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

To investigate whether downstream transmembrane regions could substitute for the sorting determinant in the first transmembrane region we made an internally truncated LBR construct in which residues 206 to 279 were deleted. This construct (p58ATMl+2) was deliberately designed to lack transmembrane regions 1 and 2, rather than just transmembrane region 1, because it would be expected to yield a similar membrane orientation as full-length LBR. Immunolocalization showed a diffuse staining throughout the cytoplasm, Figure 7 . Transmembrane regions 3 to 8 do not contain a sorting signal. COS-1 cells transfected with p58zxTMl+2 were fixed and permeabilized 40 h after transfection and processed for indirect immtmofluorescence microscopy by staining with rabbit anti-p58 antibodies, followed by anti-rabbit IgG-FITC. Bar, 10 ~m. but no exclusive nuclear rim staining (Fig. 7) . These data support the assignment of LBR's first transmembrane region as a determinant for sorting to the inner nuclear membrane and further demonstrate that the nucleoplasmic domain, per se, lacks determinants for sorting to the nuclear rim (see above).