Jeffrey Williams

SH2 Signaling in a Lower Eukaryote: A STAT Protein That Regulates Stalk Cell Differentiation in Dictyostelium

The TTGA-binding factor is a transcriptional regulator activated by DIF, the chlorinated hexaphenone that induces prestalk cell differentiation in Dictyostelium. The same activity also functions as a repressor, controlling stalk cell differentiation. We show that the TTGA-binding factor is a STAT protein. Like the metazoan STATs, it functions via the reciprocal interaction of a phosphotyrosine residue on one molecule with an SH2 domain on a dimerizing partner. Furthermore, it will bind specifically to a mammalian interferon-stimulated response element. In Saccharomyces cerevisiae, where the entire genomic sequence is known, SH2 domains have not been identified. It would seem, therefore, that SH2 signaling pathways arose very early in the evolution of multicellular organisms, perhaps to facilitate intercellular comunication.

EVIDENCE FOR POSITIONAL DIFFERENTIATION OF PRESTALK CELLS AND FOR A MORPHOGENETIC GRADIENT IN DICTYOSTELIUM

We present evidence that Dictyostelium slug tip cells, the pstA cells, may arise by positional differentiation, but at a site remote from that which they will eventually occupy. When first detectable, the pstA cells form a peripheral ring surrounding the other prestalk cell subtype, the pstO cells, but subsequently move above the pstO cells to form the tip. Because pstA cell differentiation requires a 10-fold higher concentration of differentiation-inducing factor, the stalk cell inducer, the initial patterning seems likely to reflect the existence of a morphogenetic gradient. The subsequent redistribution of the two cell types is explicable by their different rates of chemotaxis to cyclic AMP. These results help reconcile the two apparently opposing views of pattern formation in Dictyostelium, that there is positional differentiation and that pattern formation occurs by cell sorting.

Developmentally and spatially regulated activation of a Dictyostelium STAT protein by a serpentine receptor

T.Araki and M.Gamper contributed equally to this work

Morphogenesis in Dictyostelium: new twists to a not-so-old tale

Prespore differentiation requires both cAMP-dependent protein kinase and the transcription factor GBF, and for one class of prespore genes the two form part of a single pathway. It seems that differentiation-inducing factor, the inducer of prestalk cell differentiation, may operate via a calcium signalling pathway, and terminal stalk cell differentiation is in part regulated by glycogen synthase kinase 3.

RNGB: a Dictyostelium RING finger protein that is specifically located in maturing spore cells

The RING finger is a form of zinc finger motif found in proteins of widely varying biological function. The Dictyostelium RNGB protein contains a RING finger and also a K‐box, a sequence motif found in several plant homeotic proteins. The rngB mRNA is present at low concentration in growing cells and gradually increases in abundance throughout development. However, the RNGB protein is not detected until culmination and we present evidence that suggests it is translationally regulated. The protein is specifically localised in maturing spore cells and is cytoplasmic, suggesting that the RING finger does not function as a DNA binding domain.