Identification and Characterization of a Major Lysosomal Membrane Glycoprotein, LGP85/LIMP II in Mouse Liver

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Abstract
We previously have purified and characterized a major lysosomal membrane glycoprotein termed LGP85 (LIMP II) in rat liver lysosomes. In this study, LGP85 in mouse liver lysosomes was identified and characterized by biochemical and molecular biological methods. Lysosomal membranes were isolated from murine liver by differential centri-fugation. LGP85 was present in the lysosomal membrane fraction from mouse liver in a comparable amount to another lysosomal membrane glycoprotein, lamp-2. Mouse LGP85 (M-LGP85) from liver lysosomal membranes exhibited an Mr of 80,000 on SDS-PAGE, which is smaller by 5,000 than that of rat LGP85 (R-LGP85). M-LGP85 was immunochemi-cally detected in the extracts of brain, heart, lung, liver, and kidney. A cDNA encoding M-LGP85 was cloned from mouse liver cDNA library. The primary protein structure deduced from a nucleotide sequence of M-LGP85 cDNA indicated that M-LGP85 consists of 478 amino acids with Mr of 54,069. M-LGP85 showed 93.3 and 86.0% sequence similarities to its rat and human counterparts in amino acids, respectively. M-LGP85 contains 11 potential N-glycosylation sites which are heavily glycosylated, resulting in the increased Mr of M-LGP85 present in the mouse liver lysosomes. It is likely that M-LGP85 traverses the lysosomal membrane twice, with an NH2-terminal transmembrane domain, and another hydrophobic domain near the COOH-terminus. M-LGP85 has a protruding COOH-terminal cytoplasmic tail consisting of amino acid residues including the leucine-isoleu-cine sequence shown to be the lysosomal targeting signal of R-LGP85 and human LGP85 (H-LGP85). The high level of expression of M-LGP85 in the lysosomal membrane, the high structural similarities among M-, R-, and H-LGP85, and the occurrence of M-LGP85 in all the mouse tissues examined suggest the essential and constitutive function of LGP85 in lysosomes.

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