Sequence and expression of the discoidin I gene family in Dictyostelium discoideum

Studies on the transcription, translation, and structure of alpha-actinin in Dictyostelium discoideum

A clone coding for the F-actin cross-linking protein alpha-actinin was obtained by screening a genomic library of Dictyostelium discoideum DNA in lambda gt11 with monoclonal antibodies specific for Dictyostelium alpha-actinin. The 1.2-kilobase (kb) genomic clone was confirmed as containing part of the alpha-actinin gene by comparing its nucleotide sequence with the amino acid sequence of tryptic peptides from purified alpha-actinin. The clone recognized a 3.0-kb message in a Northern blot. Hybridization to RNA isolated from different developmental stages of several D. discoideum strains indicated that the mRNA content increased during early development. A similar result was obtained when the alpha-actinin content of the cells was followed by Western blot analysis. Hybridization of the clone to DNA from different wild-type strains of D. discoideum indicated a polymorphism on the DNA level that coincided with a polymorphism on the protein level. The data suggest continuous transcription of the alpha-actinin gene throughout the development of D. discoideum, up- and down-regulation of the levels of alpha-actinin mRNA and protein with maximum levels at the onset of aggregation, and a high diversity of alpha-actinin at the DNA and protein level among different D. discoideum strains. The structural data make it conceivable that the highly conserved nature of alpha-actinin resides only at the functional sites, whereas the helical portions of the alpha-actinin molecule allow a higher level of diversity throughout evolution.

Nucleotide sequence of chick 14K beta-galactoside-binding lectin mRNA

cDNA for chick 14K beta-galactoside-binding lectin mRNA was cloned and the nucleotide sequence determined. The deduced amino acid sequence and the results of in vitro translation of its mRNA suggest that this lectin does not include any cleavable signal sequence while it exists in extracellular matrix. Comparison of the primary structures has shown that chick 14K lectin includes some regions homologous to those in discoidin I, which is also known to be located in extracellular matrix and lack signal peptide. The results imply some relation between these two lectins in spite of their great phylogenetic separation.

Phenocopy of discoidin I-minus mutants by antisense transformation in Dictyostelium

Using an antisense construct of the discoidin gene transfected into Dictyostelium, we have repressed the expression of the three endogenous discoidin genes. Transformants exhibit a greater than 90% reduction in accumulated discoidin mRNA and protein. Nuclear run-on assays show that both the endogenous and the antisense genes are transcribed. Since only minor amounts of endogenous gene transcripts and none from the antisense gene can be detected on blots, we suggest that hybrids are formed within the nucleus and are rapidly degraded. Discoidin is believed to play a role in cell-substratum interaction and exhibits homologies to fibronectin. Discoidin-minus mutants exhibit the developmental phenotype of not streaming on a plastic surface. Antisense transformants show a similar phenotype and are thus phenocopies of these mutants.

Receptor for the cell binding site of discoidin I

Discoidin I, a developmentally regulated lectin in Dictyostelium discoideum, has been implicated in cell-substratum adhesion and ordered cell migration during aggregation. This depends on the cell binding site of discoidin I, which is distinct from its carbohydrate binding site. We have isolated a receptor for the cell binding site by affinity chromatography. The receptor binds immobilized discoidin I in the presence of 0.3 M galactose and can be eluted with gly-arg-gly-asp-his-asp, a synthetic peptide the sequence of which is found in discoidin I, and which blocks cell migration into aggregates. The receptor is a developmentally regulated cell-surface glycoprotein of apparent Mr approximately 67,000. Univalent antibodies specific for this glycoprotein block aggregation.

