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The cytoskeleton in most eukaryotes consists of actin filaments, intermediate filaments, microtubules and specific associated proteins. It determines the shape and the polarity of a cell and is inevitable for the coordination of cell movement. The regulation of this complex structure requires a highly organised and specialised signalling network. Ste20-like kinases and the regulator protein Mo25 (Morula protein 25) are part of this signalling network. The main objective of this work was the functional characterisation of the regulator protein Mo25 and the Ste20-like kinases Fray1, Fray2 (Frayed kinase 1/2) and DstC (Dictyostelium serine threonine kinase C) in the amoeba Dictyostelium discoideum (D. discoideum). An additional project was to map and characterise the actin and actin related genes in the genome of the fresh water foraminifer Reticulomyxa filosa (R. filosa). Mo25 is a highly conserved 40 kDa scaffolding protein with a 60% identity from amoeba to man. The disruption of the mo25 gene in D. discoideum results in very large, multinucleated cells which are unable to complete cytokinesis. Growth as well as development is severely delayed in the Mo25-minus strain. Furthermore, in phototaxis assays performed with multicellular aggregates (slugs), the Mo25-minus slugs were unable to migrate towards the light source. These findings imply that Mo25 plays an important role in cytokinesis, growth and cell polarity. We could link the Ste 20-like kinase SvkA (severin kinase), a homolog of the human Mst3, Mst4 (Mammalian Ste20-like kinase 3/4) and Ysk1/Sok1 (Yeast Sps1/Ste20-related kinase 1, Suppressor of Kinase 1) kinases to Mo25 as a binding partner. To further elucidate the interaction of Mo25 with SvkA as well as their role in cytokinesis or polarity signalling, we generated a series of GFP–Mo25 rescue constructs with distinct point mutations in protein-protein interaction surfaces and transformed these into the Mo25-minus background. The kinase domains of the Ste20-like kinases, Fray1 and Fray2 in D. discoideum are highly homologous to the catalytic domains of OSR1 (Oxidative stress response kinase 1) and SPAK (Ste20/SPS1-related proline-alanine-rich protein kinase) in humans and Frayed in fruit fly. Here, we generated the knockout clones Fray1-minus, Fray2-minus, and the double knockout Fray2Fray1-minus in D. discoideum. In developmental studies, Fray2-minus did not show an altered phenotype, whereas Fray1-minus and Fray2Fray1-minus developed slightly slower into fruiting bodies. When grown in shaking culture, Fray1-minus and Fray2Fray1-minus showed a reduced growth rate compared to Fray2-minus and the wild type. In addition, by using a GFP-Trap resin we identified a binding partner of Fray1, a yet unknown protein that we named FRIP (Fray Interacting Protein). FRIP mainly consists of a CBS (Cystathionine beta synthase) domain pair and is 30% identical to the gamma subunit of the AMPK (5‘ adenosine mono phosphate-activated protein kinase) complex in D. discoideum. The Ste20-like kinase DstC has been described to be a regulator of the actin driven process of phagocytosis. The catalytic domain of DstC is most similar to the mammalian kinase Mst2 (Mammalian Ste20-like kinase 2) and Hippo (“Hippopotamus”-like phenotype) of D. melanogaster. We could map the sorting signal that localises DstC to phagocytic cups and acidic vesicles to about 90 amino acids. Here we present an array of distinct point mutations for the identification of the exact localisation signal. R. filosa is a fresh water protist which belongs to the the group of Foraminifera within the Rhizaria. The R. filosa genome is the first foraminiferal and only the second rhizarian genome to be deciphered. In this bioinformatics project, we could identify, map and characterise four new actin genes in addition to the already known actin and about 40 genes that code for potential actin related proteins of seven different protein classes.