Clathrin heavy chain is required for spore cell but not stalk cell differentiation in Dictyostelium discoideum

Asymmetry of single cells and where that leads

Most single animal cells have an internal vector that determines where recycling membrane is added to the cell's surface. Because of the specific molecular composition of this added membrane, a dynamic asymmetry is formed on the surface of the cell. The consequences of this dynamic asymmetry are discussed, together with what they imply for how cells move. The polarity of a single-celled embryo, such as that of the nematode Caenorhabditis elegans, is explored in a similar framework.

Dynamics of clathrin-mediated endocytosis and its requirement for organelle biogenesis in Dictyostelium

The protein clathrin mediates one of the major pathways of endocytosis from the extracellular milieu and plasma membrane. In single-cell eukaryotes, such as Saccharomyces cerevisiae, the gene encoding clathrin is not an essential gene, raising the question of whether clathrin conveys specific advantages for multicellularity. Furthermore, in contrast to mammalian cells, endocytosis in S. cerevisiae is not dependent on either clathrin or adaptor protein 2 (AP2), an endocytic adaptor molecule. In this study, we investigated the requirement for components of clathrin-mediated endocytosis (CME) in another unicellular organism, the amoeba Dictyostelium. We identified a heterotetrameric AP2 complex in Dictyostelium that is similar to that which is found in higher eukaryotes. By simultaneously imaging fluorescently tagged clathrin and AP2, we found that, similar to higher eukaryotes, these proteins colocalized to membrane puncta that move into the cell together. In addition, the contractile vacuole marker protein, dajumin-green fluorescent protein (GFP), is trafficked via the cell membrane and internalized by CME in a clathrin-dependent, AP2-independent mechanism. This pathway is distinct from other endocytic mechanisms in Dictyostelium. Our finding that CME is required for the internalization of contractile vacuole proteins from the cell membrane explains the contractile vacuole biogenesis defect in Dictyostelium cells lacking clathrin. Our results also suggest that the machinery for CME and its role in organelle maintenance appeared early during eukaryotic evolution. We hypothesize that dependence of endocytosis on specific components of the CME pathway evolved later, as demonstrated by internalization independent of AP2 function.

Bestatin inhibits cell growth, cell division, and spore cell differentiation in Dictyostelium discoideum

Bestatin methyl ester (BME) is an inhibitor of Zn(2+)-binding aminopeptidases that inhibits cell proliferation and induces apoptosis in normal and cancer cells. We have used Dictyostelium as a model organism to study the effects of BME. Only two Zn(2+)-binding aminopeptidases have been identified in Dictyostelium to date, puromycin-sensitive aminopeptidase A and B (PsaA and PsaB). PSA from other organisms is known to regulate cell division and differentiation. Here we show that PsaA is differentially expressed throughout growth and development of Dictyostelium, and its expression is regulated by developmental morphogens. We present evidence that BME specifically interacts with PsaA and inhibits its aminopeptidase activity. Treatment of cells with BME inhibited the rate of cell growth and the frequency of cell division in growing cells and inhibited spore cell differentiation during late development. Overexpression of PsaA-GFP (where GFP is green fluorescent protein) also inhibited spore cell differentiation but did not affect growth. Using chimeras, we have identified that nuclear versus cytoplasmic localization of PsaA affects the choice between stalk or spore cell differentiation pathway. Cells that overexpressed PsaA-GFP (primarily nuclear) differentiated into stalk cells, while cells that overexpressed PsaAΔNLS2-GFP (cytoplasmic) differentiated into spores. In conclusion, we have identified that BME inhibits cell growth, division, and differentiation in Dictyostelium likely through inhibition of PsaA.

A single β adaptin contributes to AP1 and AP2 complexes and clathrin function in Dictyostelium

The assembly of clathrin-coated vesicles is important for numerous cellular processes, including nutrient uptake and membrane organization. Important contributors to clathrin assembly are four tetrameric assembly proteins, also called adaptor proteins (APs), each of which contains a β subunit. We identified a single β subunit, named β1/2, that contributes to both the AP1 and AP2 complexes of Dictyostelium. Disruption of the gene encoding β1/2 resulted in severe defects in growth, cytokinesis and development. Additionally, cells lacking β1/2 displayed profound osmoregulatory defects including the absence of contractile vacuoles and mislocalization of contractile vacuole markers. The phenotypes of β1/2 null cells were most similar to previously described phenotypes of clathrin and AP1 mutants, supporting a particularly important contribution of AP1 to clathrin pathways in Dictyostelium cells. The absence of β1/2 in cells led to significant reductions in the protein amounts of the medium-sized subunits of the AP1 and AP2 complexes, establishing a role for the β subunit in the stability of the medium subunits. Dictyostelium β1/2 could resemble a common ancestor of the more specialized β1 and β2 subunits of the vertebrate AP complexes. Our results support the essential contribution of a single β subunit to the stability and function of AP1 and AP2 in a simple eukaryote.

