Defining the intracellular localization pathways followed by lysosomal enzymes in Dictyostelium discoideum

A general purpose method for extracting RNA from Dictyostelium cells

Here we present a protocol for the extraction of RNA from Dictyostelium discoideum. Dictyostelium is a social amoeba that undergoes a basic developmental program, and therefore analysis of RNA levels over a time course is a commonly used technique. This procedure is similar to other guanidine thiocyanate-based methods; however, it has been adjusted because of the large quantities of carbohydrate and nucleases found in Dictyostelium cells. After cell lysis and phenol:chloroform extraction, the resulting high-quality RNA isolated with the described protocol allows the molecular genetic analysis of wild-type and genetically modified cells. The purified RNA can be used for analyses such as northern blotting, RT-PCR and microarrays. This procedure requires approximately 2 h to complete.

A reliable general purpose method for extracting genomic DNA from Dictyostelium cells

In this protocol, we present a standard method for extracting DNA from cells of the social amoeba Dictyostelium discoideum. While this procedure is similar to other phenol:chloroform-based purification methods, it is modified to account for the high level of carbohydrate and nucleases found in Dictyostelium cells. Genomic DNA can be isolated from wild-type and genetically modified cells using the described protocol, allowing molecular genetic analyses to be performed. Following cell lysis, nucleic acid extraction, and precipitation, the isolated DNA is suitable for digestion by restriction enzymes, amplification by PCR and Southern blotting. This procedure takes approximately 3 h to complete.

Recruitment of a specific amoeboid myosin I isoform to the plasma membrane in chemotactic Dictyostelium cells

The Dictyostelium class I myosins, MyoA, -B, -C, and -D, participate in plasma membrane-based cellular processes such as pseudopod extension and macropinocytosis. Given the existence of a high affinity membrane-binding site in the C-terminal tail domain of these motor proteins and their localized site of action at the cortical membrane-cytoskeleton, it was of interest to determine whether each myosin I was directly associated with the plasma membrane. The membrane association of a myosin I heavy chain kinase that regulates the activity of one of the class I myosins, MyoD was also examined. Cellular fractionation experiments revealed that the majority of the Dicyostelium MyoA, -B, -C and -D heavy chains and the kinase are cytosolic. However, a small, but significant, fraction (appr. 7. -15%) of each myosin I and the kinase was associated with the plasma membrane. The level of plasma membrane-associated MyoB, but neither that of MyoC nor MyoD, increases up to 2-fold in highly motile, streaming cells. These results indicate that Dictyostelium specifically recruits myoB to the plasma membrane during directed cell migration, consistent with its known role in pseudopod formation.

Role of esterase gp70 and its influence on growth and development of Dictyostelium discoideum

Gp70 is an esterase originally called crystal protein because of its presence in crystalline structures in aggregation-competent Dictyostelium discoideum cells. Although postulated to break down spore coats, the function of gp70 in vivo was incompletely investigated. Our immunolocalization and biochemical studies of vegetative D. discoideum amoebae show that gp70 was recruited to phagosomes and found in lysosomes. Purified gp70 was effective at hydrolyzing naphthyl substrates with acyl chains typical of lipids and lipopolysaccharides, indicating that the gp70 was involved in digesting endocytosed molecules. The activity of purified gp70 was inhibited by reductants that retarded its electrophoretic mobility and verified the presence of intramolecular disulfide bonds predicted by its amino acid sequence. Compared to wild-type cells, cells overexpressing gp70 were more phagocytically active, had shorter generation times, and produced more fruiting bodies per unit area, while cells lacking gp70 were phagocytically less active with longer doubling times, developed more slowly, and had significantly fewer fruiting bodies per unit area. Consistent with the phenotype of a disrupted metabolism, one-third of the gp70-minus cells were large and multinucleated. Together, these results indicated that despite its crystalline appearance, gp70 was an active esterase involved in both the growth and the development of D. discoideum.

A potential mechanism for regulating myosin I binding to membranes in vivo

Myosin Is are associated with specific membranes, however, the mechanism for regulating their intracellular localization is unclear. As a first step towards understanding this mechanism, membrane rebinding assays using Dictyostelium myoB were performed. Crude, cytosolic myoB bound to intact, but not to NaOH-treated plasma membranes. In contrast, partially purified myoB binds to both intact and NaOH-treated plasma membranes. Chemical cross-linking of cytosolic myoB yielded several products, whereas none were found with the partially purified myoB. These results suggest a model where proteins regulating the specific binding of myoB to the plasma membrane may exist both in the cytosol and on the plasma membrane.

