A developmentally regulated glycoprotein complex from Dictyostelium discoideum

Proteomics opens doors to the mechanisms of developmentally regulated secretion

The program of multicellular development in Dictyostelium discoideum culminates with the assembly of a rugged, environmentally resistant spore coat around each spore cell. After synthesis, the proteins that will constitute the coat are stored in prespore vesicles (PSVs) until an unknown developmental signal triggers the PSVs to move to the cell surface where they fuse with the plasma membrane and secrete their cargo by exocytosis. These events occur synchronously in 80% of the cells in each developing multicellular aggregate, and thus the system offers a unique opportunity to study the developmental regulation of protein secretion in situ. Proteomic analysis of purified PSVs identified many of the constituent proteins, which in turn has lead to novel hypotheses and new experimental avenues regarding the molecular mechanisms regulating secretion from the PSVs.

Comparative analysis of spore coat formation, structure, and function in Dictyostelium

Dictyostelium produces spores at the end of its developmental cycle to propagate the lineage. The spore coat is an essential feature of spore biology contributing a semipermeable chemical and physical barrier to protect the enclosed amoeba. The coat is assembled from secreted proteins and a polysaccharide, and from cellulose produced at the cell surface. They are organized into a polarized molecular sandwich with proteins forming layers surrounding the microfibrillar cellulose core. Genetic and biochemical studies are beginning to provide insight into how the deliveries of protein and cellulose to the cell surface are coordinated and how cysteine-rich domains of the proteins interact to form the layers. A multidomain inner layer protein, SP85/PsB, seems to have a central role in regulating coat assembly and contributing to a core structural module that bridges proteins to cellulose. Coat formation and structure have many parallels in walls from plant, algal, yeast, protist, and animal cells.

The cellulose-binding activity of the PsB multiprotein complex is required for proper assembly of the spore coat and spore viability in Dictyostelium discoideum

The terminal event of spore differentiation in the cellular slime mould Dictyostelium discoideum is the assembly of the spore coat, which surrounds the dormant amoeba and allows the organism to survive during extended periods of environmental stress. The spore coat is a polarized extracellular matrix composed of glycoproteins and cellulose. The process of spore coat formation begins by the regulated secretion of spore coat proteins from the prespore vesicles (PSVs). Four of the major spore coat proteins (SP96, PsB/SP85, SP70 and SP60) exist as a preassembled multiprotein complex within the PSVs. This complete complex has an endogenous cellulose-binding activity. Mutant strains lacking either the SP96 or SP70 proteins produce partial complexes that do not have cellulose-binding activity, while mutants lacking SP60 produce a partial complex that retains this activity. Using a combination of immunofluorescence microscopy and biochemical methods we now show that the lack of cellulose-binding activity in the SP96 and SP70 mutants results in abnormally assembled spore coats and spores with greatly reduced viability. In contrast, the SP60 mutant, in which the PsB complex retains its cellulose-binding activity, produces spores with apparently unaltered structure and viability. Thus, it is the loss of the cellulose-binding activity of the PsB complex, rather than the mere loss of individual spore coat proteins, that results in compromised spore coat structure. These results support the idea that the cellulose-binding activity associated with the complete PsB complex plays an active role in the assembly of the spore coat.

Differential developmental expression and cell type specificity of Dictyostelium catalases and their response to oxidative stress and UV-light

