Transduction of chemical signals in dictyostelium cells

Three different functions of cyclic AMP in D discoideum are known: (1) cAMP acts as a chemoattractant during cell aggregation, (2) it controls cell development, particularly the acquisition of aggregation competence, and (3) it is involved in terminal cell differentiation. In this report we will concentrate on the functions 1 and 2 of cAMP. Chemotaxis requires the recognition of concentration gradients in the environment by attractant binding to cell surface receptors, the processing of signals from the receptors to the contractile system of the cells, extension of pseudopods at one part, and contraction at other parts of the cells in accord with the external gradient. One pathway of signal processing from the receptors to the contractile system involves the regulation of a myosin kinase. The control of development up to aggregation competence is largely dependent on the temporal pattern of cAMP application: Only repetitive pulses enhance development. This effect has been studied using the expression of a membrane glycoprotein called contact site A as a differentiation marker

A Plasma Membrane Pool of Phosphatidylinositol 4-Phosphate Is Generated by Phosphatidylinositol 4-Kinase Type-III Alpha: Studies with the PH Domains of the Oxysterol Binding Protein and FAPP1

The PH domains of OSBP and FAPP1 fused to GFP were used to monitor PI(4)P distribution in COS-7 cells during manipulations of PI 4-kinase (PI4K) activities. Both domains were associated with the Golgi and small cytoplasmic vesicles, and a small fraction of OSBP-PH was found at the plasma membrane (PM). Inhibition of type-III PI4Ks with 10 μM wortmannin (Wm) significantly reduced but did not abolish Golgi localization of either PH domains. Downregulation of PI4KIIα or PI4KIIIβ by siRNA reduced the localization of the PH domains to the Golgi and in the former case any remaining Golgi localization was eliminated by Wm treatment. PLC activation by Ca(2+) ionophores dissociated the domains from all membranes, but after Ca(2+) chelation, they rapidly reassociated with the Golgi, the intracellular vesicles and with the PM. PM association of the domains was significantly higher after the Ca(2+) transient and was abolished by Wm pretreatment. PM relocalization was not affected by down-regulation of PI4KIIIβ or -IIα, but was inhibited by down-regulation of PI4KIIIα, or by 10 μM PAO, which also inhibits PI4KIIIα. Our data suggest that these PH domains detect PI(4)P formation in extra-Golgi compartments under dynamic conditions and that various PI4Ks regulate PI(4)P synthesis in distinct cellular compartments