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

copy using appropriate antibodies. The large cisternal domain of gp210 (95% of its mass) did not reveal any sorting determinants. Surprisingly, the TM of gp210 is sufficient for sorting to the pore membrane. The CT also contains a sorting determinant, but it is weaker than that of the TM. We propose specific lateral association of the transmembrane helices of two proteins to yield either a gp210 homodimer or a heterodimer of gp210 and another protein. The cytoplasmically oriented tails of these dimers may bind cooperatively to the adjacent NPCs. In addition, we demonstrate that gp210 co-localizes with cytoplasmically dispersed nucleoporins, suggesting a cytoplasmic association of these components. T hE nuclear envelope (NE) ~ consists of three morphologically and biochemically distinct domains (for review see 9, 12) . The outer nuclear membrane domain contains bound ribosomes and is continuous with and biochemically indistinguishable from the RER. The inner nuclear membrane domain is adjacent to the nuclear lamina and the underlying chromatin, and contains a distinct set of integral membrane proteins (1, 29, 36) . Finally, the outer and inner membranes of the NE are connected with each other at numerous sites forming circular openings in the NE (nuclear pores) of "~100 nm diam. The connecting membrane is sharply bent (180~ and is referred to as the "pore membrane" domain of the NE. The nuclear pore is occupied by the nuclear pore complex (NPC) and the pore membrane is adjacent to the NPC (for review see 9, 21) . The pore membrane contains at least one distinct glycoprotein, referred to as gp210 (10, 37) . This, and perhaps other yet to be identified pore membrane-specific integral proteins, may function in nuclear pore formation and in the assembly and attachment of the NPC to the pore membrane.