Cells of Dictyostelium discoideum are highly resistant to DNA damaging agents such as UV-light, gamma-radiation and chemicals. The genes encoding nucleotide excision repair (NER) and base excision repair (BER) enzymes are rapidly upregulated in response to UV-irradiation and DNA-damaging chemicals, suggesting that this is at least partially responsible for the resistance of this organism to these agents. Although Dictyostelium is also unusually resistant to high concentrations of H(2)O(2), little is known about the response of this organism to oxidative stress. To determine if transcriptional upregulation is a common mechanism for responding to DNA-damaging agents, we have studied the Dictyostelium catalase and Cu/Zn superoxide dismutase antioxidant enzymes. We show that there are two catalase genes and that each is differentially regulated both temporally and spatially during multicellular development. The catA gene is expressed throughout growth and development and its corresponding enzyme is maintained at a steady level. In contrast, the catB gene encodes a larger protein and is only expressed during the final stages of morphogenesis. Cell type fractionation showed that the CatB enzyme is exclusively localized to the prespore cells and the CatA enzyme is found exclusively in the prestalk cells. Each enzyme has a different subcellular localization. The unique developmental timing and cell type distribution suggest that the role for catB in cell differentiation is to protect the dormant spores from oxidative damage. We found that exposure to H(2)O(2) does not result in the induction of the catalase, superoxide dismutase, NER or BER mRNAs. A mutant with greatly reduced levels of catA mRNA and enzyme has greatly increased sensitivity to H(2)O(2) but normal sensitivity to UV. These results indicate that the natural resistance to oxidative stress is not due to an ability to rapidly raise the level of antioxidant or DNA repair enzymes and that the response to UV-light is independent from the response to reactive oxygen compounds.

Crossing the finish line of development: regulated secretion of Dictyostelium proteins

The genesis of the spore coat of Dictyostelium represents an exquisite example of developmentally regulated protein secretion. The proteins that are destined to be assembled into the extracellular matrix of the spore coat are stored in unique prespore vesicles that are triggered to secrete their contents at terminal differentiation. The regulation of this process is being revealed by the identification of the individual proteins in these vesicles.

Multiple O-glycoforms on the spore coat protein SP96 in Dictyostelium discoideum. Fuc(alpha1-3)GlcNAc-alpha-1-P-Ser is the major modification

A decreased level of fucosylation on certain spore coat proteins of Dictyostelium discoideum alters the permeability of the spore coat. Here the post-translational modifications of a major spore coat protein, SP96, are studied in a wild type strain (X22) and a fucosylation-defective mutant (HU2470). A novel phosphoglycan structure on SP96 of the wild type strain, consisting of Fuc(alpha1-3)GlcNAc-alpha-1-P-Ser(,) was identified by electrospray ionization mass spectrometry and NMR. It was shown using monosaccharide and gas chromatography mass spectrometry analysis that SP96 in the mutant HU2470 contained approximately 20% of wild type levels of fucose, as a result of a missing terminal fucose on the novel glycan structure. The results support previous predictions, based on inhibition studies on different fucose-deficient strains, about the nature of monoclonal antibody epitopes identified by monoclonal antibodies MUD62 and MUD166, which are known to identify O-linked glycans (Champion, A., Griffiths, K., Gooley, A. A., Gonzalez, B. Y., Gritzali, M., West, C. M., and Williams, K. L. (1995) Microbiology 141, 785-797). Quantitative studies on wild type SP96 indicated that there were approximately 60 sites with phosphodiester-linked N-acetylglucosamine-fucose disaccharide units and a further approximately 20 sites with fucose directly linked to the protein. Over 70% of the serine sites are modified, with less than 1% of these sites as phosphoserine. Threonine and tyrosine residues were not found to be modified.

The prespore vesicles of Dictyostelium discoideum. Purification, characterization, and developmental regulation

The coordinate fusion of the prespore vesicles (PSVs) with the plasma membrane at the terminal stage of spore differentiation in Dictyostelium discoideum is an important example of developmentally regulated protein secretion. However, little is known about the composition of the vesicles, the molecular signals regulating secretion, or the mechanics of the membrane fusion. Taking a biochemical approach, we purified PSVs from different developmental stages. These preparations are highly enriched for their specific cargo of spore coat proteins while devoid of markers for other cellular compartments. Electron microscopic observations show that the PSV preparations are homogenous, with the soluble spore coat protein PsB/SP85 distributed throughout the lumen and the acid mucopolysaccharide localized in the central core. During development the PSVs increase in size and density concomitant with an increase in their protein cargo. The PSVs contain approximately 80 proteins, and we have identified a PSV-specific GTP-binding protein that may be involved in regulating vesicle fusion. The PSVs are not clathrin-coated and do not contain the SpiA spore coat protein. The PSV preparations are ideal for a global proteome analysis to identify proteins involved in signal reception, vesicle movement, docking, and fusion in this developmentally regulated organelle.

