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

Some integral membrane proteins synthesized on the ER are targeted to the various membrane domains of the nuclear envelope. The nuclear envelope is composed of three distinct membrane domains: the inner nuclear membrane, the outer nuclear membrane and the nuclear pore membranes (Franke et al., 1981; Gerace and Blobel, 1982) . These three membrane domains are interconnected, but unique integral membrane proteins are found in each that are not found in the others Worman et al., 1988 Worman et al., , 1990 Senior and Gerace, 1988; Harel et al., 1989; Courvalin et al., 1990) . Proteins of the outer nuclear membrane are shared with the continuous ER membrane (Amar-Costesec et al., 1974) and outer nuclear membrane proteins have not been identified that are not also found in the ER. As the ER, outer, pore, and inner nuclear membrane domains are continuous, integral membrane proteins synthesized on the ER can reach all of the nuclear envelope membrane domains by diffusion in the proteolipid bilayer. The signals that target integral membrane proteins to their specific nuclear envelope membrane domains are not known.