Collective cell migration requires vesicular trafficking for chemoattractant delivery at the trailing edge

Chemoattractant signaling induces the polarization and directed movement of cells secondary to the activation of multiple effector pathways. In addition, chemotactic signals can be amplified and relayed to proximal cells via the synthesis and secretion of additional chemoattractant. The mechanisms underlying such remarkable features remain ill defined. We show that the asymmetrical distribution of adenylyl cyclase (ACA) at the back of Dictyostelium discoideum cells, an essential determinant of their ability to migrate in a head-to-tail fashion, requires vesicular trafficking. This trafficking results in a local accumulation of ACA-containing intracellular vesicles and involves intact actin, microtubule networks, and de novo protein synthesis. We also show that migrating cells leave behind ACA-containing vesicles, likely secreted as multivesicular bodies and presumably involved in the formation of head-to-tail arrays of migrating cells. We propose that similar compartmentalization and shedding mechanisms exist in mammalian cells during embryogenesis, wound healing, neuron growth, and metastasis.

The ENTH and C-terminal domains of Dictyostelium epsin cooperate to regulate the dynamic interaction with clathrin-coated pits

Epsin contains a phospholipid-binding ENTH domain coupled to C-terminal domain motifs that bind coated pit proteins. We examined how these domains interact to influence epsin function and localization in Dictyostelium. Although not required for global clathrin function, epsin was essential for constructing oval spores during development. Within the epsin protein, we found that features important for essential function were distinct from features targeting epsin to clathrin-coated pits. On its own, the phospholipid-binding ENTH domain could rescue the epsin-null phenotype. Although necessary and sufficient for function, the isolated ENTH domain was not targeted within clathrin-coated pits. The C-terminal domain containing the coated-pit motif was also insufficient, highlighting a requirement for both domains for targeting to coated pits. Replacement of the ENTH domain by an alternative membrane-binding domain resulted in epsin that sequestered clathrin and AP2 and ablated clathrin function, supporting a modulatory role for the ENTH domain. Within the ENTH domain, residues important for PtdIns(4,5)P2 binding were essential for both epsin localization and function, whereas residue T107 was essential for function but not coated pit localization. Our results support a model where the ENTH domain coordinates with the clathrin-binding C-terminal domain to allow a dynamic interaction of epsin with coated pits.

Dictyostelium Hip1r contributes to spore shape and requires epsin for phosphorylation and localization

Clathrin-coated pits assemble on the plasma membrane to select and sequester proteins within coated vesicles for delivery to intracellular compartments. Although a host of clathrin-associated proteins have been identified, much less is known regarding the interactions between clathrin-associated proteins or how individual proteins influence the function of other proteins. In this study, we present evidence of a functional relationship between two clathrin-associated proteins in Dictyostelium, Hip1r and epsin. Hip1r-null cells form fruiting bodies that yield defective spores that lack the organized fibrils typical of wild-type spores. This spore coat defect leads to formation of round, rather than ovoid, spores in Hip1r-null cells that exhibit decreased viability. Like Hip1r-null cells, epsin-null cells also construct fruiting bodies with round spores, but these spores are more environmentally robust. Double-null cells that harbor deletions in both epsin and Hip1r form fruiting bodies, with spores identical in shape and viability to Hip1r single-null cells. In the growing amoeba, Hip1r is phosphorylated and localizes to puncta on the plasma membrane that also contain epsin. Both the phosphorylation state and localization of Hip1r into membrane puncta require epsin. Moreover, expression of the N-terminal ENTH domain of epsin is sufficient to restore both the phosphorylation and the restricted localization of Hip1r within plasma membrane puncta. The results from this study reveal a novel interaction between two clathrin-associated proteins during cellular events in both growing and developing Dictyostelium cells.

