Lysosomal targeting of Limp II membrane glycoprotein requires a novel Leu-Ile motif at a particular position in its cytoplasmic tail

Abstract

A class of integral membrane glycoproteins specific to lysosomes has been identified, and they are classified into two separate groups depending on whether or not their cytoplasmic sequence contains a tyrosine residue. Lamp-1 and lamp-2 have a tyrosine-containing motif in their cytoplasmic segments, and this motif was found to direct the glycoproteins to lysosomes. Limp II glycoprotein, on the other hand, lacks a tyrosine in its cytoplasmic segment and it must be directed to lysosomes by a different signal (Fukuda, M. (1991) J. Biol. Chem. 266, 21327-21330). In order to elucidate the targeting signal of Limp II, a cDNA encoding its cytoplasmic segment was fused with a reporter molecule, a chimeric protein of human gonadotropin alpha chain-vesicular stomatitis G-protein transmembrane. After various mutations its expression was examined by immunofluorescence. First it was shown that a chimeric protein with a Limp II wild-type tail is transported to lysosomes. Deletion of the three amino acids of the cytoplasmic tail at the carboxyl terminus abolished this sorting to lysosomes. Substitution of individual amino acids revealed that the Leu-Ile motif in the Leu-Ile-Arg-Thr sequence at the carboxyl terminus is crucial to the sorting signal. When this motif was brought closer to the transmembrane domain by deletion of nine amino acids next to the transmembrane domain, this sorting function was abolished. In addition, substitution of alanine for the serine, which is at 5 residues from the transmembrane also abolished the sorting capacity, although there was no evidence that the phosphorylation of serine is involved in sorting. Altered proteins that were not transported to lysosomes were found to accumulate at the cell surface and, unlike proteins with a wild-type cytoplasmic tail, were unable to undergo endocytosis. These results indicate that the carboxyl-terminal amino acid sequence, including the Leu-Ile motif and the sequence that connects the motif to the transmembrane domain, is critical for the sorting of Limp II to lysosomes.