Evidence that a glycolipid tail anchors antigen 117 to the plasma membrane of Dictyostelium discoideum cells

Cytoskeleton interactions involved in the assembly and function of glycoprotein-80 adhesion complexes in dictyostelium

Adhesion complexes typically assemble from clustered receptors that link to the cytoskeleton via cytoplasmic adapter proteins. However, it is unclear how phospholipid-anchored adhesion molecules, such as the Dictyostelium receptor gp80, interact with the cytoskeleton. gp80 has been found to form adhesion complexes from raftlike membrane domains, which can be isolated as a Triton X-100-insoluble floating fraction (TIFF). We report here that the actin-binding protein ponticulin mediates TIFF-cytoskeleton interactions. Analysis of gp80-null cells revealed that these interactions were minimal in the absence of gp80. During development, gp80 was required to enhance these interactions as its adhesion complexes assembled. Whereas ponticulin and gp80 could partition independently into TIFF, gp80 was shown to recruit ponticulin to cell-cell contacts and to increase its partitioning into TIFF. However, these proteins did not co-immunoprecipitate. Furthermore, sterol sequestration abrogated the association of ponticulin with TIFF without affecting gp80, suggesting that sterols may mediate the interactions between ponticulin and gp80. In ponticulin-null cells, large gp80 adhesion complexes assembled in the absence of ponticulin despite the lack of cytoskeleton association. We propose that such nascent gp80 adhesion complexes produce expanded raftlike domains that recruit ponticulin and thereby establish stable cytoskeleton interactions to complete the assembly process.

Reciprocal raft-receptor interactions and the assembly of adhesion complexes

Cell adhesion complexes are critical for the physical coordination of cell-cell interactions and the morphogenesis of tissues and organs. Many adhesion receptors are anchored to the plasma membrane by a glycosylphosphatidylinositol (GPI) moiety and are thereby partitioned into membrane rafts. In this review, we focus on reciprocal interactions between rafts and adhesion molecules, leading to receptor clustering and raft expansion and stability. A model for a three-stage adhesion complex assembly process is also proposed. First, GPI-anchored adhesion molecules are recruited into rafts, which in turn promote receptor cis-oligomerization and thereby produce precursory complexes primed for avid trans-interactions. Second, trans-interactions of the receptors cross-link and stabilize large amalgams of rafts at sites of adhesion complex assembly. Finally, the enlarged and stabilized rafts acquire enhanced abilities to recruit the cytoskeleton and induce signaling. This process exemplifies how the domain structure of the plasma membrane can impact the function of its receptors.

Assembly of glycoprotein-80 adhesion complexes in Dictyostelium. Receptor compartmentalization and oligomerization in membrane rafts

The phospholipid-anchored membrane glycoprotein (gp)-80 mediates cell-cell adhesion through a homophilic trans-interaction mechanism during Dictyostelium development and is enriched in a Triton X-100-insoluble floating fraction. To elucidate how gp80 adhesion complexes assemble in the plasma membrane, gp80-gp80 and gp80-raft interactions were investigated. A low density raft-like membrane fraction was isolated using a detergent-free method. It was enriched in sterols, the phospholipid-anchored proteins gp80, gp138, and ponticulin, as well as DdCD36 and actin, corresponding to components found in the Triton X-100-insoluble floating fraction. Chemical cross-linking revealed that gp80 oligomers were enriched in the raft-like membrane fraction, implicating stable oligomer-raft interactions. However, gp80 oligomers resisted sterol sequestration and were partially dissociated with Triton X-100, suggesting that compartmentalization in rafts was not solely responsible for their formation. The trans-dimer known to mediate adhesion was identified, but cis-oligomerization predominated and displayed greater accumulation during development. In fact, oligomerization was dependent on the level of gp80 expression and occurred among isolated gp80 extracellular domains, indicating that it was mediated by direct gp80-gp80 interactions. Rafts existed in gp80-null cells and such pre-existent membrane domains may provide optimal microenvironments for gp80 cis-oligomerization and the assembly of adhesion complexes.

