Endocytosis: Actin in the Driving Seat

Targets and Potential Mechanism of Chondroitin Sulfate A-selenium Nanoparticle on Kashin-Beck Disease Chondrocytes

Kashin-Beck disease (KBD) is a chronic and deformable osteoarthropathy linked to low selenium. Limited cartilage regeneration poses challenges for its treatment. Previous studies have found that chondroitin sulfate A-selenium nanoparticle (CSA-SeNP) protects chondrocytes. This study used label-free LC-MS/MS quantitative proteomics to identify differentially expressed proteins and pathways in KBD chondrocytes post-CSA-SeNP treatment. Western blot (WB) was used to verify the key differential proteins, and transmission electron microscopy (TEM) was used to observe the ultrastructure of chondrocytes. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed proteins mainly involved in RNA processing, translation, peptide biosynthesis, gene expression, rRNA metabolism, and ribonucleoprotein complex biogenesis. Notably, 121 proteins were up-regulated, 27 down-regulated, and 117 specifically expressed in the CSA-SeNP group. Enriched KEGG pathways included ribosome biogenesis, mRNA surveillance, endoplasmic reticulum protein processing, and endocytosis. Proteins related to autophagy, ER stress, cell homeostasis, protein processing and transport, including PELO, WES1, PLAA, RRBP1, ARC1B, ARFGAP2, and SH3KBP1, were significantly up-regulated in the CSA-SeNP group. In conclusion, our results demonstrated that CSA-SeNP may exert protective effects on chondrocytes in adult patients with KBD mainly through the regulation of target proteins and pathways related to ribosome biogenesis, mRNA surveillance, endoplasmic reticulum protein processing, endocytosis, autophagy, ER stress, and cell homeostasis. Further studies with larger sample size and in vivo to identify, screen and verify the regulatory effects of target proteins and pathways may provide more information for elucidating the mechanism of CSA-SeNP in the treatment of KBD.

Knockdown of BmTCP-1β Delays BmNPV Infection in vitro

The molecular mechanism of silkworm resistance to Bombyx mori nucleopolyhedrovirus (BmNPV) infection remains unclear. The chaperonin containing t-complex polypeptide 1 (TCP-1) is essential for the folding of tubulin and actin to produce stable and functional competent protein conformation. However, little is known about this protein in silkworm. In the present study, a gene encoding the TCP-1β protein in silkworm was characterized, which has an open reading fragment of 1,611 bp encoding a predicted 536 amino acid residue-protein with a molecular weight of approximately 57.6 kDa containing a Cpn60_TCP1 functional domain. The sequence conservation is 81.52%. The highest level of BmTCP-1β mRNA expression was found in the midgut, while the lowest was in the hemolymph. To further study the function of BmTCP-1β, expression was knocked down with siRNA in vitro, resulting in significant downregulation of the selected cytoskeletal-related genes, actin and tubulin, which was also confirmed by overexpression of BmTCP-1β in BmN cells using the pIZT/V5-His-mCherry insect vector. Moreover, knockdown of BmTCP-1β significantly prolonged the infection process of BmNPV in BmN cells, which was also verified by overexpression of BmTCP-1β in BmN cells. Based on the results of the present study, we concluded that BmTCP-1β plays a vital role in BmNPV infection by regulating the expression of tubulin and actin. Taken together, our work provides valuable data for the clarification of the molecular mechanism of silkworm resistance to BmNPV infection.

The effect of G-CSF on F-actin reorganization in HL-60 and K562 cell lines

The aim of this investigation was to show the influence of G-CSF (G-CSF) on the F-actin cytoskeleton and the morphology of G-CSFR-proficient HL-60 and G-CSFR-deficient K562 cell lines. In the present study, we show changes in F-actin distribution in HL-60 cells after treatment with 5 and 10 ng/ml concentration of G-CSF but also changes in the organization and fluorescence intensity of F-actin in the K562 cell line. After treatment of HL-60 cells with 5 ng/ml concentration of G-CSF we observed an increase in F-actin levels. Additionally, a higher labeling of nuclear F-actin under TEM was observed. Moreover, changes in the cell cycle indicate cell differentiation. On the other hand, in the K562 cell line we observed an increase in the percentage sub-G1 cells following treatment with both concentration of G-CSF. Furthermore, an increase in the percentage of late apoptotic cells after G-CSF treatment was observed. A statistically significant difference in the cytoplasmic F-actin levels was not detected, but nuclear levels were decreased. In conclusion, we suggest that the G-CSF-based reorganization of actin filaments in HL-60 cells is involved in the differentiation process. Moreover, we suggest that the G-CSF-induced changes observed in K562 cells are associated with a G-CSF receptor-independent pathway or its binding to other similar receptors.