A 125/115-kDa cell surface receptor specific for vitronectin interacts with the arginine-glycine-aspartic acid adhesion sequence derived from fibronectin

Affinity chromatography was used to identify a cell surface receptor for the adhesive protein vitronectin. Detergent extracts of human osteosarcoma (MG-63) cells were chromatographed on either vitronectin-Sepharose or Sepharose linked to the synthetic peptide Gly-Arg-Gly-Asp-Ser-Pro, which includes the fibronectin cell attachment sequence Arg-Gly-Asp. Two cell surface proteins with apparent molecular mass of 125 and 115 kDa bound to both columns and were specifically eluted with a solution containing the Gly-Arg-Gly-Asp-Ser-Pro peptide. These proteins could be incorporated into phosphatidylcholine liposomes and mediated the specific binding of these liposomes to vitronectin but not to fibronectin. In contrast, liposomes containing a previously identified 140-kDa fibronectin receptor, which interacts with the Arg-Gly-Asp sequence in fibronectin, did not bind to vitronectin. Thus, the fibronectin and vitronectin receptors each recognize the Gly-Arg-Gly-Asp-Ser-Pro peptide but exhibit mutually exclusive reactivities toward fibronectin and vitronectin. These receptors appear to belong to a family of proteins that mediate cell substratum adhesion via related but subtly different specificities.

Discoidin I, an endogenous lectin, is externalized from Dictyostelium discoideum in multilamellar bodies

Discoidin I, a soluble lectin synthesized by aggregating Dictyostelium discoideum and implicated in their adhesion to the substratum, is localized in multilamellar bodies both intracellularly and upon externalization. These structures also contain a glycoconjugate that binds discoidin I. The multilamellar bodies apparently serve to package the lectin for externalization, and may then gradually release it to function extracellularly.

High-copy-number transformants and co-transformation in Dictyostelium

We have recently established a DNA-mediated transformation system for Dictyostelium. The vector (pB10) contains the promoter from the Dictyostelium actin 6 gene fused to the NmR gene from Tn5 which confers resistance to antibiotic G418. Dictyostelium cells can be stably transformed and express kanamycin phosphotransferase (APHII). There is an average of three to five copies of vector DNA in transformed populations. We have fused an A + T-rich region containing the 3' end of the Dictyostelium actin (Act) 8 gene to the end of the Act6-NmR fusion. Though the fragment is inserted in reverse orientation, this adds a transcription termination and/or 3' processing site and results in the formation of a discretely sized mRNA from the Act6-NmR gene fusion. Using this vector, the number of transformants increases by approx. 5-10-fold. We also describe conditions that allow for the isolation of transformants having a high copy number of vector DNA per cell (approx. 150 copies/cell). In addition, we show that cells can be co-transformed with the transformation vector and other pBR322 derivatives. Both plasmid DNAs are present in transformed Dictyostelium cells in high-Mr DNA. When cells are grown under selective conditions in the presence of the antibiotic G418, both DNAs are present in high copy number and Dictyostelium genes present on both vectors are transcribed and are properly regulated under the conditions examined. These modifications of the original transformation system should facilitate the introduction of modified genes into Dictyostelium to study gene regulation during development and allow one to examine the effects of high gene dosage.

Discoidin I is implicated in cell-substratum attachment and ordered cell migration of Dictyostelium discoideum and resembles fibronectin

All three forms of discoidin I, an endogenous N-acetylgalactosamine-binding lectin from D. discoideum, contain the amino acid sequence gly-arg-gly-asp also found in fibronectin and implicated in its attachment to cells. Synthetic peptides containing these and adjacent amino acids of discoidin I block organized streaming during aggregation of D. discoideum and, at higher concentrations, block cell attachment and spreading on a plastic surface and formation of fruiting bodies. Pure discoidin I (with or without N-acetylgalactosamine) and univalent anti-discoidin I also block formation of streams during aggregation. Two mutants of D. discoideum with low levels of discoidin I apparently reflect the deficiency of this endogenous lectin by failing to form streams or to spread on plastic and by a partial failure to enter aggregates. Together, the results indicate that discoidin I functions like fibronectin to promote cell attachment and spreading as well as ordered cellular migration during morphogenesis.

Structure of human factor VIII

The deduced amino acid sequence of human factor VIII, obtained from the DNA sequence, predicts a mature polypeptide of 2,332 amino acids containing a triplicated domain structure. The polypeptide has 35% sequence homology with the copper-binding plasma protein, ceruloplasmin. Determination of the thrombin cleavage sites in plasma-derived factor VIII polypeptides allows prediction of the domains involved in the associated activation and inactivation of the protein.

