A specific DIF binding protein in Dictyostelium

Differentiation Inducing Factor (DIF-1), a small chlorinated organic molecule which is produced during Dictyostelium development, is believed to be the morphogen that controls the stalk-specific pathway of differentiation. We describe the identification and characterization of a protease-sensitive activity from cell lysates which binds tritiated DIF-1 with the properties expected of a DIF receptor. Scatchard and linear subtraction plots show a single class of binding sites, of high affinity (Kd = 1.8 nM) and low abundance (1100 sites/cell). The activity elutes from heparin-agarose as a single peak. Various DIF-1 analogues compete for binding in proportion to their activities in a stalk cell differentiation bioassay. The amount of binding activity is developmentally regulated, peaking shortly before the appearance of the prestalk-prespore pattern and before the developmental rise in DIF concentration; the rise occurs at the same time that prestalk-specific genes become DIF inducible. Addition of cyclic AMP to aggregated cells, which induces post-aggregative gene expression in general, also induces the binding activity.     

Cell-fate choice in Dictyostelium: intrinsic biases modulate sensitivity to DIF signaling

Cell fate in Dictyostelium development depends on intrinsic differences between cells, dating from their growth period, and on cell interactions occurring during development. We have sought for a mechanism linking these two influences on cell fate. First, we confirmed earlier work showing that the vegetative differences are biases, not commitments, since cells that are stalky-biased when developed with one partner are sporey with another. Then we tested the idea that these biases operate by modulating the sensitivity of cells to the signals controlling cell fate during development. Cells grown without glucose are stalky-biased when developed with cells grown with glucose. We find, using monolayer culture conditions, that they are more sensitive to each of the stalk-inducing signals, DIFs 1-3. Mixing experiments show that this bias is a cell-intrinsic property. Cells initiating development early in the cell cycle are stalky compared to those initiating development later in the cycle. Likewise, they are more sensitive to DIF-1. Assays of standard markers for prestalk and prespore cell differentiation reveal similar differences in DIF-1 sensitivity between biased cells; DIF-1 dechlorinase (an early prestalk cell marker enzyme) behaves in a consistent manner. We propose that cell-fate biases are manifest as differences in sensitivity to DIF.     

A Dictyostelium SH2 adaptor protein required for correct DIF-1 signaling and pattern formation

Dictyostelium is the only non-metazoan with functionally analyzed SH2 domains and studying them can give insights into their evolution and wider potential. LrrB has a novel domain configuration with leucine-rich repeat, 14-3-3 and SH2 protein–protein interaction modules. It is required for the correct expression of several specific genes in early development and here we characterize its role in later, multicellular development. During development in the light, slug formation in LrrB null (lrrB-) mutants is delayed relative to the parental strain, and the slugs are highly defective in phototaxis and thermotaxis. In the dark the mutant arrests development as an elongated mound, in a hitherto unreported process we term dark stalling. The developmental and phototaxis defects are cell autonomous and marker analysis shows that the pstO prestalk sub-region of the slug is aberrant in the lrrB- mutant. Expression profiling, by parallel micro-array and deep RNA sequence analyses, reveals many other alterations in prestalk-specific gene expression in lrrB- slugs, including reduced expression of the ecmB gene and elevated expression of ampA. During culmination ampA is ectopically expressed in the stalk, there is no expression of ampA and ecmB in the lower cup and the mutant fruiting bodies lack a basal disc. The basal disc cup derives from the pstB cells and this population is greatly reduced in the lrrB- mutant. This anatomical feature is a hallmark of mutants aberrant in signaling by DIF-1, the polyketide that induces prestalk and stalk cell differentiation. In a DIF-1 induction assay the lrrB- mutant is profoundly defective in ecmB activation but only marginally defective in ecmA induction. Thus the mutation partially uncouples these two inductive events. In early development LrrB interacts physically and functionally with CldA, another SH2 domain containing protein. However, the CldA null mutant does not phenocopy the lrrB- in its aberrant multicellular development or phototaxis defect, implying that the early and late functions of LrrB are affected in different ways. These observations, coupled with its domain structure, suggest that LrrB is an SH2 adaptor protein active in diverse developmental signaling pathways.     

A myb-related protein required for culmination in Dictyostelium.

The avian retroviral v-myb gene and its cellular homologues throughout the animal and plant kingdoms contain a conserved DNA binding domain. We have isolated an insertional mutant of Dictyostelium unable to switch from slug migration to fruiting body formation i.e. unable to culminate. The gene that is disrupted, mybC, codes for a protein with a myb-like domain that is recognized by an antibody against the v-myb repeat domain. During development of myb+ cells, mybC is expressed only in prestalk cells. When developed together with wild-type cells mybC- cells are able to form both spores and stalk cells very efficiently. Their developmental defect is also bypassed by overexpressing cAMP-dependent protein kinase. However even when their defect is bypassed, mybC null slugs and culminates produce little if any of the intercellular signalling peptides SDF-1 and SDF-2 that are believed to be released by prestalk cells at culmination. We propose that the mybC gene product is required for an intercellular signaling process controlling maturation of stalk cells and spores and that SDF-1 and/or SDF-2 may be implicated in this process.