The neuronal ceroid lipofuscinosis CLN8 membrane protein is a resident of the endoplasmic reticulum

Progressive epilepsy with mental retardation (EPMR) is a new member of the neuronal ceroid lipofuscinoses (NCLs), The CLN8 gene underlying EPMR was re cently identified. it encodes a novel 286 amino acid transmembrane protein that contains an endoplasmic reticulum (ER)-retrieval signal (KKRP) in its C-terminus. A homozygous mutation in the orthologous mouse gene (Cln8) unde rlies the phenotype of a naturally occurring NCL model, the motor neuron de generation mouse (mnd). To characterize the product of the CLN8 gene and to determine its intracellular localization, we expressed CLN8 cDNA in BHK, H eLa and CHO cell lines. In western blotting and pulse-chase analyses an sim ilar to 33 kDa protein that does not undergo proteolytic processing steps w as detected. Using CLN8 and cell organelle specific antibodies with confoca l immunofluorescence microscopy the CLN8 protein was shown to localize in t he ER. Partial localization to the ER-Golgi intermediate compartment (ERGIC ) was also observed. The ER-ERGIC localization was not altered in the CLN8 protein representing the EPMR mutation. However, mnd mutant protein was onl y found in the ER. Mutations in the ER retrieval signal KKRP resulted in lo calization of CLN8 to the Golgi apparatus. Taken together, these data stron gly suggest that CLN8 is an ER resident protein that recycles between ER an d ERGIC.