The first membrane spanning region of the lamin B receptor is sufficient for sorting to the inner nuclear membrane

So far, the data have localized LBR's sorting determinant to the first transmembrane region. To test whether this region is sufficient we made a chimeric construct in which the bacterial protein, ~galactosidase, replacing the nucleoplasmic domain, was linked to LBR's first transmembrane region comprising residues 201-246 of LBR (Fig. 6) . Immunolocalization of this protein 05gal-TM1) yielded nuclear rim staining ( Fig. 6 A) . In addition, there was accumulation in a distinct region in the cytoplasm, which did not colocalize with the Golgi complex or the RER (based upon double immunofluorescence using wheat germ agglutinin or anti-SSRo~ antibody, respectively), data not shown. The additional cytoplasmic staining may reflect an accumulation of unassembled or aggregated fusion protein (Rizzolo et al., 1985; Hobman et al., 1992) . The nuclear rim staining suggests that the first transmembrane region of LBR is sufficient for sorting/3-galactosidase to the inner nuclear membrane.