LvsA, a protein related to the mouse beige protein, is required for cytokinesis in Dictyostelium

We isolated a Dictyostelium cytokinesis mutant with a defect in a novel locus called large volume sphere A (lvsA). lvsA mutants exhibit an unusual phenotype when attempting to undergo cytokinesis in suspension culture. Early in cytokinesis, they initiate furrow formation with concomitant myosin II localization at the cleavage furrow. However, the furrow is later disrupted by a bulge that forms in the middle of the cell. This bulge is bounded by furrows on both sides, which are often enriched in myosin II. The bulge can increase and decrease in size multiple times as the cell attempts to divide. Interestingly, this phenotype is similar to the cytokinesis failure of Dictyostelium clathrin heavy-chain mutants. Furthermore, both cell lines cap ConA receptors but form only a C-shaped loose cap. Unlike clathrin mutants, lvsA mutants are not defective in endocytosis or development. The LvsA protein shares several domains in common with the molecules beige and Chediak-Higashi syndrome proteins that are important for lysosomal membrane traffic. Thus, on the basis of the sequence analysis of the LvsA protein and the phenotype of the lvsA mutants, we postulate that LvsA plays an important role in a membrane-processing pathway that is essential for cytokinesis.

Characterization of Dictyostelium discoideum cathepsin D

Previous studies using magnetic purification of Dictyostelium discoideum endocytic vesicles led us to the identification of some major vesicle proteins. Using the same purification procedure, we have now focused our interest on a 44 kDa soluble vesicle protein. Microsequencing of internal peptides and subsequent cloning of the corresponding cDNA identified this protein as the Dictyostelium homolog of mammalian cathepsins D. The only glycosylation detected on Dictyostelium cathepsin D (CatD) is common antigen 1, a cluster of mannose 6-sulfate residues on N-linked oligosaccharide chains. CatD intracellular trafficking has been studied, showing the presence of the protein throughout the entire endocytic pathway. During the differentiation process, the catD gene presents a developmental regulation, which is also observed at the protein level. catD gene disruption does not alter significantly the cell behaviour, either in the vegetative form or the differentiation stage. However, modifications in the SDS-PAGE profiles of proteins bearing common antigen 1 were detected, when comparing parental and catD(-) cells. These modifications point to a possible role of CatD in the maturation of a few Dictyostelium lysosomal proteins.

Phagosomal proteins of Dictyostelium discoideum

In recognizing food particles. Dictyostelium cell-surface molecules initiate cytoskeletal rearrangements that result in phagosome formation. After feeding D. discoideum cells latex beads, early phagosomes were isolated on sucrose step gradients. Protein analyses of these vesicles showed that they contained glycoproteins and surface-labeled species corresponding to integral plasma membrane proteins. Cytoskeletal proteins also were associated with phagosomes, including myosin II, actin and a 30 kDa-actin bundling protein. As seen by the acridine orange fluorescence of vesicles containing bacteria, phagosomes were acidified rapidly by a vacuolar H(+)-ATPase that was detected by immunoblotting. Except for the loss of cytoskeletal proteins, few other changes over time were noted in the protein profiles of phagosomes, suggesting that phagosome maturation was incomplete. The indigestibility of the beads possibly inhibited further endocytic processing, which has been observed by others. Since nascent phagosomes contained molecules of both the cytoskeleton and plasma membrane, they will be useful in studies aimed at identifying specific protein associations occurring between membrane proteins and the cytoskeleton during phagocytosis.

A role for a Rab4-like GTPase in endocytosis and in regulation of contractile vacuole structure and function in Dictyostelium discoideum

