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

To determine the immunofluorescence patterns for fulllength LBR, chicken hepatic lectin and a-globin, three proteins of known cellular localization, COS-7 cells were transfected with plasmids LMBR, CHL, and AG. LBR was detected only in the nuclear envelope of transfected cells as demonstrated by rim fluorescence of the nucleus (Fig. 3 a) . A tim-staining fluorescence pattern is characteristic of extrinsic and integral proteins of the inner nuclear membrane (Gerace et al., 1978; Worrnan et al., 1988; Senior and Gerace, 1988; Courvalin et al., 1990 ). a-globin was detected in the cytoplasm of cells transfected with plasmid AG (Fig. 3 b) . The fluorescence pattern of discrete dots throughout the AG-LMBR ::::::::::::::::::::u, :::::::::::::::::::::::: cytoplasm may result from precipitation or aggregation of the expressed polypeptide. Cells expressing chicken hepatic lectin demonstrated fluorescent labeling of the ER and nuclear envelope (Fig. 3 c) , a pattern that has been previously reported for its expression in transfected fibroblasts (Mellow et al., 1988) and is expected for a protein of the ER and outer nuclear membrane. COS-7 cells were transfected with plasmid AT to determine if the nucleoplasmic amino-terminal domain of LBR lacking transmembrane segments would be transported to the nucleus. This polypeptide was detected only in the nucleus of transfected cells (Fig. 4) . A diffuse fluorescence labeling of the entire nucleus, similar to that seen with the DNA-binding dye DAPI, was observed. The fluorescence intensity of the nucleolus was relatively decreased indicating that the polypeptide is excluded from this organelle. After transfection and growth in culture for up to 72 h, only nuclear fluorescence was observed without significant cytoplasmic labeling or disruption of nuclear architecture. These experiments demonstrate that the LBR amino-terminal domain is transported to the nucleus after synthesis in the cytoplasm.