Selection of Dictyostelium discoideum transformants and analysis of vector maintenance using live bacteria resistant to G418

Protocols for growth and development of Dictyostelium discoideum

Dictyostelium discoideum, a unicellular organism capable of developing into a multicellular structure, is a powerful model system to study a variety of biological processes. Because it is inexpensive and relatively easy to grow, Dictyostelium is also frequently used in teaching laboratories. Here we describe conditions for successfully growing and developing Dictyostelium cells and methods for long-term storage of Dictyostelium amoebae and spores.

A homologue of Cdk8 is required for spore cell differentiation in Dictyostelium

The Cdk8 proteins are kinases which phosphorylate the carboxy terminal domain (CTD) of RNA polymerase II (Pol II) as well as some transcription factors and, therefore, are involved in the regulation of transcription. Here, we report that a Cdk8 homologue from Dictyostelium discoideum is localized in the nucleus where it forms part of a high molecular weight complex that has CTD kinase activity. Insertional mutagenesis was used to abrogate gene function, and analysis of the null strain revealed that the DdCdk8 protein plays an important role in spore formation during late development. As previously reported [Dev. Growth Differ. 44 (2002) 213] Ddcdk8- cells also exhibit impaired aggregation, although we report that the severity of the defect depends upon experimental conditions. When aggregation occurs, Ddcdk8- cells form abnormal terminally differentiated structures within which the Ddcdk8- cells differentiate into stalk cells but fail to form spores, indicating a role for DdCdk8 in cell differentiation. When Ddcdk8 is expressed from its own promoter, the protein is able to rescue both the late developmental defect and the impaired aggregation. However, when expressed from an heterologous promoter, only the impaired aggregation is rescued. This result demonstrates that the defect during late development is not a consequence of impaired aggregation and indicates a direct role for DdCdk8 in spore formation.

Loss of a member of the aquaporin gene family, aqpA affects spore dormancy in Dictyostelium

We isolated and characterized a gene from Dictyostelium discoideum, which encodes a protein of 279 amino acids (30.6kDa) containing six transmembrane domains with two highly conserved motifs of asparagine-proline-alanine (NPA) found in the aquaporin family of water-channel proteins, although the second motif of the protein has been modified into NPV (asparagine-proline-valine). The deduced amino acid sequence of the gene, which we have named aqpA, is 39% identical to D. discoideum WacA, 26% identical to human Aqp5, 26% identical to Oryza sativa PIP2a, 25% identical to yeast Aqy1 and 24% identical to E.coli AqpZ. Southern analyses indicated that aqpA is present as a single copy in the genome. Northern blot analysis showed that the developmentally regulated 1kb mRNA transcript first appears at the tight mound stage (12h), and is abundant in fingers (16h) and late culminants (20h). In-situ hybridization of slugs revealed that aqpA mRNA accumulated in cells of the prespore region but not in those of the prestalk region. Disruption of aqpA by homologous recombination did not significantly affect growth or developmental morphogenesis. Although mutant spores were viable, when assayed soon after encapsulation, they became permeable to propidium iodide and lost viability after a week on the top of a fruiting body. Thus, AqpA is essential to maintain spore dormancy perhaps through the regulation of water flow.

Isolation and characterization of glycogen synthase in Dictyostelium discoideum

We have partially purified the protein and isolated the glcS gene for glycogen synthase in Dictyostelium. glcS mRNA is present throughout development and is the product of a single gene coding for 775 amino acids, with a predicted molecular mass of 87 kD. The sequence is highly similar to glycogen synthase from human muscle, yeast, and rat liver, diverging significantly only at the amino and carboxy termini. Phosphorylation and UDPG binding sites are conserved, with K(m) values for UDPG being comparable to those determined for other organisms, but in vitro phosphorylation failing to convert between the G6P-dependent (D) and -independent (I) forms. Enzyme activity is relatively constant throughout the life cycle: the I form of the enzyme isolates with the soluble fraction in amoebae, switches to the D form, becomes pellet-associated during early development, and finally reverts during late development to the I form, which again localizes to the soluble fraction. Deletion analysis of the promoter reveals a GC-rich element which, when deleted, abolishes expression of glcS.

