1: Eukaryotic cells chemotax in a wide range of chemoattractant concentration gradients, and thus need inhibitory processes that terminate cell responses to reach adaptation while maintaining sensitivity to higher-concentration stimuli. However, the molecular mechanisms underlying inhibitory processes are still poorly understood. Here, we reveal a locally controlled inhibitory process in a GPCR-mediated signaling network for chemotaxis in Dictyostelium discoideum. We discover a novel negative regulator of Ras signaling, C2GAP1, which localizes at the leading edge of chemotaxing cells, is activated by and is essential for GPCR-mediated Ras adaptation. We show that both C2 and GAP domains are required for the membrane targeting of C2GAP1, and that GPCR-triggered Ras activation recruits C2GAP1 from cytosol and retains it on the membrane to locally inhibit Ras signaling. The altered Ras activation results in impaired gradient sensing and excessive polymerization of F-actin in c2gap1 knockout (c2gap1-) cells, leading to chemotaxis defects. Remarkably, c2gap1- cells display altered cell response, impaired directional sensing, and chemotaxis defects in a chemoattractant concentration-dependent fashion. Thus, we have uncovered a novel inhibitory mechanism required for the adaptation and long-range chemotax.(Xu et al., PNAS 2017). 2: How eukaryotic cells find and interact with bacteria is a fundamental question in biology. Eukaryotic phagocytes and their interactions with bacteria began when single-celled life forms, protozoans, appeared about 2.5 billion years ago. Since then, multicellular organisms endowed with increasingly complex genomes gradually formed, and phagocytic cells from these organisms, such as invertebrates and vertebrates, patrol in a host body to detect, recognize, and eliminate invading pathogenic bacteria for host immunity. The current dogma is that phagocytic cells use at least two types of receptors for defense against invading pathogens: one for detecting and chasing pathogens via chemotaxis and another one for recognizing and eliminating them via phagocytosis. Detection and chasing is facilitated by G-protein-coupled receptors (GPCRs), which sense diffusible chemoattractants derived from bacteria. Recognition and elimination employ pattern-recognition receptors (PPRs), such as Toll-like receptors, for recognizing microbial-associated molecular patterns and/or phagocytic receptors for bacterial surface-bound complements or immunoglobulins. However, the social amoeba Dictyostelium discoideum does not encode orthologs of any known pattern recognition receptors or phagocytic receptors; yet, they are highly evolved as professional phagocytes that chase bacteria via chemotaxis and consume them as food through phagocytosis. Here, we show that this stereotypical phagocyte utilizes fAR, a class C GPCR, to simultaneously detect bacterial secreted folate for chasing bacteria and microbial-associated molecular patterns (MAMPs), lipopolysaccharide (LPS), for engulfing and consuming them (Pan et al. PlOS Biology 2018).
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