Rhodopseudomonas blastica atp operon. Nucleotide sequence and transcription

The nucleotide sequence has been determined of a 12,368 base-pair region of DNA cloned from the non-sulphur photosynthetic bacterium Rhodopseudomonas blastica. It contains a cluster of six genes of which five encode the subunits of F1-ATPase; the sixth codes for an unknown protein. The genes are arranged in the same order as in the Escherichia coli unc operon, except that the unknown gene is placed between those for gamma and beta subunits. Neither the genes for F0 subunits, nor a homologue of the E. coli uncI gene is associated with this locus. The six genes are transcribed from a single promoter and we have designated this region the R. blastica atp operon. The two distal genes, beta and epsilon, may also be transcribed from a second promoter. Initiation and termination points for transcription have been identified by primer extensions and S1 nuclease mapping experiments. Signals involved in initiation of translation (Shine and Dalgarno sequences) and termination of transcription in the photosynthetic bacterium resemble those in E. coli. However, no common features can be identified in these two bacteria between 5' regions adjacent to sites of initiation of transcription. The sequence also contains a gene that encodes a protein homologous to discoidin, a cell surface lectin of Dictyostelium discoideum thought to be involved in cell--cell aggregation. Seven other reading frames have not been identified.

Variants of the cell recognition site of fibronectin that retain attachment-promoting activity

A tetrapeptide sequence, Arg-Gly-Asp-Ser, is the minimal structure recognized by cells in the large, adhesive glycoprotein fibronectin. We now have defined the structural requirements for this cell recognition site by testing several synthetic variants of the active tetrapeptide sequence. The conservative substitutions of lysine for arginine, alanine for glycine, or glutamic acid for aspartic acid each resulted in abrogation of the cell attachment-promoting activity characteristic of the natural sequence. However, in the position of the serine residue, some alterations were compatible with activity. Assay of peptides containing the structure Arg-Gly-Asp-X (where X = another amino acid residue) showed that an Arg-Gly-Asp-Val sequence predicted to be present in some, but not all, fibronectin molecules as a result of alternative RNA splicings could potentially create a second cell attachment site in those fibronectin polypeptide chains carrying that sequence. Other proteins with potentially active Arg-Gly-Asp-X sequences include several proteins that are known to interact with the cell surface. Among these are various types of collagens, thrombin, and discoidin, a slime-mold protein that may be involved in cell aggregation. The result presented here show that the arginine, glycine, and aspartic acid residues are absolutely required for the cell recognition, and that the surrounding amino acids may play a role in the expression of cell attachment activity in fibronectin and other proteins having this sequence. We suggest, based on these data, that this recognition mechanism may be common to a number of biological systems.

Overproduction of discoidin I by a temperature-sensitive motility mutant of Dictyostelium discoideum

Dictyostelium discoideum MC2 is a temperature-sensitive motility mutant of AX3. Mutant cells are incapable of growth, phagocytosis, and migration under restrictive conditions (Kayman et al., J. Cell Biol. 92:705-711, 1982). We show here that at the restrictive temperature MC2 cells grown axenically or on bacteria synthesized excessive quantities of the lectin discoidin I. By two-dimensional gel electrophoresis and peptide mapping, the proteins overproduced by MC2 cells were indistinguishable from discoidin I synthesized at lower levels in AX3 cells. At least two of the three species of discoidin I were overproduced. This protein family constituted 9% of the total protein in cells that were incubated overnight at 27 degrees C in axenic medium. Although MC2 cells are defective in nutrient uptake under restrictive conditions, the overproduction of discoidin I did not appear to be part of a pleiotropic response to starvation. We propose that transcription of the coordinately regulated discoidin I genes is altered in mutant cells. This alteration may be related to the motility defects manifested by MC2.

