The single transmembrane segment of gp210 is sufficient for sorting to the pore membrane domain of the nuclear envelope

Most integral proteins of the membranes in the exo-and endocytotic pathway are asymmetrically integrated into the ER by signal-and stop-transfer sequence-mediated mechanisms (14, 17) . After integration into the ER membrane, sorting determinants specify retention within the ER or various downstream membranes of the exo-and endocytotic pathways (3) . For the few examples of resident integral ER proteins that have so far been examined, COOH-terminal and, generally, cytoplasmically exposed regions, have been shown to be sorting determinants for their retention in the ER (13, 24, 26, 32) . In this paper we have analyzed determinants that sort gp210 to a subdomain of the ER, the pore membrane domain of the NE. As the pore membrane is continuous with the ER, sorting to this domain is likely to proceed by lateral diffusion in the plane of the membrane rather than by vesicular traffic, gp210 is a bitopic integral membrane protein with a large NH2-terminal cisternal domain (1,783 residues), a single TM, and a short COOH-terminal CT of 58 residues. The latter is topologically poised to interact with the NPC and a priori appeared to be the most likely candidate to harbor the determinant for sorting to the pore membrane. Surprisingly, we found that the TM of gp210 is sufficient for sorting to the pore membrane. Although the CT, particularly its COOH-terminal 20 residues, also contained a determinant for sorting to the pore membrane, the TM appeared to be the dominant sorting determinant. By itself, it was sufficient for sorting to the pore membrane. The ectoplasmic domain of gp210 that contributes 95% of its mass does not appear to contain determinants for sorting to the pore membrane. It appears to be retained in the ER, but our data do not rule out secretion.