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

As the NH2-terminal ectoplasmic domain of gp210 is not localized to the pore membrane, the topogenic determinants for pore membrane localization of gp210 must reside either in its TM, or in its CT, or in both. To distinguish between these possibilities, we constructed a eDNA coding for a chimeric protein, referred to as CD8/gp(TM + CT) (see Fig. 1 ) that consists of the ectoplasmic domain of CD8, used as a reporter, and the TM plus the CT of gp210. CD8 is an integral protein of the plasma membrane present in a subset of human T lymphocytes (27) . Like gp210, CD8 is a bitopic integral membrane protein. Its NH2-terminal ectoplasmic domain (161 residues) is followed by a TM and a CT (28 residues) (18, 31) . After signal-and stop-transfer sequencemediated integration into the RER, CD8 ends up, via the exocytotic pathway, in the plasma membrane. If the TM and the CT (TM + CT) portions of gp210 contain the determinants for sorting to the pore membrane, the chimera CD8/ gp(TM + CT) should be localized to the pore membrane. Indeed, staining of cells expressing this fusion protein with an anti-CD8 antibody showed a characteristic punctate pore