Isolation and sequencing of a cDNA clone encoding 85kDa sialoglycoprotein in rat liver lysosomal membranes

We used the oligonucleotide probe corresponding to the internal amino acid sequence of a lysosomal membrane glycoprotein with a molecular weight of 85 K (LGP85) and isolated and characterized cDNA clones containing the entire coding region. The isolated cDNA comprised 2065 nucleotides. The predicted amino acid sequences of LGP85 consisted of 478 amino acid residues (Mr.54,090) and the protein has 11 potential N-glycosylation sites. Since the NH2 terminal sequence determined from purified LGP85 was identical to the NH2 terminal sequence deduced from the nucleotide sequence of the cDNA, except for the lack of initiator methionine which is likely to be cleaved off posttranslationally, it is likely that LGP85 has an uncleavable signal peptide at the NH2 terminus. Hydropathy plots show that LGP85 possesses two strong hydrophobic regions at the NH2 terminus (residues 4-26) and near the COOH terminus (residues 433-457), respectively. Either one or both of the domains might be used for membrane anchoring. A comparison of the sequences of the other lysosomal membrane glycoproteins with that of LGP85 revealed no homology. Glycine-tyrosine residues (so-called GY motif) which are thought an important signal for delivery of lysosomal membrane glycoproteins to lysosomes were not contained in the cytoplasmic tail of LGP85 (residues 458-478). LGP85 appears to be an unique lysosomal membrane glycoprotein that does not require tyrosine residues for targeting to lysosomes. Tyrosine residue may not be an essential signal for delivering newly synthesized lysosomal membrane glycoproteins to lysosomes.