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

The lamin B receptor (LBR)' is a polytopic protein of the inner nuclear membrane with a basic nucleoplasmic aminoterminal domain and a hydrophobic domain that contains eight putative transmembrane segments . LBR is synthesized without a cleavable aminoterminal signal sequence, and like type II integral membrane proteins of the ER/plasma membrane that contain uncleaved internal signal sequences, its amino-terminal domain faces the contralumenal or nucleo-cytoplasmic side of the mem-brane . To identify a nuclear envelope targeting signal of LBR, we have transfected cells with plasmids that transiently express different domains and chimeric constructs of this protein.