The Dictyostelium LIM domain-containing protein LIM2 is essential for proper chemotaxis and morphogenesis

We have identified limB, a gene encoding a novel LIM domain-containing protein, LIM2, in a screen for genes required for morphogenesis. limB null cells aggregate, although poorly, but they are unable to undergo morphogenesis, and the aggregates arrest at the mound stage. limB null cells exhibit an aberrant actin cytoskeleton and have numerous F-actin-enriched microspikes. The cells exhibit poor adhesion to a substratum and do not form tight cell-cell agglomerates in suspension. Furthermore, limB null cells are unable to properly polarize in chemoattractant gradients and move very poorly. Expression of limB from a prestalk-specific but not a prespore-specific promoter complements the morphogenetic defects of the limB null strain, suggesting that the limB null cell developmental defect results from an inability to properly sort prestalk cells. LIM2 protein is enriched in the cortex of wild-type cells, although it does not colocalize with the actin cytoskeleton. Our analysis indicates that LIM2 is a new regulatory protein that functions to control rearrangements of the actin cytoskeleton and is required for cell motility and chemotaxis. Our findings may be generally applicable to understanding pathways that control cell movement and morphogenesis in all multicellular organisms. Structure function studies on the LIM domains are presented.

Tdd-4, a DNA transposon of Dictyostelium that encodes proteins similar to LTR retroelement integrases

Tdd-4 is the first DNA transposon to be isolated from Dictyostelium discoideum. This element was isolated by insertion into a target plasmid. Two classes of elements were identified which include a 3.8 kb version and a 3.4 kb deleted version. Sequence analysis reveals that the 145 bp inverted terminal repeats contain the 5'-TGellipsisCA-3' conserved terminal dinucleotides found in prokaryotic transposons and integrated LTR retroelement DNA sequences. Tdd-4 open reading frames are assembled by removal of six introns. Introns 1-5 conform to the GT-AG rule, whereas intron 6 appears to be an AT-AA intron. Also, intron 6 undergoes an alternative 5' splicing reaction. The alternatively spliced region encodes 15 tandem SPXX repeats that are proposed to function as a DNA binding motif. By analogy to other transposons that encode two proteins from the same gene, the full-length Tdd-4 protein is the putative transposase and the truncated Tdd-4 protein is the putative transposition inhibitor. Protein database searches demonstrate Tdd-4 encoded proteins are unique for a DNA element by containing similarities to retroviral/retrotransposon integrases. The putative Tdd-4 transposase contains the same structural relationship as integrases by possessing an N-terminal HHCC motif, a central DDE motif and a C-terminal DNA-binding domain composed of the SPXX motif.

Mechanism of action of the Rep protein from the Dictyostelium Ddp2 plasmid family

The two-hybrid system was used to show that the Rep proteins from three members of the Dictyostelium discoideum Ddp2 plasmid family, Ddp2, Ddp5, and Ddp6, form homomultimers but not heteromultimers when expressed in yeast cells. The results with deletion mutations suggest that multiple regions of the Rep proteins are involved in the multimerization. Electrophoretic mobility shift assays with heterologously expressed and purified Ddp2 Rep protein showed that it is a DNA binding protein. The nucleosomal organization of Ddp2 and Ddp6 in their inverted repeat and promoter regions was investigated. Analysis of mutants derived from the Ddp6 plasmid revealed that its Rep protein is required for nucleosome positioning (i.e., phasing) to occur in the promoter region. On the other hand, nucleosome positioning in the inverted repeat regions of both plasmids is not dependent on Rep protein but on either a feature of the DNA sequence or the binding of cellular factors, perhaps the Dictyostelium origin recognition complex. Rep protein is likely involved in transcription regulation and control of DNA replication, specifically amplification of plasmid at low copy numbers. The formation of homomultimers may be required for their regulatory activity.

A novel, putative MEK kinase controls developmental timing and spatial patterning in Dictyostelium and is regulated by ubiquitin-mediated protein degradation

