Mitogen-activated protein (MAP) kinases are involved in controlling a cell's responses to a variety of stimuli and can be activated by both protein tyrosine kinase and G protein-coupled receptors. It was shown previously that Dictyostelium MAP kinase ERK2 is required for normal activation of adenylyl cyclase and erk2 null cells are aggregation-deficient. In this manuscript, we show that the Dictyostelium MAP kinase ERK2 is rapidly and transiently activated in response to the chemoattractant cAMP. This response requires cAMP receptors, but is independent of the coupled G alpha2 subunit and the only known G beta subunit. These data indicate that ligand-mediated receptor activation of adenylyl cyclase requires two receptor-dependent pathways, one of which requires heterotrimeric G proteins, including G alpha2 and the only known G beta subunit, and the second of which requires ERK2. Our results suggest that ERK2 may be activated by a novel receptor-mediated pathway.

In Dictyostelium amoebae, cell-type differentiation, spatial patterning, and morphogenesis are controlled by a combination of cell-autonomous mechanisms and intercellular signaling. A chemotactic aggregation of approximately 10(5) cells leads to the formation of a multicellular organism. Cell-type differentiation and cell sorting result in a small number of defined cell types organized along an anteroposterior axis. Finally, a mature fruiting body is created by the terminal differentiation of stalk and spore cells. Analysis of the regulatory program demonstrates a role for several molecules, including GSK-3, signal transducers and activators of transcription (STAT) factors, and cAMP-dependent protein kinase (PKA), that control spatial patterning in metazoans. Unexpectedly, two component systems containing histidine kinases and response regulators also play essential roles in controlling Dictyostelium development. This review focuses on the role of cAMP, which functions intracellularly to mediate the activity of PKA, an essential component in aggregation, cell-type specification, and terminal differentiation. Cytoplasmic cAMP levels are controlled through both the regulated activation of adenylyl cyclases and the degradation by a phosphodiesterase containing a two-component system response regulator. Extracellular cAMP regulates G-protein-dependent and -independent pathways to control aggregation as well as the activity of GSK-3 and the transcription factors GBF and STATa during multicellular development. The integration of these pathways with others regulated by the morphogen DIF-1 to control cell fate decisions are discussed.

Membrane-associated NADPH oxidase complexes catalyse the production of the superoxide anion radical from oxygen and NADPH. In mammalian systems, NADPH oxidases form a family of at least seven isoforms that participate in host defence and signalling pathways. We report here the cloning and the characterisation of slime mould Dictyostelium discoideum homologs of the mammalian heme-containing subunit of flavocytochrome b (gp91(phox)) (NoxA, NoxB and NoxC), of the small subunit of flavocytochrome b (p22(phox)) and of the cytosolic factor p67(phox). Null-mutants of either noxA, noxB, noxC or p22(phox) show aberrant starvation-induced development and are unable to produce spores. The overexpression of NoxA(myc2) in noxA null strain restores spore formation. Remarkably, the gene alg-2B, coding for one of the two penta EF-hand proteins in Dictyostelium, acts as a suppressor in noxA, noxB, and p22(phox) null-mutant strains. Knockout of alg-2B allows noxA, noxB or p22(phox) null-mutants to return to normal development. However, the knockout of gene encoding NoxC, which contains two penta EF-hands, is not rescued by the invalidation of alg-2B. These data are consistent with a hypothesis connecting superoxide and calcium signalling during Dictyostelium development.

Endocytosis of ligand-activated plasma membrane receptors has been shown to contribute to the regulation of their downstream signaling. β-arrestins interact with the phosphorylated tail of activated receptors and act as scaffolds for the recruitment of adaptor proteins and clathrin, that constitute the machinery used for receptor endocytosis. Visual- and β-arrestins have a two-lobe, immunoglobulin-like, β-strand sandwich structure. The recent resolution of the crystal structure of VPS26, one of the retromer subunits, unexpectedly evidences an arrestin fold in this protein, which is otherwise unrelated to arrestins. From a functional point of view, VPS26 is involved in the retrograde transport of the mannose 6-P receptor from the endosomes to the trans-Golgi network. In addition to the group of genuine arrestins and Vps26, mammalian cells harbor a vast repertoire of proteins that are related to arrestins on the basis of their PFAM Nter and Cter arrestin- domains, which are named Arrestin Domain- Containing proteins (ADCs). The biological role of ADC proteins is still poorly understood. The three subfamilies have been merged into an arrestin-related protein clan.This paper provides an overall analysis of arrestin clan proteins. The structures and functions of members of the subfamilies are reviewed in mammals and model organisms such as Drosophila, Caenorhabditis, Saccharomyces and Dictyostelium.