The gp70 envelope glycoproteins of ecotropic murine leukemia viruses bind to receptors that occur only on mouse and rat cells and on interspecies hybrid cells that contain mouse chromosome 5. A substantial fraction of the gp70 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 gp70-receptor complexes were uniformly dispersed on mouse fibroblast plasma membranes. These complexes were mobile, and they aggregated into patches when crosslinked by antibodies at 37°, but not when membrane lipid fluidity was frozen at 0°. Ecotropic receptors still bound gp70 specifically after cells were fixed with 3.7% formaldehyde, but these receptors could not be patched, indicating that they were nondiffusible. Viable cells slowly endocytosed gp70-receptor complexes at 37° (approximate half-life 5-7 hr) and the gp70 was then proteolytically degraded in lysosomes. In the presence of 20 μM chloroquine, a lysosomal inhibitor, undegraded gp70 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 gp70 onto surfaces of uninfected cells may explain important features of viral-induced leukemia, the host immune response, and immunosuppression.
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