Our laboratory uses a combination of genetic and cell biological approaches to dissect the molecular mechanisms regulating endocytic trafficking. Cell surface molecules play a central role in establishing the variety of cell types found in neuronal networks and in regulating their function. Endocytosis is one of the important mechanisms by which the level of cell surface proteins is regulated. In Drosophila, the specific expression of the Bride of Sevenless ligand (Boss) on the surface of R8 neurons provides an excellent opportunity for the genetic dissection of endocytosis. Binding of Boss to the Sevenless receptor tyrosine kinase induces development of the R7 neuron in the developing eye. Upon binding, the complex between the Boss ligand and the Sevenless receptor is internalized into R7 neurons. Using this system we have identified several genes necessary for normal endocytic trafficking in R7 cells and found them to be required for endocytic trafficking and autophagy.
A first class of mutations, exemplified by deep orange and carnation, interferes with the delivery of ligands to lysosomes and results in the accumulation of internalized ligands. Such mutations also interfere with the delivery of cargo to pigment granules in the compound eye and therefore can be easily identified as eye color mutations. We are currently exploiting this eye color phenotype for a systematic genetic approach towards the identification and analysis of genes necessary for normal endocytic trafficking. The connection of lysosomal delivery and pigmentation is not unique to flies. Several mutations that effect both of these pathways in humans cause genetic diseases grouped together as Hermansky-Pudlak syndrome.
A second class of mutations, exemplified by hook, appears to increase the apparent rate of lysosomal delivery to a level that endocytosed ligands can no longer be detected. Molecular cloning of hook identified the founding member of a new family of coiled-coil proteins which we are analyzing in flies and vertebrates. A common theme of Hook proteins is their role in the microtubule-dependent positioning of different organelles in the cell. For example, Hook3 plays a role in the positioning of the Golgi complex in vertebrate cells. The molecular mechanisms by which different Hook proteins modulate microtubule-based trafficking of different organelles is under active investigation.
RESEARCH INTERESTS
Research in Dr. Kramer's lab focuses on the genetic and molecular dissection of late endocytic trafficking. The lab uses the Drosophila compound eye as a model system to identify mutations altering membrane traffic from the cell surface to different organelles. One class of genes, exemplified by deep orange and carnation, is necessary for the delivery of cargo to lysosomes and pigment granules.
The hook gene represents a second class of mutations necessary for normal lysosomal delivery. Hook proteins are a novel family of microtubule-binding proteins that play a role in the positioning of different organelles. To investigate the molecular mechanisms by which these proteins effect membrane trafficking we are combining genetic and cell biological approaches in Drosophila and mammalian cells.
RECENT PUBLICATIONS
Szebenyi,, G., Hall B., Yu, R., Hashim A. I, and Kramer, H., "Hook2 localizes to the centrosome, binds directly to Centriolin/CEP110, and contributes to centrosomal function." Traffic, 8:32-46, 2007
Szebenyi G, Wigley WC, Hall B, Didier A, Yu M, Thomas P, Kramer H., "Hook2 contributes to aggresome formation." MMC Cell Biol, 8:19-29, May 2007
Sevrioukov, E., Moghrabi, N., Kuhn, M., and Kramer, H., "A mutation in dVps28 reveals a link between the ESCRT-I complex and the actin cytoskeleton in Drosophila" Mol. Biol. Cell, 16:2301?2312, 2005
Pulipparacharuvil, S., Akbar, A. Ray, S., Sevrioukov, E.A., Haberman A.S. and Kramer H, "Drosophila Vps16A is required for trafficking to lysosomes and pigment granules biogenesis" J. Cell Science, 118:3663-3673, 2005
Shotland, Y, Kramer H., and Groisman, E.A., "The Salmonella SpiC protein targets the mammalian Hook3 protein to alter cellular trafficking" Molecular Microbiology, 49:1565-1576, 2003
SIGNIFICANT PUBLICATIONS
Walenta, J., Didier, A., Liu, X. and Kramer, H., "The Golgi-Associated Hook3 Protein is a Member of a Novel Family of Microtubule-Binding Proteins." J. Cell Biol., 152:923-934, March 2001
Sevrioukov, E., He, J.-P., Sunio, A, Moghrabi N. and Kramer H., "A role for the deep orange and carnation eye-color genes in lysosomal delivery in Drosophila" Molecular Cell, 4:479-486, November 1999
Sunio, A., Metcalf, A. and Kramer, H., "Genetic dissection of endocytic trafficking in Drosophila using a horseradish peroxidase-bride of sevenless chimera: hook is required for normal maturation of multivesicular endosomes" Mol. Biol. Cell, 10:847-859, April 1999
Kramer, H. and Phistry, M., "Mutations in the Drosophila hook gene inhibit endocytosis of the Boss transmembrane ligand into multivesicular bodies" J. Cell Biology, 133:1205-1216, June 1996
Kramer, H., Cagan, R.L. and Zipursky, S.L., "Interaction of bride of sevenless membrane-bound ligand and the sevenless tyrosine-kinase receptor." Nature, 352:207-212, May 1991
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