Cell-cell adhesion and signal transduction duringDictyosteliumdevelopment

The development of the non-metazoan eukaryote Dictyostelium discoideum displays many of the features of animal embryogenesis, including regulated cell-cell adhesion. During early development, two proteins, DdCAD-1 and csA, mediate cell-cell adhesion between amoebae as they form a loosely packed multicellular mass. The mechanism governing this process is similar to epithelial sheet sealing in animals. Although cell differentiation can occur in the absence of cell contact, regulated cell-cell adhesion is an important component of Dictyostelium morphogenesis, and a third adhesion molecule, gp150, is required for multicellular development past the aggregation stage.

Cell-cell junctions that appear to be adherens junctions form during the late stages of Dictyostelium development. Although they are not essential to establish the basic multicellular body plan, these junctions are required to maintain the structural integrity of the fruiting body. The Dictyostelium β-catenin homologue Aardvark (Aar) is present in adherens junctions, which are lost in its absence. As in the case of its metazoan counterparts, Aar also has a function in cell signalling and regulates expression of the pre-spore gene psA.

It is becoming clear that cell-cell adhesion is an integral part of Dictyostelium development. As in animals, cell adhesion molecules have a mechanical function and may also interact with the signal-transduction processes governing morphogenesis.

Sequential activities of phosphoinositide 3-kinase, PKB/Aakt, and Rab7 during macropinosome formation in Dictyostelium

Macropinocytosis plays an important role in the internalization of antigens by dendritic cells and is the route of entry for many bacterial pathogens; however, little is known about the molecular mechanisms that regulate the formation or maturation of macropinosomes. Like dendritic cells, Dictyostelium amoebae are active in macropinocytosis, and various proteins have been identified that contribute to this process. As described here, microscopic analysis of null mutants have revealed that the class I phosphoinositide 3-kinases, PIK1 and PIK2, and the downstream effector protein kinase B (PKB/Akt) are important in regulating completion of macropinocytosis. Although actin-rich membrane protrusions form in these cell lines, they recede without forming macropinosomes. Imaging of cells expressing green fluorescent protein (GFP) fused to the pleckstrin homology domain (PH) of PKB (GFP-PHPKB) indicates that D3 phosphoinositides are enriched in the forming macropinocytic cup and remain associated with newly formed macropinosomes for <1 minute. A fusion protein, consisting of GFP fused to an F-actin binding domain, overlaps with GFP-PHPKB in the timing of association with forming macropinosomes. Although macropinocytosis is reduced in cells expressing dominant negative Rab7, microscopic imaging studies reveal that GFP-Rab7 associates only with formed macropinosomes at approximately the time that F-actin and D3 phosphoinositide levels decrease. These results support a model in which F-actin modulating proteins and vesicle trafficking proteins coordinately regulate the formation and maturation of macropinosomes.

Rab7 regulates phagosome maturation in Dictyostelium

A Dictyostelium Rab7 homolog has been demonstrated to regulate fluid-phase influx, efflux, retention of lysosomal hydrolases and phagocytosis. Since Rab7 function appeared to be required for efficient phagocytosis, we sought to further characterize the role of Rab7 in phagosomal maturation. Expression of GFP-Rab7 resulted in labeling of both early and late phagosomes containing yeast, but not forming phagocytic cups. In order to determine if Rab7 played a role in regulating membrane traffic between the endo/lysosomal system and maturing phagosomes, latex bead containing (LBC) phagosomes were purified from wild-type cells at various times after internalization. Glycosidases, cysteine proteinases, Rab7 and lysosomally associated membrane proteins were delivered rapidly to nascent phagosomes in control cells. LBC phagosomes isolated from cells overexpressing dominant negative (DN) Rab7 contained very low levels of LmpA (lysosomal integral membrane protein) and α-mannosidase was not detectable. Interestingly, cysteine proteinases were delivered to phagosomes as apparent pro-forms in cells overexpressing DN Rab7. Despite these defects, phagosomes in cells overexpressing DN Rab7 matured to form multi-particle spacious phagosomes, except that these phagosomes remained significantly more acidic than control phagosomes. These results suggested that Rab7 regulates both an early and late steps of phagosomal maturation, similar to its role in the endo/lysosomal system.