Genomic instability and mobile genetic elements in regions surrounding two discoidin I genes of Dictyostelium discoideum

We have found that the genomic regions surrounding the linked discoidin I genes of various Dictyostelium discoideum strains have undergone rapid changes. Wild-type strain NC-4 has three complete discoidin I genes; its axenic derivative strain Ax-3L has duplicated a region starting approximately 1 kilobase upstream from the two linked genes and extending for at least 8 kilobases past the genes. A separately maintained stock, strain Ax-3K, does not have this duplication but has undergone a different rearrangement approximately 3 kilobases farther upstream. We show that there are repeat elements in these rapidly changing regions. At least two of these elements, Tdd-2 and Tdd-3, have characteristics associated with mobile genetic elements. The Tdd-3 element is found in different locations in related strains and causes a 9- to 10-base-pair duplication of the target site DNA. The Tdd-2 and Tdd-3 elements do not cross-hybridize, but they share a 22-base-pair homology near one end. At two separate sites, the Tdd-3 element has transposed into the Tdd-2 element, directly adjacent to the 22-base-pair homology. The Tdd-3 element may use this 22-base-pair region as a preferential site of insertion.

Overproduction of discoidin I by a temperature-sensitive motility mutant of Dictyostelium discoideum

Dictyostelium discoideum MC2 is a temperature-sensitive motility mutant of AX3. Mutant cells are incapable of growth, phagocytosis, and migration under restrictive conditions (Kayman et al., J. Cell Biol. 92:705-711, 1982). We show here that at the restrictive temperature MC2 cells grown axenically or on bacteria synthesized excessive quantities of the lectin discoidin I. By two-dimensional gel electrophoresis and peptide mapping, the proteins overproduced by MC2 cells were indistinguishable from discoidin I synthesized at lower levels in AX3 cells. At least two of the three species of discoidin I were overproduced. This protein family constituted 9% of the total protein in cells that were incubated overnight at 27 degrees C in axenic medium. Although MC2 cells are defective in nutrient uptake under restrictive conditions, the overproduction of discoidin I did not appear to be part of a pleiotropic response to starvation. We propose that transcription of the coordinately regulated discoidin I genes is altered in mutant cells. This alteration may be related to the motility defects manifested by MC2.

Protein synthesis during development and differentiation in the cellular slime mould Dictyostelium discoideum

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Genomic instability and mobile genetic elements in regions surrounding two discoidin I genes of Dictyostelium discoideum

We have found that the genomic regions surrounding the linked discoidin I genes of various Dictyostelium discoideum strains have undergone rapid changes. Wild-type strain NC-4 has three complete discoidin I genes; its axenic derivative strain Ax-3L has duplicated a region starting approximately 1 kilobase upstream from the two linked genes and extending for at least 8 kilobases past the genes. A separately maintained stock, strain Ax-3K, does not have this duplication but has undergone a different rearrangement approximately 3 kilobases farther upstream. We show that there are repeat elements in these rapidly changing regions. At least two of these elements, Tdd-2 and Tdd-3, have characteristics associated with mobile genetic elements. The Tdd-3 element is found in different locations in related strains and causes a 9- to 10-base-pair duplication of the target site DNA. The Tdd-2 and Tdd-3 elements do not cross-hybridize, but they share a 22-base-pair homology near one end. At two separate sites, the Tdd-3 element has transposed into the Tdd-2 element, directly adjacent to the 22-base-pair homology. The Tdd-3 element may use this 22-base-pair region as a preferential site of insertion.

Monoclonal antibody against cytoplasmic lectins of Dictyostelium discoideum: cross-reactivity with a membrane glycoprotein, contact site A, and with E. coli beta-galactosidase and lac repressor

Monoclonal antibodies were raised against two soluble, galactose-binding lectins from cells of Dictyostelium discoideum, discoidin I and II. These antibodies reacted not only with both discoidins, but also with a plasma membrane glycoprotein of aggregation competent cells, called contact site A, and with two carbohydrate-binding proteins of E. coli, beta-galactosidase and lac repressor. The possibility that the antibody recognizes a structure common to different carbohydrate-binding proteins is discussed. The two carbohydrate-binding proteins of E. coli share with discoidin I the sequence -Ser-X-X-Ile-His(Pro)-Pro(His)-Leu-Thr- which might be responsible for the cross-reactivity.