The small Mr Rab4-like GTPase, RabD, localizes to the endosomal pathway and the contractile vacuole membrane system in Dictyostelium discoideum. Stably transformed cell lines overexpressing a dominant negative functioning RabD internalized fluid phase marker at 50% of the rate of wild-type cells. Mutant cells were also slower at recycling internalized fluid. Microscopic and biochemical approaches indicated that the transport of fluid to large postlysosome vacuoles was delayed in mutant cells, resulting in an accumulation in acidic smaller vesicles, probably lysosomes. Also, RabD N121I-expressing cell lines missorted a small but significant percentage of newly synthesized lysosomal alpha-mannosidase precursor polypeptides. However, the majority of the newly synthesized alpha-mannosidase was transported with normal kinetics and correctly delivered to lysosomes. Subcellular fractionation and immunofluorescent microscopy indicated that in mutant cells contractile vacuole membrane proteins were associated with compartments morphologically distinct from the normal reticular network. Osmotic tests revealed that the contractile vacuole functioned inefficiently in mutant cells. Our results suggest that RabD regulates membrane traffic along the endosomal pathway, and that this GTPase may play a role in regulating the structure and function of the contractile vacuole system by facilitating communication with the endosomal pathway.

Subcellular distribution of "intersecting' beta-N-acetylglucosaminyltransferase in Dictyostelium discoideum. A likely marker for the Golgi apparatus

Glycoprotein processing in Dictyostelium discoideum is characterized by enzyme catalyzed steps not reported in other organisms. One of these is the formation of a beta 1 --> 4 linkage between GlcNAc and the mannose linked to the core mannose in the alpha 1 --> 6 position of N-glycosides. A simple and sensitive assay for this GlcNAc transferase activity, using a tri-mannose acceptor and a low concentration of UDP-GlcNAc, was developed. Homogenates of the organism were subjected to sub-cellular fractionation by centrifugation in discontinuous sucrose gradients. The specific activity was enriched 4-5-fold in a crude membrane fraction. The transferase was purified 10-12-fold in a membrane fraction that bands on top of 1.1 M sucrose. This fraction was also enriched in nucleotidyldiphosphatase. The enriched fraction was deficient in glucose-6-phosphatase, an endoplasmic reticulum marker. Approx. 80% of the transferase activity was latent, and unavailable to protease. Purified membranes were either subjected to phase separation in Triton X-114, or sodium carbonate extraction or sonication. In each case, the transferase behaved as an intrinsic membrane protein. Several secreted and lysosomal proteins are modified by the enzyme. These data support the idea that the GlcNAc transferase is present as an integral Golgi membrane protein and that at least the catalytic center of the transferase is on the lumenal side of the vesicles.

Clathrin heavy chain functions in sorting and secretion of lysosomal enzymes in Dictyostelium discoideum

The clathrin heavy chain is a major component of clathrin-coated vesicles that function in selective membrane traffic in eukaryotic cells. We disrupted the clathrin heavy chain gene (chcA) in Dictyostelium discoideum to generate a stable clathrin heavy chain-deficient cell line. Measurement of pinocytosis in the clathrin-minus mutant revealed a four-to five-fold deficiency in the internalization of fluid-phase markers. Once internalized, these markers recycled to the cell surface of mutant cells at wild-type rates. We also explored the involvement of clathrin heavy chain in the trafficking of lysosomal enzymes. Pulse chase analysis revealed that clathrin-minus cells processed most alpha-mannosidase to mature forms, however, approximately 20-25% of the precursor molecules remained uncleaved, were missorted, and were rapidly secreted by the constitutive secretory pathway. The remaining intracellular alpha-mannosidase was successfully targeted to mature lysosomes. Standard secretion assays showed that the rate of secretion of alpha-mannosidase was significantly less in clathrin-minus cells compared to control cells in growth medium. Interestingly, the secretion rates of another lysosomal enzyme, acid phosphatase, were similar in clathrin-minus and wild-type cells. Like wild-type cells, clathrin-minus mutants responded to starvation conditions with increased lysosomal enzyme secretion. Our study of the mutant cells provide in vivo evidence for roles for the clathrin heavy chain in (a) the internalization of fluid from the plasma membrane; (b) sorting of hydrolase precursors from the constitutive secretory pathway to the lysosomal pathway; and (c) secretion of mature hydrolases from lysosomes to the extracellular space.

