The amino-terminal domain of the lamin B receptor is a nuclear envelope targeting signal

In unfixed, digitonin-permeabilized cells, protein antigens Figure 6 . Accessibility of LBR, LBR-chicken hepatic lectin, and c~-globin-LBR to antibodies in digitonin-perrneabilized cells. COS-'/ ceils transfected with LMBR (a), LMBR-CHL (b), and AG-LMBR (c) are shown. Upper panels show representative immunofluorescence micrographs of fixed cells permeabilized with Triton X-100. Bottom panels show representative immunofluorescence micrographs for similarly transfected cells that were permeabilized with digitonin, not fixed and not exposed to Triton X-100. Antibodies against LBR were used in a and b and antibodies against c~-globin in c. Bar, 20 ~m. located in the nuclear pore complexes and cytoplasm are recognized by antibodies in immunofluorescence microscopy, whereas antigens located on the inner side of the nuclear envelope are not accessible to the antibodies (Adam et al., 1990) . COS-7 cells were transfected with plasmids LMBR, LMBR-CHL, and AG-LMBR, respectively, and the transfected cells were split into two petri dishes. Cells in one petri dish of each pair were prepared for immunofluorescence microscopy in standard fashion with fixation and exposure to Triton X-100 that permits antibodies access to intranuclear antigens. The other half of the cells were not fixed, were permeabilized with digitonin and prepared for immunofluorescence microscopy in the absence of Triton X-100. For cells transfected with LMBR (Fig. 6 a) and LMBR-CHL (Fig. 6 b) that were fixed and treated with Triton X-100 (upper panels), rim fluorescence was observed in transfected cells.