gdt1, a new signal transduction component for negative regulation of the growth-differentiation transition in Dictyostelium discoideum

Discoidin I expression was used as a marker to screen for mutants affected in the growth-differentiation transition (GDT) of Dictyostelium. By REMI mutagenesis we have isolated mutant 2-9, an overexpressor of discoidin I. It displays normal morphogenesis but shows premature entry into the developmental cycle. The disrupted gene was denominated gdt1. The mutant phenotype was reconstructed by disruptions in different parts of the gene, suggesting that all had a complete loss of function. gdt1 was expressed in growing cells; the levels of protein and mRNA appear to increase with cell density and rapidly decrease with the onset of development. gdt1 encodes a 175-kDa protein with four putative transmembrane domains. In the C terminus, the derived amino acid sequence displays some similarity to the catalytic domain of protein kinases. Mixing experiments demonstrate that the gdt1(-) phenotype is cell autonomous. Prestarvation factor is secreted at wild-type levels. The response to folate, a negative regulator of discoidin expression, was not impaired in gdt1 mutants. Cells that lack the G protein alpha2 display a loss of discoidin expression and do not aggregate. gdt1(-)/Galpha2(-) double mutants show no aggregation but strong discoidin expression. This suggests that gdt1 is a negative regulator of the GDT downstream of or in a parallel pathway to Galpha2.

The Dictyostelium discoideum beta-1,4-mannosyltransferase gene, mntA, has two periods of developmental expression

The precise roles of protein glycosylation in multicellular development are poorly understood. We have characterized the mntA gene from Dictyostelium discoideum which encodes the beta-1,4-mannosyltransferase enzyme that catalyzes the reaction: GDP-Man + dolichol-PP-GlcNAc2 --> dolichol-PP-GlcNAc2-Man + GDP. This gene has a central role in the synthesis of the lipid-linked oligosaccharide precursor which becomes the core of all asparagine-linked (N-linked) glycans. The mntA gene contains a single small intron and encodes a 493 aa protein with a predicted molecular size of 56 kDa. It is located 5' to the repE gene on chromosome IV and is transcribed in the opposite orientation to repE with which it shares a 585 bp of upstream intergenic region. The predicted mntA gene product shares 38% homology with the S. cerevisiae ALG1 gene product. The MntA protein has a region homologous to the putative dolichol-binding region in the yeast ALG1 protein, but it is located in a different part of the molecule. Northern analysis revealed that the expression of the mntA gene is regulated during multicellular development with two periods of mRNA accumulation. The mntA gene product has a classical endoplasmic reticulum retention motif, and is the first Dictyostelium gene encoding a protein that is active in this organelle. The identification of this gene will allow expanded studies of the role of N-linked glycans in multicellular development.

Differential developmental expression of the rep B and rep D xeroderma pigmentosum related DNA helicase genes from Dictyostelium discoideum

DNA helicases are essential to many cellular processes including recombination, replication and transcription, and some helicases function in multiple processes. The helicases encoded by the Xeroderma pigmentosum (XP) B and D genes function in both nucleotide excision repair and transcription initiation. Mutations that affect the repair function of these proteins result in XP while mutations affecting transcription result in neurological and developmental abnormalities, although the underlying molecular and cellular basis for these phenotypes is not well understood. To better understand the developmental roles of these genes, we have now identified and characterized the rep B and rep D genes from the cellular slime mold Dictyostelium discoideum . Both genes encode DNA helicases of the SF2 superfamily of helicases. The rep D gene contains no introns and the rep B gene contains only one intron, which makes their genomic structures dramatically different from the corresponding genes in mammals and fish. However the predicted Dictyostelium proteins share high homology with the human XPB and XPD proteins. The single copy of the rep B and D genes map to chromosomes 3 and 1, respectively. The expression of rep B and D (and the previously isolated rep E) genes during multicellular development was examined, and it was determined that each rep gene has a unique pattern of expression, consistent with the idea that they have specific roles in development. The pattern and extent of expression of these genes was not affected by the growth history of the cells, implying that the expression of these genes is tightly regulated by the developmental program. The expression of the rep genes is a very early step in development and may well represent a key event in the initiation of development in this organism.

PsB multiprotein complex of Dictyostelium discoideum. Demonstration of cellulose binding activity and order of protein subunit assembly

The differentiated spores of Dictyostelium are surrounded by an extracellular matrix, the spore coat, which protects them from environmental factors allowing them to remain viable for extended periods of time. This presumably is a major evolutionary advantage. This unique extracellular matrix is composed of cellulose and glycoproteins. Previous work has shown that some of these spore coat glycoproteins exist as a preassembled multiprotein complex (the PsB multiprotein complex) which is stored in the prespore vesicles (Watson, N., McGuire, V., and Alexander, S (1994) J. Cell Sci. 107, 2567-2579). Later in development, the complex is synchronously secreted from the prespore vesicles and incorporated into the spore coat. We now have shown that the PsB complex has a specific in vitro cellulose binding activity. The analysis of mutants lacking individual subunits of the PsB complex revealed the relative order of assembly of the subunit proteins and demonstrated that the protein subunits must be assembled for cellulose binding activity. These results provide a biochemical explanation for the localization of this multiprotein complex in the spore coat.