We have identified a developmentally regulated, putative MEK kinase (MEKKalpha) that contains an F-box and WD40 repeats and plays a complex role in regulating cell-type differentiation and spatial patterning. Cells deficient in MEKKalpha develop precociously and exhibit abnormal cell-type patterning with an increase in one of the prestalk compartments (pstO), a concomitant reduction in the prespore domain, and a loss of the sharp compartment boundaries, resulting in overlapping prestalk and prespore domains. Overexpression of MEKKalpha or MEKKalpha lacking the WD40 repeats results in very delayed development and a severe loss of compartment boundaries. Prespore and prestalk cells are interspersed throughout the slug. Analysis of chimeric organisms suggests that MEKKalpha function is required for the proper induction and maintenance of prespore cell differentiation. We show that the WD40 repeats target MEKKalpha to the cortical region of the cell, whereas the F-box/WD40 repeats direct ubiquitin-mediated MEKKalpha degradation. We identify a UBC and a UBP (ubiquitin hydrolase) that interact with the F-box/WD40 repeats. Our findings indicate that cells lacking the ubiquitin hydrolase have phenotypes similar to those of MEKKalpha null (mekkalpha-) cells, further supporting a direct genetic and biochemical interaction between MEKKalpha, the UBC, and the UBP. We demonstrate that UBC and UBP differentially control MEKKalpha ubiquitination/deubiquitination and degradation through the F-box/WD40 repeats in a cell-type-specific and temporally regulated manner. Our results represent a novel mechanism that includes targeted protein degradation by which MAP kinase cascade components can be controlled. More importantly, our findings suggest a new paradigm of spatial and temporal control of the kinase activity controlling spatial patterning during multicellular development, which parallels the temporally regulated degradation of proteins required for cell-cycle progression.

Spalten, a protein containing Galpha-protein-like and PP2C domains, is essential for cell-type differentiation in Dictyostelium

We have identified a novel gene, Spalten (Spn) that is essential for Dictyostelium multicellular development. Spn encodes a protein with an amino-terminal domain that shows very high homology to Galpha-protein subunits, a highly charged inter-region, and a carboxy-terminal domain that encodes a functional PP2C. Spn is essential for development past the mound stage, being required cell autonomously for prestalk gene expression and nonautonomously for prespore cell differentiation. Mutational analysis demonstrates that the PP2C domain is the Spn effector domain and is essential for Spn function, whereas the Galpha-like domain is required for membrane targeting and regulation of Spn function. Moreover, Spn carrying mutations in the Galpha-like domain that do not affect membrane targeting but affect specificity of guanine nucleotide binding in known GTP-binding proteins are unable to fully complement the spn- phenotype, suggesting that the Galpha-like domain regulates Spn function either directly or indirectly by mediating its interactions with other proteins. Our results suggest that Spn encodes a signaling molecule with a novel Galpha-like regulatory domain.

Coexpression of a constitutively active plasma membrane calcium pump with GFP identifies roles for intracellular calcium in controlling cell sorting during morphogenesis in Dictyostelium

To examine the potential role of calcium in regulating Dictyostelium development, we reduced free cytosolic and total cell Ca2+ in Dictyostelium cells by expressing a constitutively active form of a human erythrocyte plasma membrane calcium pump. The pump-expressing cells lacked a thapsigargin-mediated increase in cytoplasmic calcium, consistent with a reduced level of total cellular Ca2+. During aggregation, the cells initially formed a large number of aggregation centers, many of which coalesced to form mounds that were smaller than those of wild-type cells, and the cells did not exhibit the normal formation of elongated aggregation streams. The majority of the mounds either arrested at this stage with the formation of small protrusions or formed very aberrant finger-like structures, indicating an essential role for cellular calcium in morphogenesis. We used pump and wild-type cells differentially labeled by expressing different wavelength (green and blue) forms of green fluorescent protein and three-dimensional (3-D) reconstruction of serial fluorescent imaging to visualize the movement of pump and wild-type cells within the aggregate. The results showed that the pump cells exhibited very aberrant cell movement and sorting within the forming mound, suggesting that the reduced cytosolic calcium affects movement required for tip formation. When allowed to form chimeric organisms with wild-type cells, pump cells preferentially localized to two bands, one at the prestalk/prespore boundary and the other in the very posterior of the organism, suggesting that pump cells are unable to properly sort. Expression of the calcium pump had little effect on the induction of prestalk- or prespore-specific genes, whereas extended treatment with EGTA blocked induction of both classes of cell-type-specific genes. Our results suggest a role for intracellular Ca2+ in controlling cell sorting and morphogenesis in Dictyostelium.

