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

As the nueleoplasmic domain was a prime candidate for being the determinant for sorting to the inner nuclear membrane, we made two carboxy-terminaUy truncated LBR constructs. One of these was truncated at residue 246. It contained the nucleoplasmic domain, the first transmembrane segment and 20 adjacent residues but lacked the remaining seven transmembrane segments and we therefore refer to it as p58ATM2-8 (see cartoon in Fig. 4,4) . Expression Figure 5 . The amino terminal domain of LBR is not required for targeting to the nuclear envelope. COS-I cells transfected with p58zxN (A) or Ru-p58 (B) were fixed and permeabilized 40 h (A) or 24 h (B) after transfection. Cells were processed for indirect immunofluorescence microscopy by staining with rabbit anti-p58 antibodies (A) or rabbit antibodies against the small subunit of RuBPCase (B), followed by anti-rabbit IgG-FITC. Bar, 10/~m. in COS cells yielded the characteristic nuclear rim staining (Fig. 4 A) indistinguishable from that of the full-length LBR (Fig. 3, A and B) . These data indicated that the carboxyterminal part of LBR including seven of its eight transmembrahe segments are not required for sorting to the inner nuclear membrane.