Cell-density-dependent repression of discoidin in Dictyostelium discoideum

When Dictyostelium discoideum cells are grown on bacteria, their natural food source, the discoidin genes are induced by cell-density-sensing factors before the food supply is exhausted [11, 18], and expression increases continuously thereafter. This regulation pattern is changed when cells are grown in axenic medium: the discoidins are induced at a considerably lower cell density and are no longer expressed in stationary phase [13]. We have investigated this phenomenon further and show that repression begins when cells are still in exponential growth. It occurs at the level of transcription and involves an element of the discoidin I gamma promoter for which no function has previously been described. Since the effect of high cell density can be mimicked by conditioned medium, it appears that the repression is due to an extracellular signal. This signal is neither ammonia, nor folate, nor cAMP, the known repressors of discoidin expression.

Multiple signal transduction pathways regulate discoidin I gene expression in Dictyostelium discoideum

The expression of the discoidin I genes in Dictyostelium discoideum is regulated by the concerted action of the extracellular factors cyclic adenosine monophosphate (cAMP), folate, prestarvation factor (PSF) and conditioned media factor (CMF). However, the pathways by which these signals are transduced and the interactions between the pathways have been unexplored so far. We have analysed wild-type and mutant cells with defined lesions in signal transduction to elucidate these regulatory processes, and shown that different pathways are used for the down-regulation and induction of these genes. The cAMP receptor cARI is required for the cAMP-mediated down-regulation of discoidin I gene expression but not for the induction of discoidin I expression during development. Surprisingly, induction of the discoidin I genes requires G alpha 2, the G-protein subunit which is generally believed to couple to cARI, to control the expression of cAMP-inducible genes. Thus, our data suggest that G alpha 2 interacts with different receptors to regulate gene expression in early development. Furthermore, the analysis shows that discoidin induction in bacterially grown cells occurs in two sequential steps. The first is a low basal induction which occurs in late log-phase growth prior to starvation. PSF can induce the basal level, and the induction is independent of G alpha 2. The developmental induction following starvation is much stronger, dependent on G alpha 2 and probably signaled by CMF, which is secreted at that time.(ABSTRACT TRUNCATED AT 250 WORDS)

Folate responsiveness during growth and development of Dictyostelium: separate but related pathways control chemotaxis and gene regulation

Folate-controlled gene expression and chemotaxis have been examined in Dictyostelium wild-type and mutant strains. We show that regulation of the discoidin genes is sensitive to folate in growing cells as well as in suspension development. The signal is transferred via the N10-methylfolate-sensitive folate receptor sites, which also appear to confer the chemotactic response. The strain HG5145 has previously been isolated as a mutant that does not display chemotactic movement towards folate. Nevertheless, these cells are fully functional in folate-mediated downregulation of discoidin I expression. The strain ga 93 has been isolated as an overproducer mutant of the cyclic nucleotide phosphodiesterase inhibitor. Simultaneously, these cells fail to downregulate discoidin I in response to folate but are fully functional in folate chemotaxis. Therefore we conclude that the pathways for chemotaxis and for gene regulation diverge downstream of a common receptor type.

Discoidin proteins of Dictyostelium are necessary for normal cytoskeletal organization and cellular morphology during aggregation

The onset of aggregation of bacterially-grown Dictyostelium discoideum amoebae is accompanied by the accumulation of the discoidin proteins. An immunofluorescent analysis demonstrates that discoidin is distributed throughout the cytoplasm, but is excluded from vesicles and nucleoli. There is no indication of either extracellular or membrane localization. Translocating amoebae of mutants lacking discoidin form more dispersed pseudopodial regions at the cell periphery, possess an abnormally centered microtubule organizing center, are blunt rather than elongate, and lack the tapered posterior uropod characteristic of translocating wild-type cells. However, in spite of the loss of the normal elongate morphology, discoidinless mutants translocate with instantaneous velocities and directional persistence comparable to wild-type cells, and they respond normally to the rapid addition of cAMP. These results demonstrate that the discoidin proteins are cytoplasmic components essential for the maintenance of the elongate cell morphology, cytoskeletal organization and the ability to align with other cells during aggregation. However, the elongate morphology is not a requisite for rapid and persistent single cell translocation.