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 membr ane protein specifically located in the pore membrane domain of the nu clear envelope with its short N-terminal tail exposed on the luminal s ide and its major C-terminal portion adjoining the nuclear pore comple x, In order to locate a signal for targeting of POM121 to the nuclear pores, we overexpressed selected regions of POM121 alone or fused to t he green fluorescent protein (GFP) in transiently transfected COS-1 ce lls or in a stably transfected neuroblastoma cell line. Microscopic an alysis of the GFP fluorescence or immunostaining was used to determine the intracellular distribution of the overexpressed proteins. The end ofluorescent GFP tag had no effect on the distribution of POM121, sinc e the chimerical POM121-GFP fusion protein was correctly targeted to t he nuclear ports of both COS-1 cells and neuroblastoma cells. Based on the differentiated intracellular sorting of the POM121 variants, we c onclude that the first 128 amino acids of POM121 contains signals for targeting to the continuous endoplasmic reticulum/nuclear envelope mem brane system but not specifically to the nuclear pol es and that a spe cific nuclear pore targeting signal is located between amino acids 129 and 618 in the endoplasmically exposed portion of POM121.