Characterization of Human Lysosomal Membrane Glycoproteins and Evidence for their Differentiation-Related Expression in the Plasma Membrane of Myelomonocytic Leukemia Cells
JOHNS HOPKINS UNIV BALTIMORE MD SCHOOLOF MEDICINE
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Two human cell lysosomal membrane glycoproteins of approx. 120-kDa, hLAMP-1 and hLAMP-2, were identified by use of monoclonal antibodies prepared against U937 myelomonocytic leukemia cells or blood mononuclear cells. The two glycoproteins were purified by antibody affinity chromatography and each was found to a be a major constituent of human spleen cells, representing approx. 0. 05 of the total detergent-extractable protein. Both molecules were highly glycosylated, being synthesized as polypeptides of 40 to 45 kDa and cotranslationally modified by the addition of Asn-linked oligosaccharides. Amino-terminal sequence analysis indicated that each was approx. 50 identical to the corresponding mLAMP-1 or mLAMP-2 of mouse cells. Electron microscopic studies of human blood monocytes, HL-60, and U937 cells demonstrated that the principal location of these glycoproteins was intracellular, in vacuoles and lysosomal structures but not in the peroxidase-positive granules of monocytes. Transport of the proteins between organelles was evidenced by their marked accumulation in the membranes of phagolysosomes. A fraction of each glycoprotein was also detected on the plasma membrane of U937 and HL-60 cells but not on a variety of other tissue culture cells. Cell-surface expression of both glycoproteins was markedly increased in blood monocytes but not in U937 cells after exposure to the lysosomotropic reagent methylamine HCL, indicating differences in LAMP-associated membrane flow in these cell types.
- Medicine and Medical Research