Uniform cAMP stimulation of Dictyostelium cells induces localized patches of signal transduction and pseudopodia

The chemoattractant cAMP induces the translocation of cytosolic PHCrac-GFP to the plasma membrane. PHCrac-GFP is a green fluorescent protein fused to a PH domain that presumably binds to phosphatydylinositol polyphosphates in the membrane. We determined the relative concentration of PHCrac-GFP in the cytosol and at different places along the cell boundary. In cells stimulated homogeneously with 1 muM cAMP we observed two distinct phases of PHCrap-GFP translocation. The first translocation is transient and occurs to nearly the entire boundary of the cell; the response is maximal at 6-8 s after stimulation and disappears after similar to20 s. A second translocation of PHCrac-GFP starts after similar to30 s and persists as long as cAMP remains present. Translocation during this second response occurs to small patches with radius of similar to4-5 mum, each covering similar to10% of the cell surface. Membrane patches of PHCrac-GFP are both temporally and spatially closely associated with pseudopodia, which are extended at similar to10 s from the area with a PHCrac-GFP patch. These signaling patches in pseudopodia of homogeneously stimulated cells resemble the single patch of PHCrac-GFP at the leading edge of a cell in a gradient of cAMP, suggesting that PHCrac-GFP is a spatial cue for pseudopod formation also in uniform cAMP.