Properties of the membrane proteins of rat liver lysosomes. The majority of lysosomal membrane proteins are exposed to the cytoplasm

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Abstract
Rat liver lysosomes were lysed and subfractionated by differential centrifugation through 0.2 M NaCl to yield a membranous pellet. This membrane fraction contains less than 20% of the lysosomal protein, ATP activity of about 1.2 .mu.mol/min per mg of protein, 120 nmol of thiol groups/mg of protein and at least 16 protein and glycoprotein bands on sodium dodecyl sulfate/polyacrylamide-gel electrophoresis. The gel patterns of membranes isolated from lysosomes after treatment with [125I]iodide-H2O2-lactoperoxidase, toluene 2,4-di-isocyanate-activated bovine serum albumin, trypsin and subtilisin indicate that most of the membrane proteins are exposed to the cytoplasm. These exposed proteins are candidates for intracellular receptors which recognize either substances that are to be degraded or vesicles containing those substances.