Dictyostelium discoideum nuclear plasmid Ddp5 is a chimera related to the Ddp1 and Ddp2 plasmid families

The 14,955-bp Dictyostelium discoideum nuclear plasmid Ddp5 contains six transcribed open reading frames. One of these is related to the rep gene of the Ddp2 plasmid, and the other five are related to genes present on the Ddp1 plasmid. The absence of a homolog of the Ddp1 G1 gene, coupled with the presence of the Ddp2 rep gene homolog and of a 1.6-kb inverted repeat analogous to the inverted repeats on members of the Ddp2 plasmid family, suggests that Ddp5 uses Ddp2-like replication and copy number control mechanisms and that it should be assigned to the Ddp2 plasmid family. Ddp5 carries genes homologous to the D1/D3 and D2 genes of the Ddp1 plasmid as well as the Ddp1 G2/G3/D4, G5/D6, and G6/G4/D5 genes. The products of the Ddp5 G2-like, G5-like, and G6-like genes are likely to be transcription factors regulating the expression of themselves and of the other Ddp5 genes. The D1-like and D2-like genes may confer a selective advantage to plasmid-bearing cells, because they can be deleted from plasmid-based shuttle vectors with no apparent effect on vector maintenance. Updated sequence information for the Ddp1 G5/D6, D1/D3, and D2 genes as well as the Dmp1 and Dmp2 G5-like genes is presented. The locations of introns in the G5-like and D1-like genes of Ddp5 and in the homologous genes of the Ddp1, Dmp1, and Dmp2 plasmids were identified. These introns all have GU at the 5' intron border and AG at the 3' intron border, are short (59 to 71 nucleotides), and are AT-rich. A conserved HHCC domain was identified in the G5 proteins; this is a putative zinc binding domain and may be involved in protein-DNA interaction.

Antisense RNA inactivation of gene expression of a cell-cell adhesion protein (gp64) in the cellular slime mold Polysphondylium pallidum

The gp64 protein of Polysphondylium pallidum has been shown to mediate EDTA-stable cell-cell adhesion. To explore the functional role of gp64, we made an antisense RNA expression construct designed to prevent the gene expression of gp64; the construct was introduced into P. pallidum cells and the transformants were characterised. The antisense RNA-expressing clone L3mc2 which had just been harvested at the growth phase tended to re-form in aggregates smaller in size than did the parental cells in either the presence or absence of 10 mM EDTA. In contrast, 6.5-hour starved L3mc2 cells remained considerably dissociated from each other after 5 minutes gyrating, although aggregation gradually increased by 50% during a further 55 minutes gyrating in the presence of 10 mM EDTA. Correspondingly, L3mc2 lacked specifically the cell-cell adhesion protein, gp64. We therefore conclude that the gp64 protein is involved in forming the EDTA-resistant cell-cell contact. In spite of the absence of gp64, L3mc2 exhibited normal developmental processes, a fact which demonstrates that another cell-cell adhesion system exists in the development of Polysphondylium. This is the first report in which an antisense RNA technique was successfully applied to Polysphondylium.

The replication origin position and its relationship to a negative trans-acting transcription regulator encoded by Dictyostelium discoideum nuclear plasmid Ddp1

The replication origin of the Dictyostelium discoideum plasmid Ddp1 was localized to a 543-bp region. This includes most of the AT-rich intergenic region between the G1 and G5/D6 genes containing both of their promoters and multiple copies of a TTTTGACT repeat. The G5/D6 gene, which lies adjacent to, and partially overlaps, the 543-bp origin region, encodes a trans-acting factor that negatively regulates transcription of the G4/D5 gene. Inactivation of the G5/D6 gene led to expression of a transcript (G6) 0.2 kb larger than the D5 transcript from the G4/D5 gene in vegetative and developing cells. The G5/D6 gene also regulates transcription of the G1, G2/G3/D4 and G5/D6 genes either alone or in concert with other Ddp1 gene products.

Growth and developmental functions of a human immunodeficiency virus Tat-binding protein/26S protease subunit homolog from Dictyostelium discoideum

