Biomedical subjects

Helmut Krämer

Publications and source records attributed to Helmut Krämer.

5 recordsLinked to original sources

PubMed2006

Hook2 localizes to the centrosome, binds directly to centriolin/CEP110 and contributes to centrosomal function.

Centrosomes serve as microtubule-organizing centers. However, centrosome function depends on microtubule organization and protein transport because the formation, positioning and maintenance of centrosomes require microtubule-dependent retrograde transport. Linker proteins that associate with the motor protein dynein, organelles and microtubules facilitate loading of cargos for retrograde transport and thus contribute to the composition and placement of the centrosome and other juxtanuclear protein complexes. Members of the hook family of proteins may function as adaptors to link various organelle cargos to dynein for transport and have also been implicated directly in centrosome positioning. Here, we show that mammalian hook2, a previously uncharacterized member of the hook family, localizes to the centrosome through all phases of the cell cycle, the C-terminal domain of hook2 directly binds to centriolin/CEP110, the expression of the C-terminal domain of centriolin/CEP110 alters the distribution of endogenous hook2 and mislocalized wild-type or mutant hook2 proteins perturb endogenous centrosomal and pericentrosomal proteins in cultured mammalian cells. In addition, interference with hook2 function results in the loss of the radial organization of microtubules and a defect in regrowth of microtubules following their nocodazole-induced depolymerization. Thus, we propose that hook2 contributes to the establishment and maintenance of centrosomal structure and function.

Animals↗

PubMed2005

Drosophila Vps16A is required for trafficking to lysosomes and biogenesis of pigment granules.

Mutations that disrupt trafficking to lysosomes and lysosome-related organelles cause multiple diseases, including Hermansky-Pudlak syndrome. The Drosophila eye is a model system for analyzing such mutations. The eye-color genes carnation and deep orange encode two subunits of the Vps-C protein complex required for endosomal trafficking and pigment-granule biogenesis. Here we demonstrate that dVps16A (CG8454) encodes another Vps-C subunit. Biochemical experiments revealed a specific interaction between the dVps16A C-terminus and the Sec1/Munc18 homolog Carnation but not its closest homolog, dVps33B. Instead, dVps33B interacted with a related protein, dVps16B (CG18112). Deep orange bound both Vps16 homologs. Like a deep orange null mutation, eye-specific RNAi-induced knockdown of dVps16A inhibited lysosomal delivery of internalized ligands and interfered with biogenesis of pigment granules. Ubiquitous knockdown of dVps16A was lethal. Together, these findings demonstrate that Drosophila Vps16A is essential for lysosomal trafficking. Furthermore, metazoans have two types of Vps-C complexes with non-redundant functions.

Animals↗

PubMed2005

A mutation in dVps28 reveals a link between a subunit of the endosomal sorting complex required for transport-I complex and the actin cytoskeleton in Drosophila.

Proteins that constitute the endosomal sorting complex required for transport (ESCRT) are necessary for the sorting of proteins into multivesicular bodies (MVBs) and the budding of several enveloped viruses, including HIV-1. The first of these complexes, ESCRT-I, consists of three proteins: Vps28p, Vps37p, and Vps23p or Tsg101 in mammals. Here, we characterize a mutation in the Drosophila homolog of vps28. The dVps28 gene is essential: homozygous mutants die at the transition from the first to second instar. Removal of maternally contributed dVps28 causes early embryonic lethality. In such embryos lacking dVps28, several processes that require the actin cytoskeleton are perturbed, including axial migration of nuclei, formation of transient furrows during cortical divisions in syncytial embryos, and the subsequent cellularization. Defects in actin cytoskeleton organization also become apparent during sperm individualization in dVps28 mutant testis. Because dVps28 mutant cells contained MVBs, these defects are unlikely to be a secondary consequence of disrupted MVB formation and suggest an interaction between the actin cytoskeleton and endosomal membranes in Drosophila embryos earlier than previously appreciated.

Actins↗

PubMed2003

The Salmonella SpiC protein targets the mammalian Hook3 protein function to alter cellular trafficking.

The Salmonella SpiC protein is secreted into the cytosol of macrophages via a unique type III secretion system that functions intracellularly to translocate proteins across the phagosomal membrane. The SpiC protein is required for survival within macrophages and inhibition of phagosome-lysosome fusion in vivo, and it is sufficient to inhibit endosome-endosome fusion in vitro. Here, we establish that SpiC targets the function of Hook3, a mammalian protein implicated in cellular trafficking. Purified GST-SpiC pulled down Hook3 from murine macrophages, and anti-Hook3 antibodies precipitated SpiC from the cytosol of Salmonella-infected macrophages. Expression of the spiC gene disrupted Golgi morphology in Vero cells and altered the distribution of lysosomes in macrophages, mimicking the phenotype of cells expressing a hook3 dominant-negative mutant. By inactivating Hook3 function, the SpiC protein may alter the lysosome network and prevent phagosome-lysosome fusion.

Amino Acid Sequence↗

PubMed2002

Sorting out signals in fly endosomes.

Ligands and receptors that mediate cell-cell interactions during development are removed from the cell surface by endocytosis. Subsequently, many of these internalized proteins are detected in multivesicular bodies (MVBs). Recent work in different organisms has elucidated some aspects of MVB biogenesis and trafficking. This review discusses some intriguing links between these findings, the sorting of proteins in endocytic trafficking, and the regulation of signaling pathways in Drosophila.

Animals↗