Conserved structural features are found upstream from the three co-ordinately regulated discoidin I genes of Dictyostelium discoideum

The discoidin I genes of Dictyostelium form a small, co-ordinately regulated multigene family. We have sequenced and compared the upstream regions of the DiscI-alpha, -beta and -gamma genes. For the most part the upstream regions of the three genes are non-homologous. The upstream sequences of the beta and gamma genes are exceedingly A + T-rich, while those of the alpha gene are less so. All three genes have a relatively G + C-rich region 20 to 40 base-pairs in length, found approximately 200 base-pairs 5' to the messenger RNA start site. This G + C-rich region 5' to the beta and gamma genes is flanked by short inverted repeats. Within this region, there is an 11 base-pair exact homology between the alpha and gamma genes, and a less perfect homology between these genes and the beta gene. The homology is flanked at a short distance by interspersed G and T residues. The gamma gene is greater than 90% A + T for greater than 800 base-pairs upstream. Further upstream there is a G + C-rich region that is also found inverted approximately 3.5 X 10(3) base-pairs away. The gamma and beta genes are tandemly linked, and the entire approximately 500 base-pair intergene region between the 3' end of the gamma gene and the 5' end of the beta gene is A + T-rich (approximately 90%) with the exception of the homology region 5' to the gamma gene. We demonstrate also the presence of a discoidin I pseudogene fragment having only 139 base-pairs of discoidin homology with greater than 8% mismatch. It is flanked upstream by five 39 base-pair G + C-rich repeats, and downstream by sequences that are extremely A + T-rich. We discuss the possible significance of the conserved G + C-rich structures on discoidin I gene expression.

An unusual transposon encoding heat shock inducible and developmentally regulated transcripts in Dictyostelium

We have isolated and characterized a transposable element from Dictyostelium discoideum denoted Tdd-1. There are approximately 50 complete copies of the element per haploid genome and approximately 100-150 partial elements. Southern blots of DNA from different Dictyostelium discoideum strains show that Tdd-1 is a mobile element. Tdd-1 is 4.9 kb long, with 313 bp inverted repeats. These repeats lie near the termini, but unlike other transposable elements one end of Tdd-1 extends 36 bp past the repeat and the other extends 1 bp. Tdd-1 encodes a series of developmentally regulated transcripts, all with the same polarity, that increase dramatically in abundance after approximately 10 hr of development. We have also identified a heat shock inducible transcript of the opposite polarity.

Mutants of Dictyostelium discoideum blocked in expression of all members of the developmentally regulated discoidin multigene family

Mutant strains of D. discoideum are described that can complete morphogenesis and cytodifferentiation but which express vastly reduced levels of the galactose-binding lectins discoidin I and II (less than 1% and 1%-2% respectively) compared to the wild-type control. Mutant cells proceeding through development lack lectin activity, lectin protein, and specific lectin mRNA. In contrast, the genes encoding these proteins are present in their wild-type configurations in the genome. Since these proteins are encoded by four to five discrete genes, the mutations in these strains are most likely in genes involved in the regulation of the expression of members of this multigene family. The results also indicate that the discoidin lectins may not be required for fruiting body construction in this organism. Finally, coupled with the recent ability to transform D. discoideum, these mutants open the way to identification and isolation of regulatory genes and their products.

Ion dependence of the discoidin I lectin from Dictyostelium discoideum

The lectin discoidin I from Dictyostelium discoideum requires divalent cations for binding activity. The data indicate that calcium is the preferred ion in vitro. In contrast, the lectin activity of discoidin II is independent of divalent ions.

Sequence organization in Dictyostelium: unique structure at the 5'-ends of protein coding genes

We have compared the sequences which lie 5' to the coding regions of 15 Dictyostelium genes transcribed by RNA polymerase II. These sequences are extremely (approximately 90%) A + T rich and contain extensive homopolymeric regions. Like most eukaryotic genes, those in Dictyostelium possess a TATA or Goldberg-Hogness Box approximately 25-35bp 5' to the site of transcription initiation. In addition, each gene contains an oligo (dT) stretch between the TATA Box and mRNA start site; this oligo (dT) sequence is, thus far, unique to Dictyostelium. We suggest that the TATA-oligo (dT) structure is an essential component of the Dictyostelium promoter. The general sequence structure of coding, non-coding and untranscribed flanking regions in Dictyostelium is also discussed.

