Activation of HIV-1 and murine leukemia virus for cell entry - Henrik Garoff
Retroviruses like HIV-1 and murine leukemia virus (MLV) enter into cells trough fusion of the viral membrane with the cell membrane. This process is mediated by the viral spike proteins.
We like to know how the spike proteins are activated for membrane fusion and how neutralizing antibodies (Abs) are able to inhibit spike activation. In our studies Ab-bound or unliganded spikes of HIV-1 and MLV are triggered by receptor binding or other conditions to convert into structural intermediates of the activation pathway. The spikes are then solubilized with mild detergents and isolated for structural analyses using biochemical methods and cryo-electron microscopy.
Recently we showed how the newly made MLV spike matures stepwise in the infected cell into a form that is primed for receptor induced activation. Using HIV-1 we have isolated and characterized spikes in their native and activated intermediate form and elucidated how two broadly neutralizing Abs inhibit the activation. The results explain how the individual protomers of the trimeric spike coordinate their activation. Furthermore, the study also suggests novel ways to produce stabile native spikes to be used as a HIV-1 vaccine.
Maturation and activation of the MLV spike. The spike is made as a precursor trimer, which matures by two cleavages. First furin cleaves the receptor binding SU subunit from the transmembrane TM with fusion activity and then, in newly made virus, the viral protease cleaves a peptide, the R-peptide, from the endodomain of TM. 3-D structures at 18 Å resolution are shown for the MLV spike in its precursor, mature and activated intermediate forms in side and top views. The protomeric unit of the trimer forms upper, middle and lower protrusions, where the upper represents the receptor binding domain (RBD) of SU, the middle the C-terminal domain of SU and the lower the TM subunit. It is shown how the lower TM protrusions are spread out by the maturation cleavage and how the spike activation opens a hole in the spike roof through relocation of the RBDs. This supports a model where the TM subunits must be released in order to refold into fusion active forms that can reach the target membrane through a hole in the spike roof.