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

Constructs in plasmid pSVK3 were designed to transiently express in COS-7 cells the proteins shown in Fig. 2 . LMBR expressed full-length chicken LBR, a polytopic protein of the inner nuclear membrane that is synthesized without a cleavable amino-terminal signal sequence and has a nucleoplasmie amino-terminal domain followed by a hydrophobic domain with eight putative transmembrane segments that can likely function as uncleaved internal signal sequences . AG expressed chimpanzee a-globin, a 142-amino acid polypeptide that is a subunit of the cytoplasmic protein hemoglobin (Liebhaber and Begley, 1984) . CHL expressed chicken hepatic lectin, a type II integral membrane protein of the ER/plasma membrane that is synthesized without a cleavable signal sequence and has an amino terminus that faces the cytoplasm, one transmembrane segment that likely functions as an uncleaved internal signal sequence and carboxy terminus that faces the ER lumen/extracellular milieu (Chiacchia and Drickamer, 1984; Mellow et al., 1988) . AT expressed the amino-terminal domain of LBR from the initiation methionine to the arginine that immediately precedes the first putative transmembrane segment. CHL-LMBR expressed a chimeric protein that contained the first 191 amino acids of LBR followed by the transmembrane domain and a portion of the carboxyterminal domain (up to amino acid 131) of chicken hepatic lectin. AG-LMBR expressed a chimeric protein in which all 142 amino acids of chimpanzee ot-globin replaced the first 191 amino-terminal amino acids of LBR.