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

Because COS-7 cells contain endogenous LBR, antibodies were needed for transfection experiments that recognize chicken but not mammalian LBR. For this purpose, antibodies were raised against a synthetic polypeptide corresponding to amino acids 59-71 of chicken LBR. Proteins of COS-7 cell lysates and chicken erythrocyte nuclear envelopes were separated by SDS-PAGE (Fig. 1 a) and on immunoblots of these separated proteins, the anti-polypeptide antibodies recognized chicken LBR but did not recognize any proteins in COS-7 cell lysates (Fig. 1 b) . These antibodies also did not recognize proteins in nuclear envelope preparations from rat liver (data not shown) and did not give nuclear rim fluorescence in immunofluorescence microscopy of cultures of untransfected COS-7 cells (data not shown) or in numerous cells in transfection experiments that did not likely take up plasmid DNA (for example see Fig. 5 ). Since these antibodies did not apparently recognize mammalian LBR or stain the nuclear envelopes of COS-7 cells, they were suitable to follow the expression of the chicken protein in transfection experiments.