Actin-based Motility System Research Articles

Listeria monocytogenes: cell biology of invasion and intracellular growth.

The Gram-positive pathogen Listeria monocytogenes is able to promote its entry into a diverse range of mammalian host cells by triggering plasma membrane remodeling, leading to bacterial engulfment. Upon cell invasion, L. monocytogenes disrupts its internalization vacuole and translocates to the cytoplasm, where bacterial replication takes place. Subsequently, L. monocytogenes uses an actin-based motility system that allows bacterial cytoplasmic movement and cell-to-cell spread. L. monocytogenes therefore subverts host cell receptors, organelles and the cytoskeleton at different infection steps, manipulating diverse cellular functions that include ion transport, membrane trafficking, post-translational modifications, phosphoinositide production, innate immune responses as well as gene expression and DNA stability.

Arp2/3 complex ATP hydrolysis promotes lamellipodial actin network disassembly but is dispensable for assembly

We examined the role of ATP hydrolysis by the Arp2/3 complex in building the leading edge of a cell by studying the effects of hydrolysis defects on the behavior of the complex in the lamellipodial actin network of Drosophila S2 cells and in a reconstituted, in vitro, actin-based motility system. In S2 cells, nonhydrolyzing Arp2 and Arp3 subunits expanded and delayed disassembly of lamellipodial actin networks and the effect of mutant subunits was additive. Arp2 and Arp3 ATP hydrolysis mutants remained in lamellipodial networks longer and traveled greater distances from the plasma membrane, even in networks still containing wild-type Arp2/3 complex. In vitro, wild-type and ATP hydrolysis mutant Arp2/3 complexes each nucleated actin and built similar dendritic networks. However, networks constructed with Arp2/3 hydrolysis-defective mutants were more resistant to disassembly by cofilin. Our results indicate that ATP hydrolysis on both Arp2 and Arp3 contributes to dissociation of the complex from the actin network but is not strictly necessary for lamellipodial network disassembly.

THRUMIN1 Is a Light-Regulated Actin-Bundling Protein Involved in Chloroplast Motility

Chloroplast movement in response to changing light conditions optimizes photosynthetic light absorption. This repositioning is stimulated by blue light perceived via the phototropin photoreceptors and is transduced to the actin cytoskeleton. Some actin-based motility systems use filament reorganizations rather than myosin-based translocations. Recent research favors the hypothesis that chloroplast movement is driven by actin reorganization at the plasma membrane, but no proteins affecting chloroplast movements have been shown to associate with both the plasma membrane and actin filaments in vivo. Here we identified THRUMIN1 as a critical link between phototropin photoreceptor activity at the plasma membrane and actin-dependent chloroplast movements. THRUMIN1 bundles filamentous actin in vitro, and it localizes to the plasma membrane and displays light- and phototropin-dependent localization to microfilaments in vivo. These results suggest that phototropin-induced actin bundling via THRUMIN1 is important for chloroplast movement. A mammalian homolog of THRUMIN1, GRXCR1, has been implicated in auditory responses and hair cell stereocilla development as a regulator of actin architecture. Studies of THRUMIN1 will help elucidate the function of this family of eukaryotic proteins.

Capping Protein Increases the Rate of Actin-Based Motility by Promoting Filament Nucleation by the Arp2/3 Complex

Capping protein (CP) is an integral component of Arp2/3-nucleated actin networks that drive amoeboid motility. Increasing the concentration of capping protein, which caps barbed ends of actin filaments and prevents elongation, increases the rate of actin-based motility in vivo and in vitro. We studied the synergy between CP and Arp2/3 using an in vitro actin-based motility system reconstituted from purified proteins. We find that capping protein increases the rate of motility by promoting more frequent filament nucleation by the Arp2/3 complex and not by increasing the rate of filament elongation as previously suggested. One consequence of this coupling between capping and nucleation is that, while the rate of motility depends strongly on the concentration of CP and Arp2/3, the net rate of actin assembly is insensitive to changes in either factor. By reorganizing their architecture, dendritic actin networks harness the same assembly kinetics to drive different rates of motility.

Kinesin and Kinectin Can Associate with the Melanosomal Surface and Form a Link with Microtubules in Normal Human Melanocytes

Microtubuli play an important role in the organization of organelles and membrane traffic. They are present in melanocytic dendrites through which melanosomes are transported towards keratinocytes. Besides the actin-based motility systems, micro-tubuli-associated motor proteins also play a critical role in melanosome movement, as has recently been confirmed in mouse melanocytes. We investigated the in vitro expression of two forms of human conventional kinesin and its receptor kinectin in normal human epidermal melanocytes, keratinocytes, and dermal fibroblasts by reverse transcription polymerase chain reaction and northern blot analysis. In an attempt to gain insight into the subcellular distribution of kinesin and kinectin in melanocytes, double immunofluorescent staining and immunogold electron microscopy were performed. In all studied skin cells ubiquitous and neuronal kinesin are expressed as well as the kinectin receptor. Immunofluorescent staining shows distinct but partially overlapping distributions for kinesin heavy chain and melanosomes, suggesting that kinesin is associated with some but not all of the melanosomes. Similar observations for kinectin indicate that this receptor can colocalize with melanosomes, which was confirmed by immunoelectron microscopy. The latter technique allowed us to demonstrate a close association between kinesin heavy chain, microtubuli, and melanosomes. The combined data from reverse transcription polymerase chain reaction, northern blot analysis, double immunofluorescent staining, and immunogold electron microscopy suggest that kinesins and kinectin have an important role in microtubuli-based melanosome transport in human melanocytes.

