Capping of the barbed ends of actin filaments by a high-affinity profilin-actin complex

Regulation of a Coupled MARCKS-PI3K Lipid Kinase Circuit by Calmodulin: Single-Molecule Analysis of a Membrane-Bound Signaling Module

Amoeboid cells that employ chemotaxis to travel up an attractant gradient possess a signaling network assembled on the leading edge of the plasma membrane that senses the gradient and remodels the actin mesh and cell membrane to drive movement in the appropriate direction. In leukocytes such as macrophages and neutrophils, and perhaps in other amoeboid cells as well, the leading edge network includes a positive feedback loop in which the signaling of multiple pathway components is cooperatively coupled. Cytoplasmic Ca²⁺ is a recently recognized component of the feedback loop at the leading edge where it stimulates phosphoinositide-3-kinase (PI3K) and the production of its product signaling lipid phosphatidylinositol 3,4,5-trisphosphate (PIP₃). A previous study implicated Ca²⁺-activated protein kinase C (PKC) and the phosphatidylinositol 4,5-bisphosphate (PIP₂) binding protein MARCKS as two important players in this signaling, because PKC phosphorylation of MARCKS releases free PIP₂ that serves as the membrane binding target and substrate for PI3K. This study asks whether calmodulin (CaM), which is known to directly bind MARCKS, also stimulates PIP₃ production by releasing free PIP₂. Single-molecule fluorescence microscopy is used to quantify the surface density and enzyme activity of key protein components of the hypothesized Ca²⁺-CaM-MARCKS-PIP₂-PI3K-PIP₃ circuit. The findings show that CaM does stimulate PI3K lipid kinase activity by binding MARCKS and displacing it from PIP₂ headgroups, thereby releasing free PIP₂ that recruits active PI3K to the membrane and serves as the substrate for the generation of PIP₃. The resulting CaM-triggered activation of PI3K is complete in seconds and is much faster than PKC-triggered activation, which takes minutes. Overall, the available evidence implicates both PKC and CaM in the coupling of Ca²⁺ and PIP₃ signals and suggests these two different pathways have slow and fast activation kinetics, respectively.

Effect of capping protein on a growing filopodium

Filopodia, or the growth of bundles of biological fibers outwards from a biological cell surface while enclosed in a membrane tube, are implicated in many processes vital to life. This study models the effect of capping protein on such filopodia, paying close attention to the polymerization dynamics of biological fiber bundles within long membrane tubes. Due to the effects of capping protein, the number of fibers in the filopodium bundle decreases down the length of the enclosing membrane tube. This decrease in the number of fibers down the length of a growing filopodium is found to have profound implications for the dynamics and stability of filopodia in general. This study theoretically finds that the presence of even a relatively modest amount of capping protein can have a large effect on the growth of typical filopodia, such as can be found in fibroblasts, keratocytes, and neuronal growth cones. As an illustration of this modeling work, this study investigates the striking example of the acrosomal reaction in the sea cucumber Thyone, whose filopodia can grow remarkably quickly to approximately 90 mum in approximately 10 s, and where the number of fibers is known to decrease down the length of the filopodium, presumably due to progressive fiber end-capping occurring as the filopodium grows. Realistic future dynamical theories for filopodium growth are likely to rely on an accurate treatment of the kinds of capping protein effects analyzed in this work.

How depolymerization can promote polymerization: the case of actin and profilin

Rapid polymerization and depolymerization of actin filaments in response to extracellular stimuli is required for normal cell motility and development. Profilin is one of the most important actin-binding proteins; it regulates actin polymerization and interacts with many cytoskeletal proteins that link actin to extracellular membrane. The molecular mechanism of profilin has been extensively considered and debated in the literature for over two decades. Here we discuss several accepted hypotheses regarding the mechanism of profilin function as well as new recently emerged possibilities. Thermal noise is routine in molecular world and unsurprisingly, nature has found a way to utilize it. An increasing amount of theoretical and experimental research suggests that fluctuation-based processes play important roles in many cell events. Here we show how a fluctuation-based process of exchange diffusion is involved in the regulation of actin polymerization.

Profilin: emerging concepts and lingering misconceptions

Conflicting data suggest that profilin might function to promote either actin polymerization or depolymerization in cells. There are theoretical reasons and supportive data to suggest that profilin might do both. Perhaps the most accurate description of profilin emphasizes its ability to augment actin-filament dynamics, both in polymerization and in depolymerization. The effect of profilin on the critical concentration of actin, its ability to depolymerize filaments at the barbed end and the formation of a ternary complex with thymosin beta(4) all need to be accurately represented in any attempt to determine a model for profilin function.

Effects of profilin and thymosin beta4 on the critical concentration of actin demonstrated in vitro and in cell extracts with a novel direct assay

The free actin concentration at steady state, Ac, is a variable that determines how actin regulatory proteins influence the extent of actin polymerization. We describe a novel method employing fluorescence anisotropy to directly measure Ac in any sample after the addition of a trace amount of labeled thymosin beta4 or thymosin beta4 peptide. Using this assay, we confirm earlier theoretical work on the helical polymerization of actin and confirm the effects of actin filament-stabilizing drugs and capping proteins on Ac, thereby validating the assay. We also confirm a controversial prior observation that profilin lowers the critical concentration of Mg2+-actin. A general mechanism is proposed to explain this effect, and the first quantitative dose-response curve for the effect of profilin on Ac facilitates its evaluation. This mechanism also predicts the effect of profilin on critical concentration in the presence of the limited amount of capping protein, which is the condition often found in cells, and the effect of profilin on critical concentration in cell extracts is demonstrated for the first time. Additionally, nonlinear effects of thymosin beta4 on the steady state amount of F-actin are explained by the observed changes in Ac. This assay has potential in vivo applications that complement those demonstrated in vitro.