Cell-type-specific actin mRNA populations in dictyostelium discoideum

We have analyzed actin mRNA sequences present in the terminal stages of the development of Dictyostelium discoideum. Four different actin mRNA sequences were detected in migrating pseudoplasmodia. Nucleotide sequence analysis of primer-extension products derived from the four mRNA sequences showed that they each encoded an actin protein with the same eight N-terminal amino acids and that they did not derive from transcription of any previously characterized actin gene. Preculmination pseudoplasmodia of Dictyostelium contain two distinct populations of committed cells, termed prespore and prestalk cells. We show that prestalk cells contain all four of the actin mRNA sequences found in pseudoplasmodia, while prespore cells contain only three of the sequences, and mature spores contain only two. Thus there is a differential loss of actin mRNA sequences during spore-cell differentiation in Dictyostelium.

A DNA-mediated transformation system for Dictyostelium discoideum

We have established a transformation system for Dictyostelium discoideum. The transformation vector contains the protein coding region of the Tn5 neomycin resistance gene fused to the proposed promoter of the Dictyostelium actin 8 gene; the vector also contains a sequence that acts as an autonomously replicating sequence (ars) in yeast. Using this vector, we can transform Dictyostelium vegetative amoebae to be resistant to aminoglycoside G418 at a frequency of between 10(-6) and 10(-4) of the input cells. The transformed cell lines are stable and contain vector sequences integrated within chromosomal DNA.

Discoidins I and II: common and unique regions on two lectins implicated in cell--cell cohesion in Dictyostelium discoideum

As Dictyostelium discoideum amoebae differentiate from the noncohesive to the mutually cohesive state, they synthesize two galactose-binding lectins--discoidins I and II--which have been implicated as obligatory components of the morphogenetic cell-cell recognition and cohesion system. These proteins have been shown to have similar amino acid compositions and subunit Mr and overlapping but distinct carbohydrate recognition specificities. We have performed extensive immunochemical and biochemical analyses to study the structural relationships between these two molecules and to eventually identify structural and functional domains. Antisera raised against highly purified preparations of discoidin I and discoidin II were tested for their reactivities against each protein by both immunoprecipitation and double diffusion analyses. The patterns of crossreactivity indicated the presence of shared as well as unique antigenic determinants. This interpretation was supported by two-dimensional thin-layer peptide map analysis and by studies with purified peptides. Of approximately 10-12 peptides observed after exhaustive tryptic digestion of each radioiodinated lectin, 3 appeared to be common to both. These putative common peptides were purified, and the corresponding peptides from discoidins I and II were found to behave identically by two-dimensional thin-layer analysis, gel filtration, and susceptibility to chymotrypsin. The finding of common and unique regions in discoidins I and II suggests analogies with other families of recognition proteins and may have important functional implications for these cell-cell recognition molecules.

Differential expression of the members of the discoidin I multigene family during growth and development of Dictyostelium discoideum

Discoidin I and II are lectins synthesized during the aggregation of Dictyostelium discoideum amoebae which may play a role in cellular cohesion. Discoidin I was thought to consist of two major polypeptides, but we show that there are three. The N-terminal amino acid sequence of the polypeptides has been predicted by determining part of the nucleotide sequence of their respective mRNAs. We obtained the nucleotide sequences by reverse transcription of the mRNAs, using as primers, fragments derived from the coding region of two cloned discoidin I sequences, and utilizing cross hybridization to the various mRNA species and differences in the length of their 5' noncoding regions to isolate fragments for DNA sequencing. We used primer extension to measure the relative concentration of the three major discoidin I mRNA sequences. We show that during development changes in the abundance of all three mRNA sequences occur coordinately. In cells growing in nutrient broth, however, only two of the three major discoidin I mRNA sequences accumulate, and if such cells are grown to a very high density, both sequences disappear. These results indicate that the coordination of discoidin I gene expression is not obligatory and that the members of this multigene family may differ in the mode of their induction during normal development.