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

Immunofluorescence with anti-CD8 antibody (Fig. 6 B) detected CD8/gpCT at the plasma membrane, in a densely stained region near the nucleus (presumably representing the Golgi complex), and in a punctate pattern along the nuclear rim. This punctate nuclear rim pattern of CDS/gpCT matched the nucleoporin pattern when cells were double stained with mAb 414 (Fig. 6 A) . However, the match was less than perfect, apparently obscured by the Golgi and plasma membrane localization of CD8/gpCT. Additional support for a pore membrane localization of CD8/gpCT was provided by comparing its staining pattern with that of CD8/E19, a resident integral membrane protein of the ER (13, 24, and Fig. 6 C) . CD8/E19 is a chimeric protein where the CT of CD8 has been replaced by the CT of El9, a viral protein that is retained in the ER (30) . Although both chimeric proteins showed nuclear rim staining, only CD8/E19 additionally showed the typical ER staining pattern, indicating that the nuclear rim staining of CD8/gpCT is likely the result of locaiization to the pore membrane, rather than retention in the ER. Moreover, a deletion mutant of CD8/gpCT lacking 20 COOH-terminal residues from the CT (CD8/gpCT-20, see Fig. 1 ) is no longer localized to the pore membrane, but rather is present exclusively at the plasma membrane (Fig. 7, compare A and B) . Taken together, these data suggest that the CT of gp210 contains a determinant for sorting to the pore membrane and that this determinant might reside in its COOH-terminal 20 residues.