Cell surface receptors for ecotropic XXXX retroviruses: mobile membrane proteins that mediate binding and slow endocytosis of the viral envelope glycoprotein.

@entity19 @entity406 @entity15309 @entity1778 @entity35 @entity503
The @entity15309 envelope glycoproteins of ecotropic @entity19 @entity1778 viruses bind to receptors that occur only on @entity19 and @entity35 cells and on interspecies hybrid cells that contain @entity19 chromosome 5. A substantial fraction of the @entity15309 that was bound specifically by these criteria remained undegraded and accessible to extracellular labeling reagents for many hours. Accordingly, cells with ecotropic receptors could be labeled specifically. As seen by immunofluorescence microscopy, the @entity15309 -receptor complexes were uniformly dispersed on @entity19 fibroblast plasma membranes. These complexes were mobile, and they aggregated into patches when crosslinked by antibodies at 37 degrees, but not when membrane lipid fluidity was frozen at 0 degrees. Ecotropic receptors still bound @entity15309 specifically after cells were fixed with 3.7% @entity406 , but these receptors could not be patched, indicating that they were nondiffusible. Viable cells slowly endocytosed @entity15309 -receptor complexes at 37 degrees (approximate half-life 5-7 hr) and the @entity15309 was then proteolytically degraded in lysosomes. In the presence of 20 microM @entity503 , a lysosomal inhibitor, undegraded @entity15309 was seen to slowly accumulate in these intracellular organelles. These results suggest that ecotropic receptors mediate a slow internalization of attached ligand. Long-lived binding of @entity15309 onto surfaces of uninfected cells may explain important features of viral-induced @entity1778 , the host immune response, and immunosuppression.