Copine A is required for cytokinesis, contractile vacuole function, and development in Dictyostelium

Copines make up a family of soluble, calcium-dependent, membrane binding proteins found in a variety of eukaryotic organisms. In an earlier study, we identified six copine genes in the Dictyostelium discoideum genome and focused our studies on cpnA. Our previous localization studies of green fluorescent protein-tagged CpnA in Dictyostelium suggested that CpnA may have roles in contractile vacuole function, endolysosomal trafficking, and development. To test these hypotheses, we created a cpnA- knockout strain, and here we report the initial characterization of the mutant phenotype. The cpnA- cells exhibited normal growth rates and a slight cytokinesis defect. When placed in starvation conditions, cpnA- cells appeared to aggregate into mounds and form fingers with normal timing; however, they were delayed or arrested in the finger stage. When placed in water, cpnA- cells formed unusually large contractile vacuoles, indicating a defect in contractile vacuole function, while endocytosis and phagocytosis rates for the cpnA- cells were similar to those seen for wild-type cells. These studies indicate that CpnA plays a role in cytokinesis and contractile vacuole function and is required for normal development, specifically in the later stages prior to culmination. We also used real-time reverse transcription-PCR to determine the expression patterns of all six copine genes during development. The six copine genes were expressed in vegetative cells, with each gene exhibiting a distinct pattern of expression throughout development. All of the copine genes except cpnF showed an upregulation of mRNA expression at one or two developmental transitions, suggesting that copines may be important regulators of Dictyostelium development.

The monomeric clathrin assembly protein, AP180, regulates contractile vacuole size in Dictyostelium discoideum

AP180, one of many assembly proteins and adaptors for clathrin, stimulates the assembly of clathrin lattices on membranes, but its unique contribution to clathrin function remains elusive. In this study we identified the Dictyostelium discoideum ortholog of the adaptor protein AP180 and characterized a mutant strain carrying a deletion in this gene. Imaging GFP-labeled AP180 showed that it localized to punctae at the plasma membrane, the contractile vacuole, and the cytoplasm and associated with clathrin. AP180 null cells did not display defects characteristic of clathrin mutants and continued to localize clathrin punctae on their plasma membrane and within the cytoplasm. However, like clathrin mutants, AP180 mutants, were osmosensitive. When immersed in water, AP180 null cells formed abnormally large contractile vacuoles. Furthermore, the cycle of expansion and contraction for contractile vacuoles in AP80 null cells was twice as long as that of wild-type cells. Taken together, our results suggest that AP180 plays a unique role as a regulator of contractile vacuole morphology and activity in Dictyostelium.

Clathrin Light Chain: Importance of the Conserved Carboxy Terminal Domain to Function in Living Cells

Clathrin triskelions assemble into coats capable of packaging membrane and receptors for transport to intracellular destinations. A triskelion is formed from three heavy chains bound to three light chains. All clathrin light chains (clc) contain an acidic amino terminal domain, a central coiled segment, and a carboxy terminal domain conserved in amino acid sequence. To assess their functional contribution in vivo, we expressed tagged segments of the Dictyostelium clcA in clc-minus Dictyostelium (clc null) cells. We examined the ability of these clcA fragments to rescue clathrin phenotypic deficiencies, to cluster into punctae on membranes, and to bind to the heavy chain. When expressed in clc null cells, a clcA fragment containing the amino terminal domain and the central coiled domain bound heavy chain but was dispensable for clathrin function. Instead, the carboxy terminal domain of clcA was a critical determinant for association with punctae, for clathrin function and for robust binding to the heavy chain. A 70 amino acid carboxy terminal fragment was necessary and sufficient for full function, and for localization into punctae on intracellular membranes. A shorter 49 amino acid carboxy terminal fragment could distribute into punctae but failed to rescue developmental deficiencies. These results reveal the importance of the carboxy terminal domain of the light chain in vivo.

Clathrin-dependent targeting of receptors to the flagellar pocket of procyclic-form Trypanosoma brucei

