Purification and characterization of a soluble form of lysosome-associated membrane glycoprotein-2 (lamp-2) from rat liver lysosomal contents.

Lysosomal membrane of rat liver contains a highly glycosylated protein referred to as lamp-2. Lamp-2 occurs to a significant extent in a soluble fraction of rat liver lysosomes. The soluble form of lamp-2 (SF-lamp-2) was purified to electrophoretic homogeneity. An apparent molecular weight M(r) of SF-lamp-2 on sodium dodecy sulfate-polyacrylamide gel electrophoresis was determined to be 91,000 which is 5,000 less than that of the membranous form of lamp-2 (MF-lamp-2). SF- and MF-lamp-2 were very similar to each other in terms of sialic acid content, NH2-terminal amino acid sequence and isoelectric point. Gel filtration data indicated that native SF-lamp-2 has an M(r) = 360,000. Taken together, SF-lamp-2 forms a tetrameric structure consisting of a homogenous polypeptide lacking a membrane-spanning domain and a cytoplasmic tail near the COOH-terminus.