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

In thinking about possible mechanisms by which the TM of gp210 could function as a dominant sorting determinant, one should consider the following uncertainties. First, the gp210 mutants studied here were expressed in ceils that make their own "endogenous" gp210. We do not know the ratio of expressed (mutant) versus endogenous (wild-type) gp210 in individual pores. Second, although nuclear pores are doubled in $ phase (22) , mutant gp210 expression, over a period of 5 h, was assayed in unsynchronized, interphase cells. Thus, expressed mutant gp210 may have been involved in de novo formation of pores and/or may have gained ac- Fig. 1 ) were examined using the procedure described in Fig. 4 to visualize the chimeric proteins (mAb anti-CD8; A and B) and the pore complexes (mAb 414; C and D). All cells are recognized by mAb 414(C and D), whereas only those expressing the chimeric proteins are detected with the anti-CD8 antibody (A and B). Bar, 10 #m. cess to already formed pores by exchange with endogenous gp210. It should be noted that at present little is known about the dynamics of pore formation and NPC assembly or the stability of the interacting components. Whether involved in de novo formation of pores or in lateral exchange, we suggest that gp210's TM functions as the dominant sorting determinant because of the specific surface features of the a-helix that it forms. These features would allow specific lateral interactions with another transmembrane a-helix, either of gp210 or another integral membrane protein, to form a homodimer or heterodimer, respectively.