In trypanosomatids, endocytosis and exocytosis occur exclusively at the flagellar pocket, which represents about 0.43% of the pellicle membrane and is a deep invagination of the plasma membrane where the flagellum extends from the cell. Receptor molecules are selectively retained at the flagellar pocket. We studied the function of clathrin heavy chain (TbCLH) in the trafficking of the flagellar pocket receptors in Trypanosoma brucei by using the double-stranded RNA interference approach. It appears that TbCLH is essential for the survival of both the procyclic form and the bloodstream form of T. brucei, even though structures resembling large coated endocytic vesicles are absent in procyclic-form trypanosomes. Down-regulation of TbCLH by RNA interference (RNAi) for 24 h rapidly and drastically reduced the uptake of macromolecules via receptor-mediated endocytosis in procyclic-form trypanosomes. This result suggested the importance of TbCLH in receptor-mediated endocytosis of the procyclic-form trypanosome, in which the formation of large coated endocytic vesicles may not be required. Surprisingly, induction of TbCLH RNAi in the procyclic T. brucei for a period of 48 h prohibited the export of the flagellar pocket-associated transmembrane receptor CRAM from the endoplasmic reticulum to the flagellar pocket, while trafficking of the glycosylphosphatidylinositol-anchored procyclin coat was not significantly affected. After 72 h of induction of TbCLH RNAi, procyclics exhibited morphological changes to an apolar round shape without a distinct structure of the flagellar pocket and flagellum. Although trypanosomes, like other eukaryotes, use similar organelles and machinery for protein sorting and transport, our studies reveal a novel role for clathrin in the secretory pathway of trypanosomes. We speculate that the clathrin-dependent trafficking of proteins to the flagellar pocket may be essential for the biogenesis and maintenance of the flagellar pocket in trypanosomes.

Dd-Alix, a conserved endosome-associated protein, controls Dictyostelium development

We have characterized the Dictyostelium homolog of the mammalian protein Alix. Dd-Alix is encoded by a single gene and is expressed during vegetative growth and multicellular development. We showed that the alx null strain fails to complete its developmental program. Past the tight aggregate stage, morphogenesis is impaired, leading to markedly aberrant structures containing vacuolated and undifferentiated cells but no mature spores. The developmental defect is cell-autonomous as most cells remain of the PstB type even when mixed with wild-type cells. Complementation analysis with different Alix constructs allowed the identification of a 101-residue stretch containing a coiled-coil domain essential for Alix function. In addition, we showed that the protein associates in part with vesicular structures and that its distribution on a Percoll gradient overlaps that of the endocytic marker Vamp7. Dd-Alix also co-localizes with Dd-Vps32. In view of our data, and given the role of Vps32 proteins in membrane protein sorting and multivesicular body formation in yeast and mammals, we hypothesize that the developmental defects of the alx null strain result from abnormal trafficking of cell-surface receptors.

Compromise of clathrin function and membrane association by clathrin light chain deletion

While clathrin heavy chains from different species are highly conserved in amino acid sequence, clathrin light chains are much more divergent. Thus clathrin light chain may have different functions in different organisms. To investigate clathrin light chain function, we cloned the clathrin light chain, clcA, from Dictyostelium and examined clathrin function in clcA-mutants. Phenotypic deficiencies in development, cytokinesis, and osmoregulation showed that light chain was critical for clathrin function in Dictyostelium. In contrast with budding yeast, we found the light chain did not influence steady-state levels of clathrin, triskelion formation, or contribute to clathrin over-assembly on intracellular membranes. Imaging GFP-CHC in clcA- mutants showed that the heavy chain formed dynamic punctate structures that were remarkably similar to those found in wild-type cells. However, clathrin light chain knockouts showed a decreased association of clathrin with intracellular membranes. Unlike wild-type cells, half of the clathrin in clcA- mutants was cytosolic, suggesting that the absence of light chain compromised the assembly of triskelions onto intracellular membranes. Taken together, these results suggest a role for the Dictyostelium clathrin light chain in regulating the self-assembly of triskelions onto intracellular membranes, and demonstrate a crucial contribution of the light chain to clathrin function in vivo.

The AP-1 clathrin-adaptor is required for lysosomal enzymes sorting and biogenesis of the contractile vacuole complex in Dictyostelium cells

Adaptor protein complexes (AP) are major components of the cytoplasmic coat found on clathrin-coated vesicles. Here, we report the molecular and functional characterization of Dictyostelium clathrin-associated AP-1 complex, which in mammalian cells, participates mainly in budding of clathrin-coated vesicles from the trans-Golgi network (TGN). The gamma-adaptin AP-1 subunit was cloned and shown to belong to a Golgi-localized 300-kDa protein complex. Time-lapse analysis of cells expressing gamma-adaptin tagged with the green-fluorescent protein demonstrates the dynamics of AP-1-coated structures leaving the Golgi apparatus and rarely moving toward the TGN. Targeted disruption of the AP-1 medium chain results in viable cells displaying a severe growth defect and a delayed developmental cycle compared with parental cells. Lysosomal enzymes are constitutively secreted as precursors, suggesting that protein transport between the TGN and lysosomes is defective. Although endocytic protein markers are correctly localized to endosomal compartments, morphological and ultrastructural studies reveal the absence of large endosomal vacuoles and an increased number of small vacuoles. In addition, the function of the contractile vacuole complex (CV), an osmoregulatory organelle is impaired and some CV components are not correctly targeted.