The actin-binding protein comitin (p24) is a component of the Golgi apparatus

Comitin (p24) was first identified in Dictyostelium discoideum as a membrane-associated protein which binds in gel overlay assays to G and F actin. To analyze its actin-binding properties we used purified, bacterially expressed comitin and found that it binds to F actin in spin down experiments and increases the viscosity of F actin solutions even under high-salt conditions. Immunofluorescence studies, cell fractionation experiments and EM studies of vesicles precipitated with comitin-specific monoclonal antibodies showed that comitin was present in D. discoideum on: (a) a perinuclear structure with tubular or fibrillary extensions; and (b) on vesicles distributed throughout the cell. In immunofluorescence experiments using comitin antibodies NIH 3T3 fibroblasts showed a similar staining pattern as D. discoideum cells. Using bona fide Golgi markers the perinuclear structure was identified as the Golgi apparatus. The results were supported by an electron microscopic study using cryosections. Based on these data we propose that also in Dictyostelium the stained perinuclear structure is the Golgi apparatus. In vivo the perinuclear structure was found to be attached to the actin and the microtubule network. Alteration of the actin network or depolymerization of the microtubules led to its dispersal into vesicles distributed throughout the cell. These results suggest that the Golgi apparatus in D. discoideum is connected to the actin network by comitin. This protein seems also to be present in mammalian cells.

Association of calmodulin and an unconventional myosin with the contractile vacuole complex of Dictyostelium discoideum

mAbs specific for calmodulin were used to examine the distribution of calmodulin in vegetative Dictyostelium cells. Indirect immunofluorescence indicated that calmodulin was greatly enriched at the periphery of phase lucent vacuoles. The presence of these vacuoles in newly germinated (non-feeding) as well as growing cells, and the response of the vacuoles to changes in the osmotic environment, identified them as contractile vacuoles, osmoregulatory organelles. No evidence was found for an association of calmodulin with endosomes or lysosomes, nor was calmodulin enriched along cytoskeletal filaments. When membranes from Dictyostelium cells were fractionated on equilibrium sucrose density gradients, calmodulin cofractionated with alkaline phosphatase, a cytochemical marker for contractile vacuole membranes, at a density of 1.156 g/ml. Several high molecular weight calmodulin-binding proteins were enriched in the same region of the gradient. One of the calmodulin-binding polypeptides (molecular mass approximately 150 kD) cross-reacted with an antiserum specific for Acanthamoeba myosin IC. By indirect immunofluorescence, this protein was also enriched on contractile vacuole membranes. These results suggest that a calmodulin-binding unconventional myosin is associated with contractile vacuoles in Dictyostelium; similar proteins in yeast and mammalian cells have been implicated in vesicle movement.

Synthesis of a human lysosomal enzyme, beta-hexosaminidase B, using the baculovirus expression system

Human lysosomal beta-hexosaminidase exists in two major forms: the A isoform is composed of both alpha and beta chains, while the B form is a homopolymer of beta chains. Deficiency of beta-hexosaminidase underlies the GM2 gangliosidoses. We have produced active beta-hexosaminidase B in cultured insect (Sf9) cells by isolation of a recombinant insect virus (baculovirus) containing the cDNA for the beta chain within the viral polyhedron gene and infection of Sf9 cells with this construct. That portion of the enzyme secreted into the medium, 50%, was purified with concanavalin A Sepharose and subsequent affinity chromatography to yield beta-hexosaminidase B that is 75% pure. The product has an N-terminal amino acid sequence, specific activity, and size (M(r) 62,000) similar to that of the enzyme present in cultured human fibroblasts. However, endo H sensitivity studies revealed that the oligosaccharide structures present on recombinant beta-hexosaminidase B differ from those found on the enzyme synthesized in the human system. In addition, these structures lack the mannose 6-phosphate recognition marker that targets degradative hydrolases to lysosomes. Despite these differences, recombinant beta-hexosaminidase B does serve as a specific substrate for the mannose phosphorylating enzyme, N-acetylglucosaminyl phosphotransferase. Furthermore, the oligosaccharide moieties phosphorylated in vitro match those phosphorylated in vivo, pointing to the conformational integrity of the recombinant enzyme. Generous amounts of easily obtained, easily purified, and properly folded beta-hexosaminidase B will facilitate physical structural analysis of the enzyme.

