Partition of epidermal growth factor receptors on freeze-fractured plasma membranes of A431 cells is affected by the ligand

Freeze-fracture immunogold labeling

Several approaches have been developed to combine immunogold cytochemistry and freeze-fracture techniques. These methods are highly heterogeneous regarding both the sequence of the procedural steps and the aspect of the resulting images. They imply immunolabeling either before or after freeze-fracture or even immunolabeling of platinum/carbon replicas of the freeze-fractured membranes, and have been used alternatively or in parallel to address different questions related to cell membrane structure, composition and dynamics or to intracellular membrane traffic. This review will briefly describe these methods and report most of their immunogold cytochemical applications, with the aim of facilitating selection of the most appropriate approach.

Surface distribution and partition during freeze-fracture of CD8 antigens on human lymphocytes and on epithelial transfected cells

Freeze-fracture immunocytochemistry was used to analyse the surface distribution, redistribution induced by antibodies, and partition during freeze-fracture, of CD8 molecules on human T lymphocytes and rat epithelial transfected (FRT-U10) cells. Immunogold labelling of CD8 antigens was uniform over the unfractured cell surfaces of both lymphocytes and epithelial transfected cells. After freeze-fracture, the gold particles were associated with the exoplasmic outer leaflets of the plasma membranes in both cell types. In lymphocytes, incubation with antibodies at 37 degrees C up to 20 min induced patching and capping of the antigens on the unfractured cell surface. After fracture, the patched molecules appeared associated with the protoplasmic inner leaflet of the plasma membranes. Parallel antibody-treatment at 37 degrees C of FRT-U10 cells induced clustering of CD8 molecules but failed to cause further aggregation in larger patches or in caps. After freeze-fracture, the immunolabelling was clustered, but associated with the exoplasmic outer leaflet of the plasma membranes as in untreated cells. The different redistribution induced by antibodies and the different behaviour on fracture of the redistributed molecules in the two cell types may be regulated by CD8 interaction with the cytoskeleton.

Surface distribution and internalization of erbB-2 Proteins

We report the localization over the cell surface and the early steps of antibody-induced internalization of the product of the erbB-2 proto-oncogene, structurally related to the epidermal growth factor receptor (EGFR). We show that erbB-2/p 185 is mostly excluded from endocytic pits on the cell surface. Incubation at 37 degrees C with an anti-erbB-2/p185 monoclonal antibody induces the rapid entry of the protein into the cell. Similar internalization is shown by a chimeric molecule EGFR/erbB-2 in response to EGF. Both the timing and the pathway of internalization followed by the erbB-2/p185 appear totally similar to those described for the EGFR. At variance with the normal erbB-2/p185, two mutant activated erbB-2 proteins are frequently localized within endocytic pits of the cell surface, indicating that mutations in the transmembrane regions may determine constitutive internalization of the protein.