Lysosomes are found in virtually all eukaryotic cells and contain hydrolytic enzymes which function in the degradation of biological substances. The lysosomal system has been implicated in a wide variety of cellular functions; including autolysis, modulation of hormone and drug action and as mediators of tissue destruction during inflammation. Lysosomes are also involved in a large number of human genetic diseases involving a deficiency in one or more lysosomal enzyme. Current research is directed at understanding the molecular mechanisms cells use to target lysosomal enzymes to lysosomes. A model to explain the targeting of these enzymes in animal cells has recently been proposed. This model states that specific membrane receptors recognize and bind post-translational modifications unique to lysosomal enzymes. These receptors move to lysosomes where the enzymes are released. However, cell lines exist that lack these receptors and must therefore target lysosomal enzymes by a different mechanism. Dictyostelium discoideum also lack these receptors and, because of the ease in which it can be genetically and biochemically manipulated, represents an ideal organism in which to study this alternative pathway. Our research involves both a biochemical approach, consisting of subcellular fractionation and immunocytochemistry, and a genetic approach, using mutants altered in the synthesis, modification, and proteolytic processing of lysosomal enzymes. Cells altered biochemically and genetically will be analyzed with respect to their ability to properly localize lysosomal enzymes. In addition to defining the intracellular pathway followed by lysosomal enzymes, this approach will reveal the importance of both post-translational modification and proteolytic processing in the localization event. Ultimately, these studies will identify both the molecular nature of the sorting signal on lysosomal enzymes and the cellular components involved in the localization process in cells that lack phosphomannosyl receptors.