Proteomic changes in articular cartilage of human endemic osteoarthritis in China

Kashin-Beck disease (KBD) is a chronic endemic osteochondropathy with unclear pathogenesis. It is a degenerative disease similar to osteoarthritis, but with different manifestations of cartilage damage. The aim of this investigation was to show the protein changes in KBD cartilage and to identify the candidate proteins in order to understand the pathogenesis of the disease. Proteins were extracted from the media of primary cell cultures of KBD and normal chondrocytes, and separated by two-dimensional fluorescence difference gel electrophoresis (2-D DIGE). MALDI-TOF/TOF analysis revealed statistically significant differences in 27 proteins from KBD chondrocyte cultures, which consisted of 17 up-regulated and ten down-regulated proteins. The results were further validated by Western blot analysis. The proteins identified are mainly involved in cellular redox homeostasis and stress response (MnSOD, Hsp27, Peroxiredoxin-1, and Cofilin-1), glycolysis (PGK-1, PGM-1, α-enolase), and cell motility and cytoskeletal organization (Actin, Calponin-2, and Keratin). These KBD-associated proteins indicate that cytoskeletal remodeling, glycometabolism, and oxidative stress are abnormal in KBD articular cartilage.

Characterization of large yellow croaker (Pseudosciaena crocea) β-actin promoter supports β-actin gene as an internal control for gene expression modulation and its potential application in transgenic studies in fish

As a housekeeping gene, β-actin is one of the most commonly used reference gene and its promoter is widely used in transgenic studies in mammals and fish. In this study, we used genomic walker technology to clone the β-actin gene (Lycβ-actin) promoter sequence from large yellow croaker, an economically important marine fish in China. The Lycβ-actin promoter region spans 3350 nucleotides (nt) and contains several transcription factor binding sites and a conserved enhancer motif (ATGGTAATAA) in the first intron. A promoter activity assay showed that this promoter region can drive enhanced green fluorescent protein (EGFP) gene expression in the fish cell line, EPC. Luciferase activity analysis demonstrated that the activity of the Lycβ-actin promoter is not affected by poly(I:C) or lipopolysaccharide (LPS) stimulation. Absolute real-time PCR analysis of various tissues revealed that Lycβ-actin expression levels are not significantly altered by poly(I:C) or inactivated trivalent bacterial vaccine (P > 0.05). These results suggest that β-actin can be used as a suitable internal control for gene expression modulation in response to immune stimulations in large yellow croaker. In vivo transgenic experiments showed that the Lycβ-actin promoter region can drive efficient EGFP expression in large yellow croaker fries or fertilized zebrafish eggs, supporting its potential application in transgenic studies in fish.

Actin-binding proteins implicated in the formation of the punctate actin foci stimulated by the self-incompatibility response in Papaver

The actin cytoskeleton is a key target for signaling networks and plays a central role in translating signals into cellular responses in eukaryotic cells. Self-incompatibility (SI) is an important mechanism responsible for preventing self-fertilization. The SI system of Papaver rhoeas pollen involves a Ca(2+)-dependent signaling network, including massive actin depolymerization as one of the earliest cellular responses, followed by the formation of large actin foci. However, no analysis of these structures, which appear to be aggregates of filamentous (F-)actin based on phalloidin staining, has been carried out to date. Here, we characterize and quantify the formation of F-actin foci in incompatible Papaver pollen tubes over time. The F-actin foci increase in size over time, and we provide evidence that their formation requires actin polymerization. Once formed, these SI-induced structures are unusually stable, being resistant to treatments with latrunculin B. Furthermore, their formation is associated with changes in the intracellular localization of two actin-binding proteins, cyclase-associated protein and actin-depolymerizing factor. Two other regulators of actin dynamics, profilin and fimbrin, do not associate with the F-actin foci. This study provides, to our knowledge, the first insights into the actin-binding proteins and mechanisms involved in the formation of these intriguing structures, which appear to be actively formed during the SI response.

