The large C-terminal region of the integral pore membrane protein, POM121, is facing the nuclear pore complex.

POM121 is an integral membrane protein that has been specifically localized to the "pore membrane" domain of the nuclear envelope. Based on its cDNA-deduced primary structure it was suggested that POM121 contains one or two transmembrane segments and that its major C-terminal portion faces the pore side rather than the cisternal side of the pore membrane. We have investigated the membrane topology of POM121 by studying the accessibility of a C-terminal and an N-terminal epitope of POM121 for epitope-specific antibodies. The accessibility of POM121 in unfixed, semi-intact or permeabilized tissue culture cells was analyzed by indirect immunofluorescence. We found that the C-terminal epitope was accessible for antibodies in both semi-intact and permeabilized cells, whereas the N-terminal epitope was only accessible in the permeabilized cells. The results show that the large C-terminal region of POM121, containing more than 90% of its total mass, is exposed on the pore side of the nuclear membrane and suggest that the N-terminal portion is most likely localized in the perinuclear space. The data also show that at least part of the C-terminal epitopes are localized on the cytoplasmic side of the nuclear envelope. The topology suggests that the C-terminal portion of POM121, which contains a nucleoporin-like domain, interacts with the nuclear pore complex and thus, may play a role in biogenesis of the nuclear envelope and the nuclear pore complex.