Transcription profiling of Candida albicans cells undergoing the yeast-to-hyphal transition

The ability of the pathogenic fungus Candida albicans to switch from a yeast to a hyphal morphology in response to external signals is implicated in its pathogenicity. We used glass DNA microarrays to investigate the transcription profiles of 6333 predicted ORFs in cells undergoing this transition and their responses to changes in temperature and culture medium. We have identified several genes whose transcriptional profiles are similar to those of known virulence factors that are modulated by the switch to hyphal growth caused by addition of serum and a 37 degrees C growth temperature. Time course analysis of this transition identified transcripts that are induced before germ tube initiation and shut off later in the developmental process. A strain deleted for the Efg1p and Cph1p transcription factors is defective in hyphae formation, and its response to serum and increased temperature is almost identical to the response of a wild-type strain grown at 37 degrees C in the absence of serum. Thus Efg1p and Cph1p are needed for the activation of the transcriptional program that is induced by the presence of serum.

Phosphatidylinositol-4-phosphate 5-kinase activity is stimulated during temperature-induced morphogenesis in Candida albicans

Phosphoinositides are important lipid signalling molecules in eukaryotic cells. Phosphatidylinositol-4-phosphate 5-kinase (PI4P5K) catalyses the production of phosphatidylinositol 4,5-bisphosphate (PIP2), which stimulates phospholipase D1 (PLD1) activity in mammalian and yeast cells. PLD1 catalyses the formation of phosphatidic acid (PA), which has been shown to activate PI4P5Ks in mammalian and Saccharomyces cerevisiae cells. In the present study, PI4P5K activity in the opportunistic pathogen Candida albicans was identified. A gene with significant sequence homology to the S. cerevisiae PI4P5K was cloned and designated MSS4. This gene was demonstrated to encode a functional PI4P5K by expression in S. cerevisiae. This enzyme was found to be membrane-associated and was stimulated by PA. Within the first 20 min after induction of polarized hyphal growth induced by a shift to elevated temperature, PI4P5K activity increased 2.5-fold. This stimulation was not observed when hyphae were induced by a combination of elevated temperature and serum. A lack of PLD1 activity resulted in the loss of induction of PI4P5K activity during the morphogenetic switch. Furthermore, the addition of propranolol attenuated the stimulation of PI4P5K activity during morphogenesis. These results suggest that PA derived from PLD1 activity stimulates C. albicans PI4P5K during the switch to the hyphal form under some conditions.

Function of the cytoskeleton in human neutrophils and methods for evaluation

The cytoskeleton plays a critical role in the determination of cell shape and serves as a scaffold for critical cellular enzymes and adhesion molecules. It provides structural integrity for the cell and regulates the function of many biochemical events that are critical to cellular function. The microfilamentous cytoskeleton participates in force generation necessary for shape change and motion. In neutrophils and other motile cells, polymerization of actin likely drives extension of the lamellae and participates in force generation through interaction with myosin, by polymerization alone and by osmotic mechanisms. Here, we will focus on the microfilamentous cytoskeleton in the neutrophil and briefly review its function as well as some direct and indirect methods that have been used to asses its role in neutrophil function. The discussion will address general approaches and leaves the details of the methods to the references.

Actin and phosphoinositide binding by the ActA protein of the bacterial pathogen Listeria monocytogenes

The surface protein ActA of the pathogenic bacterium Listeria monocytogenes induces actin-driven movement of bacteria in the cytoplasm of infected host cells and serves as a model for actin-based motility in general. We generated and purified soluble recombinant fragments of ActA and assessed their ability to interact with the acidic phospholipids phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate, both implicated in the regulation of actin polymerization. Purified ActA consisted of biologically active, elongated molecules with an alpha-helix and beta-sheet content of 11 and 32%, respectively. In the presence of either phosphatidylinositol 4,5-bisphosphate or phosphatidylinositol 3,4,5-trisphosphate, but not phosphatidylcholine, ActA molecules underwent a structural change that raised the alpha-helix content to 19% and lowered the beta-sheet content to 27%. Co-sedimentation experiments with phosphatidylcholine vesicles containing different acidic phospholipids demonstrated that ActA binds preferentially to D-3 phosphoinositides. The D-3 phosphoinositide binding activity was mapped to a small subregion in the N-terminal domain of ActA. This subregion comprised 19 amino acids and showed homology to cecropins. In addition, we found that amino acids 33 to 74 of ActA mediated actin binding by the whole, folded ActA molecule. These findings shed new light on ActA function.

Chemoattractant-induced lamellipod extension

The mammary adenocarcinoma cell line MTLn3 is chemotactic towards epidermal growth factor (EGF), and this induced motility is thought to promote breast cancer invasion and metastasis. Stimulation of MTLn3 cells with EGF results in the extension of a flat, thin structure filled with filamentous actin and termed a lamellipod. Lamellipod extension is dependent on actin polymerization and is localized to the border of adherent cells. The structure of EGF-stimulated lamellipods in MTLn3 cells is well suited to analysis of chemoattractant-stimulated protrusion. Actin polymerization occurs within 200 nm of the extending edge of the lamellipod. Although extension of the lamellipod is not dependent upon interaction with the substratum, stabilization of the extended lamellipod is dependent on an adhesive substratum. Dorsal ruffling is suppressed during lamellipod extension. Tyrosine phosphorylation is reduced in preexisting focal contacts compared to new contacts induced by EGF stimulation. The coordination of turnover of focal contacts with lamellipod extension is proposed to result in polarized cell motility in response to gradients of chemoattractants.