The oligomerization state of the rabies virus envelope glycoprotein (G protein) was determined using electron microscopy and sedimentation analysis of detergent solubilized G. Most of the detergents used in this study solubilized G in a 4 S monomeric form. However, when CHAPS was used, G had a sedimentation coefficient of 9 S. This high sedimentation coefficient allowed its further separation from M1 and M2. Using electron microscopy of negatively stained samples, we studied the morphology of G on virus and after detergent extraction. End-on views of G on virus clearly showed triangles consisting of three dots indicating the trimeric nature of native G. End-on views of CHAPS-isolated G showed very similar triangles confirming that, using this detergent, G was solubilized in its native trimeric structure. Electron microscopy also showed that G had a "head" and a "stalk" and provided the basis for a low-resolution model of the glycoprotein structure.
A random-primed cDNA expression library constructed from the mRNA of neuroblastoma cells (NG108) was used to clone a specific rabies virus (RV) receptor. A soluble form of the RV glycoprotein (G s ) was utilized as a ligand to detect positive cells. We identified the murine low-affinity nerve-growth factor receptor, p75NTR. BSR cells stably expressing p75NTR were able to bind G s and G-expressing lepidopteran cells. The ability of the RV glycoprotein to bind p75NTR was dependent on the presence of a lysine and arginine in positions 330 and 333 respectively of antigenic site III, which is known to control virus penetration into motor and sensory neurons of adult mice. P75NTR-expressing BSR cells were permissive for a non-adapted fox RV isolate (street virus) and nerve growth factor (NGF) decreased this infection. In infected cells, p75NTR associates with the RV glycoprotein and could be precipitated with anti-G monoclonal antibodies. Therefore, p75NTR is a receptor for street RV.
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