Biochemical and genetic analysis of the biosynthesis, sorting, and secretion of Dictyostelium lysosomal enzymes

Dictyostelium discoideum is a useful system to study the biosynthesis of lysosomal enzymes because of the relative ease with which it can be manipulated genetically and biochemically. Previous studies have revealed that lysosomal enzymes are synthesized in vegetatively growing amoebae as glycosylated precursor polypeptides that are phosphorylated and sulfated on their N-linked oligosaccharide side-chains upon arrival in the Golgi complex. The precursor polypeptides are membrane associated until they are proteolytically processed and deposited as soluble mature enzymes in lysosomes. In this paper we review biochemical experiments designed to determine the roles of post-translational modification, acidic pH compartments, and proteolytic processing in the transport and sorting of lysosomal enzymes. We also describe molecular genetic approaches that are being employed to study the biosynthesis of these enzymes. Mutants altered in the sorting and secretion of lysosomal enzymes are being analyzed biochemically, and we describe recent efforts to clone the genes coding for three lysosomal enzymes in order to better understand the molecular mechanisms involved in the targeting of these enzymes.

Developing Dictyostelium cells contain the lysosomal enzyme alpha-mannosidase in a secretory granule

The prespore vesicle (PSV) is an organelle which secretes spore coat proteins and gal/galNAc polysaccharides from prespore cells of Dictyostelium. By combining the techniques of protein A-gold immunocytochemistry and ricin-gold affinity cytochemistry we have demonstrated colocalization of the lysosomal enzyme alpha-mannosidase with gal/galNAc polysaccharides in prespore vesicles and the spore coat. To determine the origin of prespore vesicles a series of pulse-chase experiments were performed. Cells were labeled with [35S]methionine or [35S]sulfate at different times during development and allowed to differentiate in the presence of unlabeled methionine or sulfate for various periods of time. The cells were homogenized and intracellular organelles were separated using Percoll density gradient centrifugation. The distribution of [35S]methionine-labeled alpha-mannosidase and [35S]sulfate-labeled glycoproteins in the Percoll gradients was determined. It was found that prespore vesicles contained protein which was previously found in lysosomes. Newly labeled protein also entered these vesicles. The data suggest that developing Dictyostelium cells either restructure preexisting lysosomes into prespore vesicles or transport protein between these two organelles. We propose that secretory granules and lysosomes may have a common biosynthetic origin and may be evolutionarily related.

A Dictyostelium discoideum mutant that missorts and oversecretes lysosomal enzyme precursors is defective in endocytosis

A mutant strain of Dictyostelium discoideum, HMW570, oversecretes several lysosomal enzyme activities during growth. Using a radiolabel pulse-chase protocol, we followed the synthesis and secretion of two of these enzymes, alpha-mannosidase and beta-glucosidase. A few hours into the chase period, HMW570 had secreted 95% of its radiolabeled alpha-mannosidase and 86% of its radiolabeled beta-glucosidase as precursor polypeptides compared to the secretion of less than 10% of these forms from wild-type cells. Neither alpha-mannosidase nor beta-glucosidase in HMW570 were ever found in the lysosomal fractions of sucrose gradients consistent with HMW570 being defective in lysosomal enzyme targeting. Also, both alpha-mannosidase and beta-glucosidase precursors in the mutant strain were membrane associated as previously observed for wild-type precursors, indicating membrane association is not sufficient for lysosomal enzyme targeting. Hypersecretion of the alpha-mannosidase precursor by HMW570 was not accompanied by major alterations in N-linked oligosaccharides such as size, charge, and ratio of sulfate and phosphate esters. However, HMW570 was defective in endocytosis. A fluid phase marker, [3H]dextran, accumulated in the mutant at one-half of the rate of wild-type cells and to only one-half the normal concentration. Fractionation of cellular organelles on self-forming Percoll gradients revealed that the majority of the fluid-phase marker resided in compartments in mutant cells with a density characteristic of endosomes. In contrast, in wild-type cells [3H]dextran was predominantly located in vesicles with a density identical to secondary lysosomes. Furthermore, the residual lysosomal enzyme activity in the mutant accumulated in endosomal-like vesicles. Thus, the mutation in HMW570 may be in a gene required for both the generation of dense secondary lysosomes and the sorting of lysosomal hydrolases.

