CREG: a novel lysosomal protein

In eukaryotic cells, dedicated subcellular structures referred to as lysosomes play a pivotal role in the metabolism of cellular macromolecules. These organelles contain a large array of hydrolases and auxiliary proteins required for efficient substrate breakdown. Deficiencies in individual lysosomal proteins result in the gradual accumulation of undigested macromolecules which ultimately interferes with the normal functions of lysosomes and the affected cells. In humans, about 45 diseases have been directly linked to the deficiency of one or several lysosomal proteins. However, the molecular etiology of some lysosomal storage diseases is still unknown. Since lysosomal dysfunction has been also observed in other human disease states including neurodegenerative diseases and cancer, considerable efforts have been recently undertaken to establish the complete proteome of lysosomes. This has led to the discovery of a range of novel lysosomal proteins with as yet unknown functions. This project aims at a thorough characterization of a particularly striking novel lysosomal protein, Cellular Repressor of E1A-stimulated Genes (CREG). The biochemical function of the protein is still unclear. The cellular roles of CREG in mammals are a matter of debate. The occurrence of CREG in non-mammalian species such as plants and insects is also not understood. Hence, the main goals of this study are to assess the consequences of CREG ablation in mammalian cells and whole organisms, to perform screens for possible interaction partners of the protein, to elucidate the relationship between CREG`s unique structural features and its biological activity, and to characterize the structural and functional properties of CREG from plants and insects as compared to their mammalian counterparts. The detailed characterization of CREG and elucidation of its cellular tasks will improve our understanding of the biology of lysosomes and related compartments. It also has the potential to provide novel insights into the pathophysiology of human diseases associated with disturbed functions of these organelles.

In eukaryotic cells, dedicated subcellular structures referred to as lysosomes play a pivotal role in the metabolism of cellular macromolecules. These organelles contain a large array of proteins required for efficient breakdown of ingested substrates. Deficiencies in individual lysosomal proteins result in the gradual accumulation of undegraded macromolecules which ultimately interferes with the normal functions of these compartments and the affected cells. In humans, about fifty lysosomal storage diseases have been directly linked to the deficiency of one or several lysosomal proteins. Lysosomal dysfunction is also associated with more common human disease states such as neurodegenerative diseases or cancer. In recent years, a range of novel lysosomal proteins has been discovered. This project has focused on the investigation of one of them, Cellular Repressor of E1A-stimulated Genes (CREG).CREG is an evolutionarily conserved protein. We have therefore set out to characterize its molecular features and cellular activities in mammals as well as non-mammalian organisms. Our results establish that CREG is not only localized in the lysosomes of mammalian cells, but also transported to the equivalent subcellular structures of insects and plants. In all model systems investigated, delivery of CREG to these compartments is associated with its proteolytic maturation by lysosomal cysteine proteinases. Intracellular trafficking of CREG in mammalian cells relies on the canonical lysosomal sorting machinery utilizing mannose 6-phosphate receptors, but this is not the case in insects and plants. Whereas the model plant Arabidopsis thaliana tolerates reduced CREG levels without signs of organ malformation or increased stress sensitivity, CREG was found to be essential for proper development of the fruitfly Drosophila melanogaster. Furthermore, increased synthesis of CREG was found to protect tumour cells against programmed cell death and to render them more likely to metastasize. The detailed characterization of CREG performed in this project provides valuable new insights into the biology of lysosomes, which could lead to a better understanding of the pathophysiology of human diseases associated with disturbed functions of these cellular compartments. The identification of CREG as a pro-survival factor of cancer cells could inform the development of more effective cancer treatment strategies.