Formation of nuclear bodies in cells overexpressing the nuclear pore protein POM121.

POM121 is an integral membrane protein specifically localized in the pore membrane domain of the nuclear envelope. We have investigated the intracellular distribution of rat POM121 heterologously overexpressed in monkey COS cells by immunofluorescence and fluorescence digital imaging microscopy. At low levels of expression overexpressed POM121 was distributed in the nuclear envelope in a punctate fashion, partially overlapping with the distribution of nuclear pores. At high levels of expression, however, the overexpressed protein accumulated in intranuclear bodies. These bodies represent a novel subnuclear structure, displaying a defined cylindrical structure and a distinct localization at or adjacent to the inner nuclear membrane. The C-terminal portion of POM121, which contains a pentapeptide repeat domain common to a subfamily of related nucleoporins, was sufficient to mediate targeting to the nuclear envelope as well as formation of intranuclear bodies.