Biogenesis of lysosomal enzymes in the alpha-glucosidase II-deficient modA mutant of Dictyostelium discoideum: retention of alpha-1,3-linked glucose on N-linked oligosaccharides delays intracellular transport but does not alter sorting of alpha-mannosidase or beta-glucosidase

The endoplasmic reticulum-localized enzyme alpha-glucosidase II is responsible for removing the two alpha-1,3-linked glucose residues from N-linked oligosaccharides of glycoproteins. This activity is missing in the modA mutant strain, M31, of Dictyostelium discoideum. Results from both radiolabeled pulse-chase and subcellular fractionation experiments indicate that this deficiency did not prevent intracellular transport and proteolytic processing of the lysosomal enzymes, alpha-mannosidase and beta-glucosidase. However, the rate at which the glucosylated precursors left the rough endoplasmic reticulum was several-fold slower than the rate at which the wild-type precursors left this compartment. Retention of glucose residues did not disrupt the binding of the precursor forms of the enzymes with intracellular membranes, indicating that the delay in movement of proteins from the ER did not result from lack of association with membranes. However, the mutant alpha-mannosidase precursor contained more trypsin-sensitive sites than did the wild-type precursor, suggesting that improper folding of precursor molecules might account for the slow rate of transport to the Golgi complex. Percoll density gradient fractionation of extracts prepared from M31 cells indicated that the proteolytically processed mature forms of alpha-mannosidase and beta-glucosidase were localized to lysosomes. Finally, the mutation in M31 may have other, more dramatic, effects on the lysosomal system since two enzymes, N-acetylglucosaminidase and acid phosphatase, were secreted much less efficiently from lysosomal compartments by the mutant strain.

Inhibition of early but not late proteolytic processing events leads to the missorting and oversecretion of precursor forms of lysosomal enzymes in Dictyostelium discoideum

Lysosomal enzymes are initially synthesized as precursor polypeptides which are proteolytically cleaved to generate mature forms of the enzymatically active protein. The identification of the proteinases involved in this process and their intracellular location will be important initial steps in determining the role of proteolysis in the function and targeting of lysosomal enzymes. Toward this end, axenically growing Dictyostelium discoideum cells were pulse radiolabeled with [35S]methionine and chased in fresh growth medium containing inhibitors of aspartic, metallo, serine, or cysteine proteinases. Cells exposed to the serine/cysteine proteinase inhibitors leupeptin and antipain and the cysteine proteinase inhibitor benzyloxycarbonyl-L-phenylalanyl-L-alanine-diazomethyl ketone (Z-Phe-AlaCHN2) were unable to complete proteolytic processing of the newly synthesized lysosomal enzymes, alpha-mannosidase and beta-glucosidase. Antipain and leupeptin treatment resulted in both a dramatic decrease in the efficiency of proteolytic processing, as well as a sevenfold increase in the secretion of alpha-mannosidase and beta-glucosidase precursors. However, leupeptin and antipain did not stimulate secretion of lysosomally localized mature forms of the enzymes suggesting that these inhibitors prevent the normal sorting of lysosomal enzyme precursors to lysosomes. In contrast to the results observed for cells treated with leupeptin or antipain, Z-Phe-AlaCHN2 did not prevent the cleavage of precursor polypeptides to intermediate forms of the enzymes, but greatly inhibited the production of the mature enzymes. The accumulated intermediate forms of the enzymes, however, were localized to lysosomes. Finally, fractionation of cell extracts on Percoll gradients indicated that the processing of radiolabeled precursor forms of alpha-mannosidase and beta-glucosidase to intermediate products began in cellular compartments intermediate in density between the Golgi complex and mature lysosomes. The generation of the mature forms, in contrast, was completed immediately upon or soon after arrival in lysosomes. Together these results suggest that different proteinases residing in separate intracellular compartments may be involved in generating intermediate and mature forms of lysosomal enzymes in Dictyostelium discoideum, and that the initial cleavage of the precursors may be critical for the proper localization of lysosomal enzymes.

Modifications of lysosomal enzymes in Dictyostelium discoideum

This paper has two purposes. The first is to review the past studies on the structure, biosynthesis, and immunological properties of a class of glycoproteins, the lysosomal enzymes, in Dictyostelium discoideum. The second purpose is to present new data on the analysis of mutant strains altered in the biosynthesis of the lipid-linked precursor of N-linked oligosaccharides, and on the characterization of new carbohydrate antigenic determinants found on multiple proteins in Dictyostelium. We will also show how a combination of genetic, biochemical and immunochemical approaches have been used to unravel a portion of the glycosylation pathway in Dictyostelium. The long-term goal of these studies is to use Dictyostelium discoideum as a model system to understand the functions of a variety of glycoconjugates in a multicellular organism. The existence of a large number of mutant strains which are altered in a variety of cellular functions, development and the posttranslational modification of multiple proteins, offers a great opportunity to explore this area.