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

The HA-tagged protein could also be detected using anti-p58 antibodies (Fig. 3 B,/) directed against a synthetic peptide (amino acid residues 2 to 17) of chick LBR. The antipeptide antibodies did not react with the endogenous COS cell LBR in immunofluorescenee microscopy or on immnnoblots (data not shown) and thus, HA tagging was not necessary to distinguish foreign from endogenous LBR. However, for unknown reasons LBR constructs lacking the HA tag, while producing a typical rim staining pattern, were expressed at low frequency and at very low levels (data not shown). COS cells do synthesize their own LBR that can be readily detected using a previously characterized human autoimmune serum (Courvalin et al., 1990) (Fig. 3 C) . Again, a characteristic nuclear rim staining was obtained (Fig. 3 C) indistinguishable from that of HA-tagged chick LBR (Fig. 3, A and B ).