Involvement of a triton-insoluble floating fraction in Dictyostelium cell-cell adhesion

We have isolated and characterized a Triton-insoluble floating fraction (TIFF) from Dictyostelium. Ten major proteins were consistently detected in TIFF, and six species were identified by mass spectrometry as actin, porin, comitin, regulatory myosin light chain, a novel member of the CD36 family, and the phospholipid-anchored cell adhesion molecule gp80. TIFF was enriched with many acylated proteins. Also, the sterol/phospholipid ratio of TIFF was 10-fold higher than that of the bulk plasma membrane. Immunoelectron microscopy showed that TIFF has vesicular morphology and confirmed the association of gp80 and comitin with TIFF membranes. Several TIFF properties were similar to those of Dictyostelium contact regions, which were isolated as a cytoskeleton-associated membrane fraction. Mass spectrometry demonstrated that TIFF and contact regions shared the same major proteins. During development, gp80 colocalized with F-actin, porin, and comitin at cell-cell contacts. These proteins were also recruited to gp80 caps induced by antibody cross-linking. Filipin staining revealed high sterol levels in both gp80-enriched cell-cell contacts and gp80 caps. Moreover, sterol sequestration by filipin and digitonin inhibited gp80-mediated cell-cell adhesion. This study reveals that Dictyostelium TIFF has structural properties previously attributed to vertebrate TIFF and establishes a role for Dictyostelium TIFF in cell-cell adhesion during development.

The carboxyl terminus of Dictyostelium discoideum protein 1I encodes a functional glycosyl-phosphatidylinositol signal sequence

The 1I gene is expressed in the prespore cells of culminating Dictyostelium discoideum. The open reading frame of 1I cDNA encodes a protein of 155 amino acids with hydrophobic segments at both its NH(2)- and COOH-termini that are indicative of a glycosyl-phosphatidylinositol (GPI)-anchored protein. A hexaHis-tagged form of 1I expressed in D. discoideum cells appeared on Western blot analysis as a doublet of 27 and 24 kDa, with a minor polypeptide of 22 kDa. None of the polypeptides were released from the cell surface with bacterial phosphatidylinositol-specific phospholipase C, although all three were released upon nitrous acid treatment, indicating the presence of a phospholipase-resistant GPI anchor. Further evidence for the C-terminal sequence of 1I acting as a GPI attachment signal was obtained by replacing the GPI anchor signal sequence of porcine membrane dipeptidase with that from 1I. Two constructs of dipeptidase with the 1I GPI signal sequence were constructed, one of which included an additional six amino acids in the hydrophilic spacer. Both of the resultant constructs were targeted to the surface of COS cells and were GPI-anchored as shown by digestion with phospholipase C, indicating that the Dictyostelium GPI signal sequence is functional in mammalian cells. Site-specific antibodies recognising epitopes either side of the expected GPI anchor attachment site were used to determine the site of GPI anchor attachment in the constructs. These parallel approaches show that the C-terminal signal sequence of 1I can direct the addition of a GPI anchor.

Evidence for a recycling role for Rab7 in regulating a late step in endocytosis and in retention of lysosomal enzymes in Dictyostelium discoideum

