Electron tomography of vitrified cells is a noninvasive three-dimensional imaging technique that opens up new vistas for exploring the supramolecular organization of the cytoplasm. We applied this technique to Dictyostelium cells, focusing on the actin cytoskeleton. In actin networks reconstructed without prior removal of membranes or extraction of soluble proteins, the cross-linking of individual microfilaments, their branching angles, and membrane attachment sites can be analyzed. At a resolution of 5 to 6 nanometers, single macromolecules with distinct shapes, such as the 26S proteasome, can be identified in an unperturbed cellular environment.

Cell-cell adhesion mediated by specific cell-surface molecules is essential for multicellular development. Here we quantify de-adhesion forces at the resolution of individual cell-adhesion molecules, by controlling the interactions between single cells and combining single-molecule force spectroscopy with genetic manipulation. Our measurements are focused on a glycoprotein, contact site A (csA), as a prototype of cell-adhesion proteins. csA is expressed in aggregating cells of Dictyostelium discoideum, which are engaged in development of a multicellular organism. Adhesion between two adjacent cell surfaces involves discrete interactions characterized by an unbinding force of 23 +/- 8 pN, measured at a rupture rate of 2.5 +/- 0.5 microm s-1.

Nuclear pore complexes (NPCs) are gateways for nucleocytoplasmic exchange. To analyze their structure in a close-to-life state, we studied transport-active, intact nuclei from Dictyostelium discoideum by means of cryoelectron tomography. Subvolumes of the tomograms containing individual NPCs were extracted in silico and subjected to three-dimensional classification and averaging, whereby distinct structural states were observed. The central plug/transporter (CP/T) was variable in volume and could occupy different positions along the nucleocytoplasmic axis, which supports the notion that it essentially represents cargo in transit. Changes in the position of the CP/T were accompanied by structural rearrangements in the NPC scaffold.

Coronin is a protein involved in cell locomotion and cytokinesis of Dictyostelium discoideum. Here we show that coronin is strongly enriched in phagocytic cups formed in response to particle attachment. A fusion of coronin with green fluorescent protein (GFP) accumulates in the cups within less than 1 min upon attachment of a particle and is gradually released from the phagosome within 1 min after engulfment is completed. Phagocytic cup formation competes with leading edge formation and can be interrupted at any stage. When the cup regresses, coronin dissociates from the site of accumulation. TRITC-labeled yeast cells have been used to assay phagocytosis quantitatively in wild-type and coronin-null cells. In the mutant, the rate of uptake is reduced to about one third, which shows that coronin contributes to the efficiency of phagocytosis to about the same extent as it improves the speed of cell locomotion.

Actin polymerization is typically initiated at specific sites in a cell by membrane-bound protein complexes, and the resulting structures are involved in specialized cellular functions, such as migration, particle uptake, or mitotic division. Here we analyze the potential of the actin system to self-organize into waves that propagate on the planar, substrate-attached membrane of a cell. We show that self-assembly involves the ordered recruitment of proteins from the cytoplasmic pool and relate the organization of actin waves to their capacity for applying force. Three proteins are shown to form distinct three-dimensional patterns in the actin waves. Myosin-IB is enriched at the wave front and close to the plasma membrane, the Arp2/3 complex is distributed throughout the waves, and coronin forms a sloping layer on top of them. CARMIL, a protein that links myosin-IB to the Arp2/3 complex, is also recruited to the waves. Wave formation does not depend on signals transmitted by heterotrimeric G-proteins, nor does their propagation require SCAR, a regulator upstream of the Arp2/3 complex. Propagation of the waves is based on an actin treadmilling mechanism, indicating a program that couples actin assembly to disassembly in a three-dimensional pattern. When waves impinge on the cell perimeter, they push the edge forward; when they reverse direction, the cell border is paralyzed. These data show that force-generating, highly organized supramolecular networks are autonomously formed in live cells from molecular motors and proteins controlling actin polymerization and depolymerization.

A soluble actin binding protein of Dictyostelium discoideum cells has been extracted and purified from precipitated actin‐myosin complexes. This protein with a relative molecular mass of 55 kDa has been named coronin because of its association with crown‐shaped cell surface projections of growth‐phase D. discoideum cells. In aggregating cells, which respond most sensitively to the chemoattractant cyclic AMP, coronin is accumulated at the front where surface projections are directed towards a cAMP source. Since these cells can quickly change shape and polarity, it follows that coronin is rapidly reshuffled within the cells during motion and chemotactic orientation. The cDNA derived sequence of coronin indicates a protein of 49 kDa, consisting of an amino‐terminal domain with similarities to the beta subunits of G proteins and a carboxy‐terminal domain with a high tendency for alpha‐helical structure. It is hypothesized that coronin is implicated in the transmission of chemotactic signals from cAMP receptors in the plasma membrane through G proteins to the cortical cytoskeleton, whose structure and activity is locally modulated.

Myosin VII plays an important role in mediating the initial binding of cells to substrata, a novel role for an unconventional myosin.

Cortexillins I and II of D. discoideum constitute a novel subfamily of proteins with actin-binding sites of the alpha-actinin/spectrin type. The C-terminal halves of these dimeric proteins contain a heptad repeat domain by which the two subunits are joined to form a two-stranded, parallel coiled coil, giving rise to a 19 nm tail. The N-terminal domains that encompass a consensus actin-binding sequence are folded into globular heads. Cortexillin-linked actin filaments form preferentially anti-parallel bundles that associate into meshworks. Both cortexillins are enriched in the cortex of locomoting cells, primarily at the anterior and posterior ends. Elimination of the two isoforms by gene disruption gives rise to large, flattened cells with rugged boundaries, portions of which are often connected by thin cytoplasmic bridges. The double-mutant cells are multinucleate owing to a severe impairment of cytokinesis.