Two proteins of the Dictyostelium spore coat bind to cellulose in vitro

The spore coat of Dictyostelium contains nine different proteins and cellulose. Interactions between protein and cellulose were investigated using an in vitro binding assay. Proteins extracted from coats with urea and 2-mercaptoethanol could, after removal of urea by gel filtration, efficiently bind to particles of cellulose (Avicel), but not Sephadex or Sepharose. Two proteins, SP85 and SP35, were enriched in the reconstitution, and they retained their cellulose binding activities after purification by ion exchange chromatography under denaturing conditions to suppress protein--protein interactions. Neither protein exhibited cellulase activity, though under certain conditions SP85 copurified with a cellulase activity which appeared after germination. Amino acid sequencing indicated that SP85 and SP35 are encoded by the previously described pspB and psvA genes. This was confirmed for SP85 by showing that natural M(r) polymorphisms correlated with changes in the number of tetrapeptide-encoding sequence repeats in pspB. Using PCR to reconstruct missing elements from the recombinogenic middle region of pspB, SP85 was shown to consist of three sequence domains separated by two groups of the tetrapeptide repeats. Expression of partial pspB cDNAs in Escherichia coli showed that cellulose-binding activity resided in the Cys-rich COOH-terminal domain of SP85. This cellulose-binding activity can explain SP85's ultrastructural colocalization with cellulose in vivo. Amino acid composition and antibody binding data showed that SP35 is derived from the Cys-rich N-terminal region of the previously described psvA protein. SP85 and SP35 may link other proteins to cellulose during coat assembly and germination.

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.

The extracellular matrix of the Dictyostelium discoideum slug

In this review, we detail the current understanding of the extracellular matrix (ECM) of the migratory slug phase of the cellular slime mould, Dictyostelium discoideum. We describe some structural and non-structural molecules which comprise the ECM, and how these molecules reflect both plant and animal ECM systems. We also describe zones of the multicellular slug that are known to make ECM components, including the role of the prestalk cells and the slug epithelium-like layer. Finally, we review the contributions of studies on mutants to our understanding of the ECM of D. discoideum, and relate this to differentiation and development in more complex eukaryotic systems.

The PsB glycoprotein complex is secreted as a preassembled precursor of the spore coat in Dictyostelium discoideum

The PsB glycoprotein in Dictyostelium discoideum is one of a diverse group of developmentally regulated, prespore-cell-specific proteins, that contain a common O-linked oligosaccharide. This post-translational modification is dependent on the wild-type modB allele. The PsB protein exists as part of a multiprotein complex of six different proteins, which have different post-translational modifications and are held together by both covalent and non-covalent interactions (Watson et al. (1993). J. Biol. Chem. 268, 22634–22641). In this study we have used microscopic and biochemical analyses to examine the cellular localization and function of the PsB complex during development. We found that the PsB complex first accumulates in prespore vesicles in slug cells and is secreted later during culmination and becomes localized to both the extracellular matrix of the apical spore mass of mature fruiting bodies and to the inner layer of the spore coat. The PsB associated with the spore coat is covalently bound by disulfide bridges. The PsB protein always exists in a multiprotein complex, but the composition of the PsB complex changes during secretion and spore maturation. Some of the PsB complex proteins have been identified as spore coat proteins. These data demonstrate that some of the proteins that form the spore coat exist as a preassembled precursor complex. The PsB complex is secreted in a developmentally regulated manner during the process of spore differentiation, at which time proteins of the complex, as well as additional spore coat proteins, become covalently associated in at least two forms of extracellular matrix: the interspore matrix and the spore coat. These and other studies show that proteins with modB dependent O-linked oligosaccharides are involved in a wide variety of processes underlying morphogenesis in this organism. These developmental processes are the direct result of cellular mechanisms regulating protein targeting, assembly and secretion, and the assembly of specific extracellular matrices.