The mammalian small molecular weight GTPase Rab7 (Ypt7 in yeast) has been implicated in regulating membrane traffic at postinternalization steps along the endosomal pathway. A cDNA encoding a protein 85% identical at the amino acid level to mammalian Rab7 has been cloned from Dictyostelium discoideum. Subcellular fractionation and immunofluorescence microscopy indicated that Rab7 was enriched in lysosomes, postlysosomes, and maturing phagosomes. Cell lines were generated that overexposed Rab7 wild-type (WT), Rab7 Q67L (constitutively active form), and Rab7 T22N (dominant negative form) proteins. The Rab7 T22N cell line internalized fluid phase markers and latex beads (phagocytosis) at one-third the rate of control cells, whereas Rab7 WT and Rab7 Q67L cell lines were normal in uptake rates but exocytosed fluid phase faster than control cells. In contrast, fluid phase markers resided in acidic compartments for longer periods of time and were more slowly exocytosed from Rab7 T22N cells as compared with control cells. Light microscopy indicated that Rab7-expressing cell lines contained morphologically altered endosomal compartments. Compared with control cells, Rab7 WT- and Rab7 Q67L-expressing cells contained a reduced number of vesicles, the size of postlysosomes (> 2.5 microns) and an increased number of smaller vesicles, many of which were nonacidic; in control cells, > 90% of the smaller vesicles were acidic. In contrast, Rab7 T22N cells contained an increased proportion of large acidic vesicles relative to nonacidic vesicles. Radiolabel pulse-chase experiments indicated that all of the cell lines processed and targeted lysosomal alpha-mannosidase normally, indicating the lack of a significant role for Rab7 in the targeting pathway; however, retention of mature lysosomal hydrolases was affected in Rab7 WT and Rab7 T22N cell lines. Contrary to the results observed for the fluid phase efflux experiments, Rab7 T22N cells oversecreted alpha-mannosidase, whereas Rab7 WT cells retained this hydrolase as compared with control cells. These data support a model that Rab7 may regulate retrograde transport of lysosomal enzymes and the V-type H(+)-ATPase from postlysosomes to lysosomes coupled with the efficient release of fluid phase from cells.

An uncleaved glycosylphosphatidylinositol signal mediates Ca(2+)-sensitive protein degradation

Inv-gp80 is a chimeric protein which contains a signal for the attachment of a glycosylphosphatidylinositol (GPI) anchor. When expressed in Dictyostelium discoideum, this protein fails to become GPI anchored and is retained within the cell as an integral membrane protein. We have compared the subcellular localization and degradation of Inv-gp80 with that of its intracellular but soluble counterpart, Inv-gp80sc. Inv-gp80sc lacks the hydrophobic C-terminal 22 amino acids of Inv-gp80. The N-linked oligosaccharides of both Inv-gp80 and Inv-gp80sc remained sensitive to endoglycosidase H, and both proteins co-fractionated with endoplasmic reticulum marker enzymes on Percoll gradients. Under normal conditions, Inv-gp80 displayed a half-life (t 1/2) of 90 min, while Inv-gp80sc displayed a t 1/2 of 120 min. The degradation of both proteins required ATP, was inhibited by tosyl phenylalanylchloromethane (Tos-Phe-CH2Cl) and was insensitive to inhibitors of lysosomal function. While depletion of Ca2+ from the endoplasmic reticulum had no effect on the degradation of Inv-gp80sc, it stimulated the degradation of Inv-gp80. When the GPI anchor signal sequence of Inv-gp80 was replaced with the transmembrane domain of the interleukin-2 receptor, the degradation of the protein was no longer influenced by Ca2+ fluxes. The data suggest that while the GPI anchor sequence of Inv-gp80 does not contain determinants regulating the degradation of the protein under basal conditions, it targets Inv-gp80 for rapid degradation under conditions where Ca2+ is depleted from the endoplasmic reticulum.

Cell adhesion in the life cycle of Dictyostelium

Three forms of cell adhesion determine the life cycle of Dictyostelium: i) adhesion of bacteria to the surface of the growing amoebae, as the prerequisite for phagocytosis; ii) cell-substrate adhesion, necessary for both locomotion of the amoebae and migration of the slug; iii) cell-cell adhesion, essential for transition from the unicellular to the multicellular stage. Intercellular adhesion has received the most attention, and fruitful approaches have been developed over the past 25 years to identify, purify and characterize cell adhesion molecules. The csA glycoprotein, in particular, which mediates adhesion during the aggregation stage, is one of the best defined cell adhesion molecules. The molecular components involved in phagocytosis and cell-substratum adhesion are less well understood, but the basis has been laid for a systematic investigation of both topics in the near future.