Regulation of Cutaneous Allergic Reaction by Odorant Inhalation

Microtubuli play an important role in the organization of organelles and membrane traffic. They are present in melanocytic dendrites through which melanosomes are transported towards keratinocytes. Besides the actin-based motility systems, microtubuli-associated motor proteins also play a critical role in melanosome movement, as has recently been confirmed in mouse melanocytes. We investigated the in vitro expression of two forms of human conventional kinesin and its receptor kinectin in normal human epidermal melanocytes, keratinocytes, and dermal fibroblasts by reverse transcription polymerase chain reaction and northern blot analysis. In an attempt to gain insight into the subcellular distribution of kinesin and kinectin in melanocytes, double immunofluorescent staining and immunogold electron microscopy were performed. In all studied skin cells ubiquitous and neuronal kinesin are expressed, as well as the kinectin receptor. Immunofluorescent staining shows distinct but partially overlapping distributions for kinesin heavy chain and melanosomes, suggesting that kinesin is associated with some but not all of the melanosomes. Similar observations for kinectin indicate that this receptor can colocalize with melanosomes, which was confirmed by immunoelectron microscopy. The latter technique allowed us to demonstrate a close association between kinesin heavy chain, microtubuli, and melanosomes. The combined data from reverse transcription polymerase chain reaction, northern blot analysis, double immunofluorescent staining, and immunogold electron microscopy suggest that kinesins and kinectin have an important role in microtubuli-based melanosome transport in human melanocytes.

Interactions between cellular actin and human respiratory syncytial virus (HRSV)

Actin the main component of the cellular microfilament network, is present in human respiratory syncytial virus (HRSV) purified virions, as an internal component. This fact and the results of immunoprecipitation studies indicate that during HRSV infection in HEp-2 cells there are interactions between cellular actin and viral components, that can promote a transitory increase in the polymerization of synthetized actin, mainly of the β isotype. This increased actin polymerization can be related with the formation of cytoplasmic extensions, that contain β actin and viral particles observed in the HRSV infected HEp-2 cells. The formation of these structures may indicate that HRSV has developed an actin-based motility system similar to that described for other viral and bacterial systems.

Calcium-independent regulation of pigment granule aggregation and dispersion in teleost retinal pigment epithelial cells.

In the eyes of teleosts and amphibians, melanin pigment granules of the retinal pigment epithelium (RPE) migrate in response to changes in light conditions. In the light, pigment granules disperse into the cells' long apical projections, thereby shielding the rod photoreceptor outer segments and reducing their extent of bleach. In darkness, pigment granules aggregate towards the base of the RPE cells. In vitro, RPE pigment granule aggregation can be induced by application of nonderivatized cAMP, and pigment granule dispersion can be induced by cAMP washout. In previous studies based on RPE-retina co-cultures, extracellular calcium was found to influence pigment granule migration. To examine the role of calcium in regulation of RPE pigment granule migration in the absence of retinal influences, we have used isolated RPE sheets and dissociated, cultured RPE cells. Under these conditions depletion of extracellular or intracellular calcium ([Ca2+]o, [Ca2+]i) had no effect on RPE pigment granule aggregation or dispersion. Using the intracellular calcium dye fura-2 and a new dye, fura-pe3, to monitor calcium dynamics in isolated RPE cells, we found that [Ca2+]i did not change from basal levels when pigment granule aggregation was triggered by cAMP, or dispersion was triggered by cAMP washout. Also, no change in [Ca2+]i was detected when dispersion was triggered by cAMP washout in the presence of 10 microM dopamine, a treatment previously shown to enhance dispersion. In addition, elevation of [Ca2+]i by addition of ionomycin neither triggered pigment movements, nor interfered with pigment granule motility elicited by cAMP addition or washout. Since other studies have indicated that actin plays a role in both pigment granule dispersion and aggregation in RPE, our findings suggest that RPE pigment granule migration depends on an actin-based motility system that is not directly regulated by calcium.

Genetic Alteration of Proteins in Actin-Based Motility Systems

Skeletal muscle fibers are endpoints of extreme structural and functional specialization, but the basic components of their thin and thick filaments, actin and myosin, are similar to the constituents of motor systems in non­ muscle cells. The same is true for various regulatory and anchoring proteins of the actomyosin system. Sequence information of cytoske1etal proteins from lower eukaryotes indicates that proteins related to those in muscles have emerged during evolution. Together these proteins constitute the actin-based motors in non-muscle cells of a great variety of organisms. Often these proteins are built of domains: units of defined structure and function that can be used as modules to generate families of related proteins. Three principal questions are posed by molecular geneticists: (a) what is the role of domains in cytoskeletal proteins, (b) what is the relationship between primary sequence and function, and (c) what are the in vivo activities of these proteins? In this review we cover actin, myosin, and proteins that reg­ ulate the activity or assembly state of these two proteins, but we do not discuss the system of proteins like vinculin and spectrin that anchor the cytoskeleton to the plasma membrane. We summarize genetic approaches

Genetic Alteration Of Proteins In Actin-Based Motility Systems

Genetic Alteration Of Proteins In Actin-Based Motility Systems