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

U 'SING solution and solid state binding assays, isolated lamin B from turkey nuclei has previously been shown to bind to an integral membrane protein of the nuclear envelope termed the lamin B receptor (LBR) 1 or p58 (because of its apparent molecular weight in SDS-PAGE) 0Vorman et al., 1988) . Cell fractionation and immunoelectron microscopy (Yuan, J., H. J. Worman, and G. Blobel, unpublished results) revealed that LBR is exclusively localized in the inner nuclear membrane. LBR's eDNA deduced primary structure showed a protein of 637 amino acid residues with two major domains: an amino terminal hydrophilic and basic domain of 205 residues followed by a predominantly hydrophobic region with eight putative transmembrane segments . The amino terminal domain of 205 residues is likely located on the nucleoplasmic (rather than the cisternal) side of the inner membrane because it contains all of LBR's consensus sites for various kinases (Feramisco et al., 1980; Moreno and Nurse, 1990) and LBR has been shown to be a phosphoprotein (Applebaum et al., 1990) . The amino terminal nucleoplasmic domain of LBR also contains SPXX motifs that are characteristic of nucleic acid binding proteins (Suzuki, 1989) . It is not known, however, whether LBR interacts with nucleic acids.