Lack of glycosyl-phosphatidylinositol anchoring leads to precursor retention by a unique mechanism in Dictyostelium discoideum

Gp80, a cell-adhesion molecule in Dictyostelium discoideum, is modified by N- and O-linked oligosaccharides, and a glycosylphosphatidylinositol (GPI) anchor. To identify sequences important for the addition of these modifications to gp80, we created a hybrid protein in which the C-terminal 136 amino acids of yeast invertase were replaced by the C-terminal 110 amino acids of gp80. When expressed in D. discoideum, this protein (Inv-gp80) was not GPI-anchored and was retained in a pre-Golgi compartment. Inv-gp80 did, however, display characteristics of a transmembrane protein, suggesting a novel mechanism for its retention. We also expressed a truncated version of the hybrid protein in which the C-terminal 22 amino acids of the Inv-gp80 were deleted. The truncated protein (Inv-gp80stop) was O-glycosylated and secreted. These observations indicate that the hybrid protein is not abnormally folded and demonstrate the importance of the C-terminal 22 amino acids in the retention of Inv-gp80. Together, the data suggest that oligomerization of the protein blocks its GPI anchoring.

Heterogeneity of glycosylphosphatidylinositol-anchored alkaline phosphatase of calf intestine

A method is described for large-scale purification of glycosylphosphatidylinositol-anchored alkaline phosphatase from intestinal mucosa and chyme to homogeneity. Both enzyme preparations contain approximately 2 mol fatty acid/mol subunit and exhibit a very similar fatty acid composition with octadecanoate and hexadecanoate as prevalent components. No significant differences between native glycosylPtdIns-anchored and hydrophilic alkaline phosphatases from both sources were found regarding Km, Vmax, the type of inhibition and inhibition constants of the amino acids L-leucine, L-phenylalanine, and L-tryptophan. The purified enzymes of both sources yield diacylglycerol and phosphatidic acid, after treatment with phosphatidylinositol-specific phospholipase C (PtdIns-PLC) and glycosylphosphatidylinositol phospholipase D (PLD), respectively. Enzyme preparations of both sources appear as heterogeneous mixtures of five fractions separable by octyl-Sepharose chromatography. Fraction I corresponds to the anchorless enzyme, fractions II-V differ in their susceptibility to phospholipases. Fractions II and IV are completely split by PtdIns-PLC or PLD action, almost 50% of fraction III is split by PtdIns-PLC, while fraction V is resistant. The susceptibility of these two fractions toward the action of PLD is considerably higher. Fatty acid analysis yields molar ratios of fatty acids/alkaline phosphatase subunit of 1.78, 2.58, 2.24, and 3.37 for fractions II, III, IV, and V, respectively. Aggregates of glycosylPtdIns-anchored alkaline phosphatase of all fractions are seen in native PAGE in the presence of Triton X-100. By gel chromatography in the presence of Brij 35, fractions II-V form stable multiple aggregates of dimers and may bind different amounts of the detergent. These data, together with fatty acid analysis, can be interpreted by the following model. Fractions II and IV are tetramers and octamers with two molecules fatty acid/subunit. Fraction III is a tetramer, bearing one additional fatty acid molecule, localized on the dimer. Fraction V is an octamer, containing glycosylPtdIns-anchor molecules with three molecules fatty acids/anchor molecule. The additional fatty acid residue is possibly located on inositol and responsible for the reduced susceptibility to PtdIns-PLC. The similarity of all measured parameters of both enzymes suggests that the glycosylPtdIns-anchored alkaline phosphatase of the mucosa is released into the chyme without changing the anchor molecule constituents.

Immobilization antigens from Tetrahymena thermophila are glycosyl-phosphatidylinositol-linked proteins

We have studied four strains of Tetrahymena thermophila, each of which expresses a different allele of the SerH gene and produces a distinctive surface protein of the immobilization antigen (i-antigen) class. Following exposure of the strains to [3H]ethanolamine or [3H]myristic acid, a protein corresponding in molecular mass to the characteristic i-antigen for that strain became highly labeled, as determined by mobility in sodium dodecylsulfate-polyacrylamide electrophoresis gels. Furthermore, antibodies raised to the i-antigens of the T. thermophila strains selectively immunoprecipitated radioactive proteins having molecular mass identical to that of the i-antigen characteristic for that particular strain. The lipid moieties labeled by [3H]myristate were not susceptible to hydrolysis by exogenous phosphatidylinositol-specific phospholipase C from bacteria. However, when protein extraction was carried out in the absence of phospholipase C inhibitors, radioactive fatty acids derived from [3H]myristate were rapidly cleaved from the putative i-antigens. On the basis of available data, it was concluded that T. thermophila i-antigens contain covalently-linked glycosyl-phosphatidylinositol anchors.