We have characterized a newly identified gene from Dictyostelium discoideum, DdTBP alpha, that encodes a member of the family of eukaryotic proteins. These proteins contain a conserved ATPase domain, include subunits of the 26S protease subunit, and are homologous to the mammalian human immunodeficiency virus Tat-binding protein TBP1. While information indicates that some family members are involved in the regulation of transcription in mammalian and yeast cells during growth, these proteins are also involved in other cellular functions, and nothing is known about their possible function in multicellular development. The Dictyostelium DdTBP alpha gene is developmentally regulated, with its expression at the highest levels occurring during growth and early development. The gene is present in two copies in the genome. Disruption of one copy by homologous recombination leads to aberrant morphogenesis, which lasts from the formation of the first finger until the onset of culmination. The gene appears to be essential for growth since we were unable to obtain a complete null phenotype and since expression of an inducible antisense construct in the partial null background resulted in cell death. Expression of the antisense construct during development accentuated the partial null phenotype and also resulted in very abnormal fruiting bodies. Overexpression of DdTBP alpha from its own promoter leads to very large multinucleated vegetative cells when the cells are grown in suspension culture. When the cells are plated onto petri dishes in growth medium, they rapidly split into multiple cells containing one to two nuclei, in a manner similar to that of wild-type cells. Overexpressing cells are significantly delayed in forming a multicellular aggregate, but development proceeds normally once the first finger stage is reached. The results indicate that DdTBP alpha plays an important role in regulating both growth and morphogenesis in D. discoideum.

An autonomously propagating luciferase-encoding vector for Dictyostelium discoideum

We have constructed a luc reporter vector for Dictyostelium discoideum using a 626-bp fragment from the nuclear-associated plasmid Ddp2. The ori from Ddp2 is localized within this fragment and was used to provide an autonomous replication sequence for the reporter vector. This reporter vector was stably retained in D. discoideum AX3K cells without alteration. The vector molecule was also found to exist in relatively low copy number compared to other Dictyostelium vectors in the transformed cells. We demonstrated the utility of this vector as a reporter vector with glycogen synthase promoter/luc fusions of varying sizes.

Expression of a myosin regulatory light chain phosphorylation site mutant complements the cytokinesis and developmental defects of Dictyostelium RMLC null cells

In a number of systems phosphorylation of the regulatory light chain (RMLC) of myosin regulates the activity of myosin. In smooth muscle and vertebrate nonmuscle systems RMLC phosphorylation is required for contractile activity. In Dictyostelium discoideum phosphorylation of the RMLC regulates both ATPase activity and motor function. We have determined the site of phosphorylation on the Dictyostelium RMLC and used site-directed mutagenesis to replace the phosphorylated serine with an alanine. The mutant light chain was then expressed in RMLC null Dictyostelium cells (mLCR-) from an actin promoter on an integrating vector. The mutant RMLC was expressed at high levels and associated with the myosin heavy chain. RMLC bearing a ser13ala substitution was not phosphorylated in vitro by purified myosin light chain kinase, nor could phosphate be detected on the mutant RMLC in vivo. The mutant myosin had reduced actin-activated ATPase activity, comparable to fully dephosphorylated myosin. Unexpectedly, expression of the mutant RMLC rescued the primary phenotypic defects of the mlcR- cells to the same extent as did expression of wild-type RMLC. These results suggest that while phosphorylation of the Dictyostelium RMLC appears to be tightly regulated in vivo, it is not essential for myosin-dependent cellular functions.

Identification and functional analysis of a developmentally regulated extracellular signal-regulated kinase gene in Dictyostelium discoideum

We have cloned a developmentally regulated mitogen-activated protein kinase (extracellular signal-regulated kinase) from Dictyostelium discoideum designated ERK1. Using anti-pTyr antibodies, we show that ERK1 is phosphorylated on tyrosine in vivo and that it will phosphorylate myelin basic protein. The gene expresses two transcripts, one that is preferentially expressed during vegetative growth and early development and one that is induced during the multicellular stages. Developmental Western blots (immunoblots) using anti-ERK1 antibodies indicate that ERK1 is present throughout development. ERK1/lacZ reporter constructs suggest that, in the multicellular stages, the gene is preferentially expressed in a subpopulation of cells scattered throughout the organism, similar to the pattern seen with anterior-like cell markers. Antisense mutagenesis from a derepressible promoter indicates that ERK1 is essential for vegetative growth. Overexpression of ERK1 from either the Actin 15 promoter or the ERK1 promoter results in abnormal morphogenesis starting at the slug stage. Overexpression of ERK1 in null mutants of the phosphotyrosine phosphatase PTP2 results in the production of large aggregation streams and subsequent abnormal morphogenesis that indicate a genetic interaction between ERK1 and PTP2. These cells produce very large aggregation streams that break up into very small mounds that undergo abnormal morphogenesis. The genetic interaction between ERK1 and PTP2 appears to be specific since overexpression of ERK1 in a ptp1- null mutant does not produce the same phenotype. Our results indicate that ERK1 plays an essential role during the growth and differentiation of D. discoideum.