The role of BEACH proteins in Dictyostelium

The BEACH family of proteins is a novel group of proteins with diverse roles in eukaryotic cells. The identifying feature of these proteins is the BEACH domain named after the founding members of this family, the mouse beige and the human Chediak-Higashi syndrome proteins. Although all BEACH proteins share a similar structural organization, they appear to have very distinct cellular roles, ranging from lysosomal traffic to apoptosis and cytokinesis. Very little is currently known about the function of most of these proteins, few binding-partner proteins have been identified, and no molecular mechanism for any of these proteins has been discovered. Thus, it is important to establish good model systems for the study of these novel proteins. Dictyostelium contains six BEACH proteins that can be classified into four subclasses. Two of them, LvsA and LvsB, have clearly distinct roles in the cell. LvsA is localized on the contractile vacuole membrane and is essential for cytokinesis and osmoregulation. LvsB is most similar in sequence to the mammalian beige/Chediak-Higashi syndrome proteins and shares with them a common function in lysosomal trafficking. Structural and functional analysis of these proteins in Dictyostelium will help elucidate the function of this enigmatic novel family of proteins.

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.

Spatially regulated recruitment of clathrin to the plasma membrane during capping and cell translocation

Clathrin-coated vesicles bud from selected cellular membranes to traffic-specific intracellular proteins. To study the dynamic properties of clathrin-coated membranes, we expressed clathrin heavy chain tagged with green fluorescent protein (GFP) in Dictyostelium cells. GFP-clathrin was functional and retained the native properties of clathrin: the chimeric protein formed classic clathrin lattices on cellular membranes and also rescued phenotypic defects of clathrin null cells. GFP-clathrin distributed into punctate loci found throughout the cytoplasm, on the plasma membrane, and concentrated to a perinuclear location. These clathrin-coated structures were remarkably motile and capable of rapid and bidirectional transport across the cell. We identified two local domains of the plasma membrane as sites for clathrin recruitment in motile cells. First, as cells translocated or changed shape and retracted their tails, clathrin was transiently concentrated on the membrane at the back of the cell tail. Second, as cells capped their cell surface receptors, clathrin was recruited locally to the membrane under the tight cap of cross-linked receptors. This suggests that local sites for clathrin polymerization on specific domains of the plasma membrane undergo rapid and dynamic regulation in motile cells.

The actin cytoskeleton of Dictyostelium: a story told by mutants

Actin-binding proteins are effectors of cell signalling and coordinators of cellular behaviour. Research on the Dictyostelium actin cytoskeleton has focused both on the elucidation of the function of bona fide actin-binding proteins as well as on proteins involved in signalling to the cytoskeleton. A major part of this work is concerned with the analysis of Dictyostelium mutants. The results derived from these investigations have added to our understanding of the role of the actin cytoskeleton in growth and development. Furthermore, the studies have identified several cellular and developmental stages that are particularly sensitive to an unbalanced cytoskeleton. In addition, use of GFP fusion proteins is revealing the spatial and temporal dynamics of interactions between actin-associated proteins and the cytoskeleton.

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.

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.

Circulation of the plasma membrane in Dictyostelium

We have developed a fluorimetric assay with the use of the dye FM1-43 to determine the rate at which Dictyostelium amoebae endocytose their surface membrane. Our results show that they do so about once each 4-10 min. A clathrin null mutant takes its surface up only approximately 30% more slowly, showing that this membrane uptake cannot be caused by clathrin-coated vesicles. Surprisingly, Ax2 and its parent, NC4, which differ in their rates of fluid-phase internalization by approximately 60-fold, take up their surfaces at the same rates. These results show that, in axenic cells, the uptake of fluid and of surface area are separate processes. The large activity of this new endocytic cycle in both Ax2 and NC4 amoebae appears capable of delivering sufficient new surface area to advance the cells' fronts during migration.

