be forced to fuse with the PM by lowering the external pH. When retroviral cores pseudotyped with the Semliki Forest Virus E1E2 glycoproteins were bound to cells and exposed to acidic pH, lipid mixing, but not viral content transfer was observed. Likewise, Avian Sarcoma and Leukosis Virus (ASLV) that enters target cells via a receptor-mediated endocytosis followed by low pH-induced fusion with endosomes failed to release its content at the PM upon reducing the pH. These results demonstrate a surprisingly strong preference for endosomal fusion among pH-dependent and pH-independent viruses, suggesting the existence of a fusion block at the PM and/or the involvement of an endosomal factor(s) facilitating the viral fusion. Consistent with the latter mechanism, inhibition of the dynamin function blocked endosomal fusion of HIV-1, ASLV and several other viruses. Together, these results are consistent with the involvement of cellular factors in facilitating the viral entry via endosomes. Supported by NIH grants GM054787 and AI053668.