Identification and functional analysis of a developmentally regulated extracellular signal-regulated kinase gene in Dictyostelium discoideum

We have cloned a developmentally regulated mitogen-activated protein kinase (extracellular signal-regulated kinase) from Dictyostelium discoideum designated ERK1. Using anti-pTyr antibodies, we show that ERK1 is phosphorylated on tyrosine in vivo and that it will phosphorylate myelin basic protein. The gene expresses two transcripts, one that is preferentially expressed during vegetative growth and early development and one that is induced during the multicellular stages. Developmental Western blots (immunoblots) using anti-ERK1 antibodies indicate that ERK1 is present throughout development. ERK1/lacZ reporter constructs suggest that, in the multicellular stages, the gene is preferentially expressed in a subpopulation of cells scattered throughout the organism, similar to the pattern seen with anterior-like cell markers. Antisense mutagenesis from a derepressible promoter indicates that ERK1 is essential for vegetative growth. Overexpression of ERK1 from either the Actin 15 promoter or the ERK1 promoter results in abnormal morphogenesis starting at the slug stage. Overexpression of ERK1 in null mutants of the phosphotyrosine phosphatase PTP2 results in the production of large aggregation streams and subsequent abnormal morphogenesis that indicate a genetic interaction between ERK1 and PTP2. These cells produce very large aggregation streams that break up into very small mounds that undergo abnormal morphogenesis. The genetic interaction between ERK1 and PTP2 appears to be specific since overexpression of ERK1 in a ptp1- null mutant does not produce the same phenotype. Our results indicate that ERK1 plays an essential role during the growth and differentiation of D. discoideum.

Plasmid maintenance functions encoded on Dictyostelium discoideum nuclear plasmid Ddp1

All of the plasmid-carried genes expressed during vegetative growth are essential for long-term maintenance of plasmid Ddp1 in the nucleus of Dictyostelium discoideum. Deletion of Ddp1 genes expressed only during development had no detectable effect on plasmid maintenance. Deletion of vegetatively expressed genes, either singly or in pairs, resulted in (i) a rapid loss of plasmid from cells grown in the absence of selection for plasmid retention, (ii) variation in the proportion of monomer to multimer forms of the plasmid molecules, and/or (iii) abnormalities in plasmid copy number. At least two plasmid-encoded gene products influence patterns of expression of plasmid genes.

Plasmid maintenance functions encoded on Dictyostelium discoideum nuclear plasmid Ddp1

All of the plasmid-carried genes expressed during vegetative growth are essential for long-term maintenance of plasmid Ddp1 in the nucleus of Dictyostelium discoideum. Deletion of Ddp1 genes expressed only during development had no detectable effect on plasmid maintenance. Deletion of vegetatively expressed genes, either singly or in pairs, resulted in (i) a rapid loss of plasmid from cells grown in the absence of selection for plasmid retention, (ii) variation in the proportion of monomer to multimer forms of the plasmid molecules, and/or (iii) abnormalities in plasmid copy number. At least two plasmid-encoded gene products influence patterns of expression of plasmid genes.

Regulation by protein-tyrosine phosphatase PTP2 is distinct from that by PTP1 during Dictyostelium growth and development

