Mondal, Subhanjan (2008) Role of modulators of small GTPases in chemotaxis, cytokinesis and development in Dictyostelium Discoideum. PhD thesis, Universität zu Köln.
The work described here shows the complexity of GTPase signalling in an apparently simple organism Dictyostelium discoideum. Ras Guanine nucleotide exchange factor RasGEF Q is one out of at least 25 RasGEFs in D. discoideum. Here we show that it specifically regulates myosin II functions by regulating myosin phosphorylation. RasGEF Q activates the Ras isoform RasB upon stimulation with cAMP. Activated RasB can directly or indirectly activate Myosin Heavy chain kinase A (MHCK A) which then phosphorylates the myosin II heavy chain. Phosphorylated myosin II cannot assemble into filaments and is thus a non-functional form. Furthermore, a DEP domain in RasGEF Q appears to be critical for activating RasGEF Q by release from an autoinhibited state. Studies on the multidomain Rac Guanine nucleotide exchange factor GxcDD show that it is required for the early phases of development in chemotactic migration and streaming behaviour. The characterization of its single domains revealed that the CH domain (Calponin homology domain) of GxcDD functions as a membrane association domain, the RhoGEF domain can physically interact with a subset of Rac GTPases and the ArfGAP-PH tandem accumulates in cortical regions of the cell and in phagosomes. Our results also suggest that a conformational change is required for activation of GxcDD, which would be important for its downstream signaling. Studies on the IQGAP related protein GAPA showed that it associates with two actin-corsslinking protein, namely Filamin and Cortexillin I. GAPA is required for cytokinesis and localizes to the cleavage furrow during cytokinesis in a cortexillin I dependent way. We also observed that GAPA is required for proper phototaxis as is its binding partner Filamin.
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