Identification and ligand-induced modification of the cAMP receptor in Dictyostelium

Signal relay during the life cycle of Dictyostelium

A fundamental property of multicellular organisms is signal relay, the process by which information is transmitted from one cell to another. The integration of external information, such as nutritional status or developmental cues, is critical to the function of organisms. In addition, the spatial organizations of multicellular organisms require intricate signal relay mechanisms. Signal relay is remarkably exhibited during the life cycle of the social amoebae Dictyostelium discoideum, a eukaryote that retains a simple way of life, yet it has greatly contributed to our knowledge of the mechanisms cells use to communicate and integrate information. This chapter focuses on the molecules and mechanisms that Dictyostelium employs during its life cycle to relay temporal and spatial cues that are required for survival.

Novel Mechanism of PTEN Regulation by Its Phosphatidylinositol 4,5-Bisphosphate Binding Motif Is Critical for Chemotaxis

In chemotaxing cells, localization of phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3) to the leading edge of the cell sets the direction and regulates the formation of pseudopods at the anterior. We show that the lipid phosphatase activity of PTEN mediates chemotaxis and that the sharp localization of PI(3,4,5)P3 requires localization of PTEN to the rear of the cell. Our data suggest that a phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) binding motif at the N terminus of PTEN serves the dual role of localizing the enzyme to the membrane and regulating its activity. Mutations in this motif enhance catalytic activity but render the enzyme inactive in vivo by preventing membrane association. The key role of this motif may explain the heretofore puzzling tumor-suppressing mutations occurring within the PI(4,5)P2 binding motif. On the other hand, the localization of PTEN does not depend on its phosphatase activity, the actin cytoskeleton, or the intracellular level of PI(3,4,5)P3, suggesting that events controlling localization are upstream of phosphoinositide signaling.

A molecular network that produces spontaneous oscillations in excitable cells of Dictyostelium

A network of interacting proteins has been found that can account for the spontaneous oscillations in adenylyl cyclase activity that are observed in homogenous populations of Dictyostelium cells 4 h after the initiation of development. Previous biochemical assays have shown that when extracellular adenosine 3',5'-cyclic monophosphate (cAMP) binds to the surface receptor CAR1, adenylyl cyclase and the MAP kinase ERK2 are transiently activated. A rise in the internal concentration of cAMP activates protein kinase A such that it inhibits ERK2 and leads to a loss-of-ligand binding by CAR1. ERK2 phosphorylates the cAMP phosphodiesterase REG A that reduces the internal concentration of cAMP. A secreted phosphodiesterase reduces external cAMP concentrations between pulses. Numerical solutions to a series of nonlinear differential equations describing these activities faithfully account for the observed periodic changes in cAMP. The activity of each of the components is necessary for the network to generate oscillatory behavior; however, the model is robust in that 25-fold changes in the kinetic constants linking the activities have only minor effects on the predicted frequency. Moreover, constant high levels of external cAMP lead to attenuation, whereas a brief pulse of cAMP can advance or delay the phase such that interacting cells become entrained.