We have cloned a gene encoding a second Dictyostelium discoideum protein-tyrosine phosphatase (PTP2) whose catalytic domain has approximately 30 to 39% amino acid identity with those of other PTPs and a 41% amino acid identity with D. discoideum PTP1. Like PTP1, PTP2 is a nonreceptor PTP with the catalytic domain located at the C terminus of the protein. PTP2 has a predicted molecular weight of 43,000 and possesses an acidic 58-amino-acid insertion 24 amino acids from the N terminus of the conserved catalytic domain. PTP2 transcripts are expressed at moderate levels in vegetative cells and are induced severalfold at the onset of development. Studies with a PTP2-lacZ reporter gene fusion indicate that PTP2, like PTP1, is preferentially expressed in prestalk and anterior-like cell types during the multicellular stages of development. PTP2 gene disruptants (ptp2 null cells) are not detectably altered in growth and show a temporal pattern of development similar to that of wild-type cells. ptp2 null slugs and fruiting bodies, however, are significantly larger than those of wild-type slugs, suggesting a role for PTP2 in regulating multicellular structures. D. discoideum strains overexpressing PTP2 from the PTP2 promoter exhibit growth rate and developmental abnormalities, the severity of which corresponds to the level of PTP2 overexpression. Strains with high overexpression of the PTP2 gene grow slowly on bacterial lawns and produce small cells in axenic medium. When development is initiated in these strains, cells are able to aggregate but then stop further morphogenesis for 6 to 8 h, after which time a variable fraction of these aggregates continue with normal timing, producing diminutive fruiting bodies. These disruption and overexpression phenotypes for PTP2 are distinct from the corresponding mutant PTP1 phenotypes. Immunoprobing PTP2 mutant strains during growth and development with antiphosphotyrosine antibodies reveals several changes in the tyrosine phosphorylation of proteins in PTP2 mutant strains compared with that in wild-type cells. These changes are different from those identified in the previously characterized corresponding PTP1 disruption and overexpression mutant strains. Thus, although PTP2 and PTP1 are nonreceptor PTPs with similar spatial patterns of expression, our findings suggest that they possess distinct regulatory functions in controlling D. discoideum growth and development.

Regulation by protein-tyrosine phosphatase PTP2 is distinct from that by PTP1 during Dictyostelium growth and development

We have cloned a gene encoding a second Dictyostelium discoideum protein-tyrosine phosphatase (PTP2) whose catalytic domain has approximately 30 to 39% amino acid identity with those of other PTPs and a 41% amino acid identity with D. discoideum PTP1. Like PTP1, PTP2 is a nonreceptor PTP with the catalytic domain located at the C terminus of the protein. PTP2 has a predicted molecular weight of 43,000 and possesses an acidic 58-amino-acid insertion 24 amino acids from the N terminus of the conserved catalytic domain. PTP2 transcripts are expressed at moderate levels in vegetative cells and are induced severalfold at the onset of development. Studies with a PTP2-lacZ reporter gene fusion indicate that PTP2, like PTP1, is preferentially expressed in prestalk and anterior-like cell types during the multicellular stages of development. PTP2 gene disruptants (ptp2 null cells) are not detectably altered in growth and show a temporal pattern of development similar to that of wild-type cells. ptp2 null slugs and fruiting bodies, however, are significantly larger than those of wild-type slugs, suggesting a role for PTP2 in regulating multicellular structures. D. discoideum strains overexpressing PTP2 from the PTP2 promoter exhibit growth rate and developmental abnormalities, the severity of which corresponds to the level of PTP2 overexpression. Strains with high overexpression of the PTP2 gene grow slowly on bacterial lawns and produce small cells in axenic medium. When development is initiated in these strains, cells are able to aggregate but then stop further morphogenesis for 6 to 8 h, after which time a variable fraction of these aggregates continue with normal timing, producing diminutive fruiting bodies. These disruption and overexpression phenotypes for PTP2 are distinct from the corresponding mutant PTP1 phenotypes. Immunoprobing PTP2 mutant strains during growth and development with antiphosphotyrosine antibodies reveals several changes in the tyrosine phosphorylation of proteins in PTP2 mutant strains compared with that in wild-type cells. These changes are different from those identified in the previously characterized corresponding PTP1 disruption and overexpression mutant strains. Thus, although PTP2 and PTP1 are nonreceptor PTPs with similar spatial patterns of expression, our findings suggest that they possess distinct regulatory functions in controlling D. discoideum growth and development.

Dictyostelium giganteum plasmid Dgp1 is a member of the Ddp2 plasmid family

Dgp1, a circular 4.4-kb plasmid found in the nuclei of Dictyostelium giganteum strain DG61, is a member of the same plasmid family as plasmids Ddp2 and pDG1. Dgp1 has sequence similarity to a conserved region of the Ddp2 and pDG1 open reading frames. As with Ddp2 and pDG1, a single large RNA is transcribed from Dgp1. This 3.3-kb transcript is present at about 350 copies per vegetative cell. The transcript abundance decreased about 10-fold in early aggregation and continued at this lower level until late culmination when it returned to the level seen in vegetative cells. Dgp1 has a repeat of several hundred base pairs in a location, relative to the transcribed region, similar to the inverted repeats found in Ddp2 and pDG1. Dgp1 cannot be maintained as a plasmid in Dictyostelium discoideum AX4 cells, suggesting that Dgp1 carries species-specific maintenance elements.