Transfer of glycosyl-phosphatidylinositol membrane anchors to polypeptide acceptors in a cell-free system

Glycosylinositol phospholipid (GPI) membrane anchors are the sole means of membrane attachment of a large number of cell surface proteins, including the variant surface glycoproteins (VSGs) of the parasitic protozoan, Trypanosoma brucei. Biosynthetic data suggest that GPI-anchored proteins are synthesized with carboxy-terminal extensions that are immediately replaced by GPI, suggesting the existence of preformed GPI species available for transfer to the nascent protein in the ER. Candidate precursor glycolipids having a linear sequence indistinguishable from the conserved core structure found on all GPI anchors, have been characterized in T. brucei. In this paper we describe the transfer of three GPI variants to endogenous VSG in vitro. GPI addition is not reduced by inhibitors of protein synthesis and does not require ATP or GTP, consistent with a transpeptidation mechanism.

Cell-cell adhesion molecules in Dictyostelium

Multicellularity in the cellular slime mold Dictyostelium discoideum is achieved by the expression of two types of cell-cell adhesion sites. The EDTA-sensitive adhesion sites are expressed very early in the development cycle and a surface glycoprotein of 24,000 Da is known to be responsible for these sites. The EDTA-resistant contact sites begin to accumulate on the cell surface at the aggregation stage of development. Several glycoproteins have been implicated in the EDTA-resistant type of cell-cell binding and the best characterized one has an Mr of 80,000 (gp80). gp80 mediates cell-cell binding via homophilic interaction and its cell binding site has been mapped to an octapeptide sequence. The mechanism by which gp80 mediates cell-cell adhesion will be discussed.

A rapid posttranslational myristylation of a 68-kD protein in D. discoideum

Cells incubated with [3H]myristate were shown to rapidly and specifically acylate a 68-kD protein, p68, in a developmentally-regulated manner. The fatty acid incorporated into p68 was identified as myristate, and is linked to the protein via an amide bond, apparently to an NH2-terminal glycine. The acylation of p68 in D. discoideum displays some unusual properties. Unexpectedly, myristylation of p68 is a posttranslational event and occurs in the presence of inhibitors of protein synthesis. Another unusual finding was that although p68 is a stable protein, the acyl moiety is removed with a half time of approximately 15 min.

Effect of phospholipase C from Bacillus cereus on the release of membrane-bound choline-O-acetyltransferase from rat hippocampal tissue

Some of the enzyme choline-O-acetyltransferase (ChAT) associated with central cholinergic nerve terminals appears to be non-ionically associated with membranes. In the present study, we tested the possibility that some membrane-bound ChAT might be anchored to membranes by a phosphatidylinositol linkage by incubating rat hippocampal tissue with phospholipase C (PLC) from Bacillus cereus. The PLC selectively augmented the release of ChAT; also, the glycosylphosphatidylinositol-PLC inhibitor, zinc, blocked this increase in release. When control and PLC-treated hippocampal tissues were subjected to Triton X-114 phase separation, a procedure that separates amphiphilic from hydrophilic proteins, the detergent-soluble, membrane-bound fraction of tissue ChAT appeared to be the source of the ChAT released by PLC into the incubation medium. Zinc also blocked the temperature-dependent release of ChAT, but not lactic dehydrogenase, from hippocampal tissue. Extracellular membrane-bound ChAT appeared to be the source of the ChAT released by a low exogenous concentration of PLC, as well as that released by a temperature-dependent process during tissue incubation. Phosphatidylinositol-specific PLC from Bacillus thuringiensis released ChAT, but not lactic dehydrogenase, from a crude synaptosomal fraction prepared from rat hippocampal tissue. These results suggest that some of the membrane-bound ChAT in rat hippocampal tissue may be extracellular and anchored to the membrane by phosphatidylinositol, and also that an endogenous factor in hippocampal tissue may function to remove this extracellular ChAT from the membrane.