Nuclear translocation of beta-actin is involved in transcriptional regulation during macrophage differentiation of HL-60 cells

The functional significance of nuclear translocation of β-actin remains unclear. Here, we demonstrate that PMA induces β-actin accumulation in the nucleus and binding to various target genes with different functions. We also find that accumulated nuclear β-actin is involved in recruitment of RNA polymerase II and in transcription regulation.

Studies have shown that nuclear translocation of actin occurs under certain conditions of cellular stress; however, the functional significance of actin import remains unclear. Here, we demonstrate that during the phorbol 12-myristate 13-acetate (PMA)-induced differentiation of HL-60 cells toward macrophages, β-actin translocates from the cytoplasm to the nucleus and that this process is dramatically inhibited by pretreatment with p38 mitogen-activated protein kinase inhibitors. Using chromatin immunoprecipitation-on-chip assays, the genome-wide maps of β-actin binding to gene promoters in response to PMA treatment is analyzed in HL-60 cells. A gene ontology-based analysis shows that the identified genes belong to a broad spectrum of functional categories such as cell growth and differentiation, signal transduction, response to external stimulus, ion channel activity, and immune response. We also demonstrate a correlation between β-actin occupancy and the recruitment of RNA polymerase II at six selected target genes, and β-actin knockdown decreases the mRNA expression levels of these target genes induced by PMA. We further show that nuclear β-actin is required for PMA-induced transactivation of one target gene, solute carrier family 11 member 1, which is important for macrophage activation. Our data provide novel evidence that nuclear accumulation of β-actin is involved in transcriptional regulation during macrophage-like differentiation of HL-60 cells.

Actin dynamics regulate multiple endosomal steps during Kaposi's sarcoma-associated herpesvirus entry and trafficking in endothelial cells

The role of actin dynamics in clathrin-mediated endocytosis in mammalian cells is unclear. In this study, we define the role of actin cytoskeleton in Kaposi's sarcoma-associated herpesvirus (KSHV) entry and trafficking in endothelial cells using an immunofluorescence-based assay to visualize viral capsids and the associated cellular components. In contrast to infectivity or reporter assays, this method does not rely on the expression of any viral and reporter genes, but instead directly tracks the accumulation of individual viral particles at the nuclear membrane as an indicator of successful viral entry and trafficking in cells. Inhibitors of endosomal acidification reduced both the percentage of nuclei with viral particles and the total number of viral particles docking at the perinuclear region, indicating endocytosis, rather than plasma membrane fusion, as the primary route for KSHV entry into endothelial cells. Accordingly, a viral envelope protein was only detected on internalized KSHV particles at the early but not late stage of infection. Inhibitors of clathrin- but not caveolae/lipid raft-mediated endocytosis blocked KSHV entry, indicating that clathrin-mediated endocytosis is the major route of KSHV entry into endothelial cells. KSHV particles were colocalized not only with markers of early and recycling endosomes, and lysosomes, but also with actin filaments at the early time points of infection. Consistent with these observations, transferrin, which enters cells by clathrin-mediated endocytosis, was found to be associated with actin filaments together with early and recycling endosomes, and to a lesser degree, with late endosomes and lysosomes. KSHV infection induced dynamic actin cytoskeleton rearrangements. Disruption of the actin cytoskeleton and inhibition of regulators of actin nucleation such as Rho GTPases and Arp2/3 complex profoundly blocked KSHV entry and trafficking. Together, these results indicate an important role for actin dynamics in the internalization and endosomal sorting/trafficking of KSHV and clathrin-mediated endocytosis in endothelial cells.

Protein kinase Cdelta and calmodulin regulate epidermal growth factor receptor recycling from early endosomes through Arp2/3 complex and cortactin

The intracellular trafficking of the epidermal growth factor receptor (EGFR) is regulated by a cross-talk between calmodulin (CaM) and protein kinase Cdelta (PKCdelta). On inhibition of CaM, PKCdelta promotes the formation of enlarged early endosomes and blocks EGFR recycling and degradation. Here, we show that PKCdelta impairs EGFR trafficking due to the formation of an F-actin coat surrounding early endosomes. The PKCdelta-induced polymerization of actin is orchestrated by the Arp2/3 complex and requires the interaction of cortactin with PKCdelta. Accordingly, inhibition of actin polymerization by using cytochalasin D or by overexpression of active cofilin, restored the normal morphology of the organelle and the recycling of EGFR. Similar results were obtained after down-regulation of cortactin and the sequestration of the Arp2/3 complex. Furthermore we demonstrate an interaction of cortactin with CaM and PKCdelta, the latter being dependent on CaM inhibition. In summary, this study provides the first evidence that CaM and PKCdelta organize actin dynamics in the early endosomal compartment, thereby regulating the intracellular trafficking of EGFR.

