Intracellular distribution of an integral nuclear pore membrane protein fused to green fluorescent protein--localization of a targeting domain.

The 121-kDa pore membrane protein (POM121) is a bitopic integral membrane protein specifically located in the pore membrane domain of the nuclear envelope with its short N-terminal tail exposed on the luminal side and its major C-terminal portion adjoining the nuclear pore complex. In order to locate a signal for targeting of POM121 to the nuclear pores, we overexpressed selected regions of POM121 alone or fused to the green fluorescent protein (GFP) in transiently transfected COS-1 cells or in a stably transfected neuroblastoma cell line. Microscopic analysis of the GFP fluorescence or immunostaining was used to determine the intracellular distribution of the overexpressed proteins. The endofluorescent GFP tag had no effect on the distribution of POM121, since the chimerical POM121-GFP fusion protein was correctly targeted to the nuclear pores of both COS-1 cells and neuroblastoma cells. Based on the differentiated intracellular sorting of the POM121 variants, we conclude that the first 128 amino acids of POM121 contains signals for targeting to the continuous endoplasmic reticulum/nuclear envelope membrane system but not specifically to the nuclear pores and that a specific nuclear pore targeting signal is located between amino acids 129 and 618 in the endoplasmically exposed portion of POM121.