Under certain conditions, extracellular chemical cues guide the locomotion of a cell in a particular direction. In some cases, the movement is guided by insoluble molecules in the underlying substratum. In other cases, the cell senses soluble molecules and follows them, along a concentration gradient, to their source. The latter response is called chemotaxis. One of the best-studied examples of chemotaxis is the migration of Dictyostelium amebas along an increasing concentration of cAMP. Following cAMP to its source, the amebas aggregate into a slug and then differentiate into a fruiting body. Many other cells also display chemotactic
Coincident Gradients of Chemoattractants, Activated G Proteins, and Ca2+ Micrographs of cAMP receptors tagged with green fluorescent protein (GFP) show that the receptors are distributed uniformally along the length of an ameba cell (Figure 19-30). Therefore an internal gradient must be established by another component of the signalling pathway. Because cAMP receptors signal through trimeric G proteins, a subunit of the trimeric G protein and other downstream signaling proteins were tagged with GFP. Fluorescence micrographs show that the concentration of trimeric G proteins is higher in the direction of the chemoattractant. Trimeric G proteins coupled to cAMP receptors can activate pathways
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