Regulatory light chain mutations affect myosin motor function and kinetics

The actin-based motor protein myosin II plays a critical role in many cellular processes in both muscle and non-muscle cells. Targeted disruption of the Dictyostelium regulatory light chain (RLC) caused defects in cytokinesis and multicellular morphogenesis. In contrast, a myosin heavy chain mutant lacking the RLC binding site, and therefore bound RLC, showed normal cytokinesis and development. One interpretation of these apparently contradictory results is that the phenotypic defects in the RLC null mutant results from mislocalization of myosin caused by aggregation of RLC null myosin. To distinguish this from the alternative explanation that the RLC can directly influence myosin activity, we expressed three RLC point mutations (E12T, G18K and N94A) in a Dictyostelium RLC null mutant. The position of these mutations corresponds to the position of mutations that have been shown to result in familial hypertrophic cardiomyopathy in humans. Analysis of purified Dictyostelium myosin showed that while these mutations did not affect binding of the RLC to the MHC, its phosphorylation by myosin light chain kinase or regulation of its activity by phosphorylation, they resulted in decreased myosin function. All three mutants showed impaired cytokinesis in suspension, and one produced defective fruiting bodies with short stalks and decreased spore formation. The abnormal myosin localization seen in the RLC null mutant was restored to wild-type localization by expression of all three RLC mutants. Although two of the mutant myosins had wild-type actin-activated ATPase, they produced in vitro motility rates half that of wild type. N94A myosin showed a fivefold decrease in actin-ATPase and a similar decrease in the rate at which it moved actin in vitro. These results indicate that the RLC can play a direct role in determining the force transmission and kinetic properties of myosin.

Disruption of a dynamin homologue affects endocytosis, organelle morphology, and cytokinesis in Dictyostelium discoideum

The identification and functional characterization of Dictyostelium discoideum dynamin A, a protein composed of 853 amino acids that shares up to 44% sequence identity with other dynamin-related proteins, is described. Dynamin A is present during all stages of D. discoideum development and is found predominantly in the cytosolic fraction and in association with endosomal and postlysosomal vacuoles. Overexpression of the protein has no adverse effect on the cells, whereas depletion of dynamin A by gene-targeting techniques leads to multiple and complex phenotypic changes. Cells lacking a functional copy of dymA show alterations of mitochondrial, nuclear, and endosomal morphology and a defect in fluid-phase uptake. They also become multinucleated due to a failure to complete normal cytokinesis. These pleiotropic effects of dynamin A depletion can be rescued by complementation with the cloned gene. Morphological studies using cells producing green fluorescent protein-dynamin A revealed that dynamin A associates with punctate cytoplasmic vesicles. Double labeling with vacuolin, a marker of a postlysosomal compartment in D. discoideum, showed an almost complete colocalization of vacuolin and dynamin A. Our results suggest that that dynamin A is likely to function in membrane trafficking processes along the endo-lysosomal pathway of D. discoideum but not at the plasma membrane.

Purification of clathrin heavy and light chain from Dictyostelium discoideum

Clathrin, a protein important for endocytosis, is a hexamer composed of three heavy chains and three light chains. We report here the purification scheme used to isolate the clathrin protein from the simple eukaryote, Dictyostelium discoideum. Using a combination of differential centrifugation and column chromatography, we isolated approximately 2 mg of clathrin triskelions from 150-200 g of Dictyostelium cells. One additional step purified the 30-kDa clathrin light chain to homogeneity. Glycerol gradient centrifugation was used to determine an S value of 7.9 for purified clathrin. Rotary shadowed images of Dictyostelium clathrin revealed trimeric molecules with extended legs measuring 48 +/- 5 nm, similar in length to the legs of mammalian and yeast clathrin triskelions. The single clathrin light chain proved resistant to heat treatment, a property also similar to light chains from other species. The conservation of these physical properties in Dictyostelium clathrin demonstrates the potential of this model organism for the study of clathrin structure and function.

A novel role for clathrin in cytokinesis

Using clathrin-minus Dictyostelium cells, we identified a novel requirement for clathrin during cytokinesis. In suspension culture, clathrin-minus cells failed to divide and became large and multinucleate. This cytokinesis deficiency was not attributable to a pleiotropic effect on the actomyosin cytoskeleton, since other cellular events driven by myosin II (e.g., cortical contraction and capping of concanavalin A receptors) remained intact in clathrin-minus cells. Examination of cells expressing myosin II tagged with green fluorescent protein showed that clathrin-minus cells failed to assemble myosin II into a functional contractile ring. This inability to localize myosin II to a particular location was specific for cytokinesis, since clathrin-minus cells moving across a substrate localized myosin II properly to their posterior cortexes. These results demonstrate that clathrin is essential for construction of a functional contractile ring during cell division.