Topology and Membrane Anchoring of the Lysosomal Storage Disease‐Related Protein CLN 5

One late infantile variant of the neurodegenerative disease neuronal ceroid lipofuscinosis (NCL) is caused by a mutation in the CLN 5 gene. CLN 5 encodes a lysosomal glycoprotein whose structure and function have not yet been clearly defined. In the present study, we used epitope‐tagged CLN 5 to determine the topology and solubility of the CLN 5 protein. Our results indicated that CLN 5 is synthesized as a type II transmembrane ( TM ) glycoprotein with a cytoplasmic N ‐terminus, one TM segment, and a large luminal C‐terminal domain containing an amphipathic helix ( AH ). The cytoplasmic and TM domains were rapidly removed following signal‐peptide cleavage, and the resulting mature CLN 5 was tightly associated with the lumen of the membrane through the AH . CLN 5 pathological mutants deprived of AH lose their membrane association, are retained in the endoplasmic reticulum, and are rapidly degraded by the proteasomal machinery. We experimentally define the topology of CLN 5 and demonstrate the existence of an AH that anchors the protein to the membrane. Our work sheds light on the basic properties of CLN 5 required to better understand its biological functions and involvement in NCL pathogenesis.