Nucleocytoplasmic trafficking is required for functioning of the adaptor protein Sla1p in endocytosis

Dual localization of proteins at the plasma membrane and within the nucleus has been reported in mammalian cells. Among these proteins are those involved in cell adhesion structures and in clathrin-mediated endocytosis. In the case of endocytic proteins, trafficking to the nucleus is not known to play a role in their endocytic function. Here, we show localization of the yeast endocytic adaptor protein Sla1p to the nucleus as well as to the cell cortex and we demonstrate the importance of specific regions of Sla1p for this nuclear localization. A role for specific karyopherins (importins and exportins) in Sla1p nuclear localization is revealed. Furthermore, endocytosis of Sla1p-dependent cargo is defective in three strains with karyopherin mutations. Finally, we investigate possible functions for nuclear trafficking of endocytic proteins. Our data reveal for the first time that nuclear transport of endocytic proteins is important for functional endocytosis in Saccharomyces cerevisiae. We determine the mechanism, involving an alpha/beta importin pair, that facilitates uptake of Sla1p and demonstrate that nuclear transport is required for the functioning of Sla1p during endocytosis.

Endocytosis machinery is involved in aggregation of proteins with expanded polyglutamine domains

The cell's failure to refold or break down abnormal polypeptides often leads to their aggregation, which could cause toxicity and various pathologies. Here we investigated cellular factors involved in protein aggregation in yeast and mammalian cells using model polypeptides containing polyglutamine domains. In yeast, a number of mutations affecting the complex responsible for formation of the endocytic vesicle reduced the aggregation. Components of the endocytic complex (EC) Sla1, Sla2, and Pan1 were seen as clusters in the polyglutamine aggregates. These proteins associate with EC at the later stages of its maturation. In contrast, Ede1 and Ent1, the elements of EC at the earlier stages, were not found in the aggregates, suggesting that late ECs are involved in polyglutamine aggregation. Indeed, stabilization of the late complexes by inhibition of actin polymerization enhanced aggregation of polypeptides with polyglutamine domains. Similarly, in mammalian cells, inhibitors of actin polymerization, as well as depletion of a mediator of actin polymerization, Arp2, strongly enhanced the aggregation. In contrast, destabilization of EC by depletion or inhibition of a scaffolding protein N-WASP effectively suppressed the aggregation. Therefore, EC appears to play a pivotal role in aggregation of cytosolic polypeptides with polyglutamine domains in both yeast and mammalian cells.

The Fps/Fes kinase regulates the inflammatory response to endotoxin through down-regulation of TLR4, NF-kappaB activation, and TNF-alpha secretion in macrophages

Fps/Fes and Fer are members of a distinct subfamily of cytoplasmic protein tyrosine kinases that have recently been implicated in the regulation of innate immunity. Previous studies showed that mice lacking Fps/Fes are hypersensitive to systemic LPS challenge, and Fer-deficient mice displayed enhanced recruitment of leukocytes in response to local LPS challenge. This study identifies physiological, cellular, and molecular defects that contribute to the hyperinflammatory phenotype in Fps/Fes null mice. Plasma TNF-alpha levels were elevated in LPS challenged Fps/Fes null mice as compared with wild-type mice and cultured Fps/Fes null peritoneal macrophages treated with LPS showed increased TNF-alpha production. Cultured Fps/Fes null macrophages also displayed prolonged LPS-induced degradation of IkappaB-alpha, increased phosphorylation of the p65 subunit of NF-kappaB, and defective TLR4 internalization, compared with wild-type macrophages. Together, these observations provide a likely mechanistic basis for elevated proinflammatory cytokine secretion by Fps/Fes null macrophages and the increased sensitivity of Fps/Fes null mice to endotoxin. We posit that Fps/Fes modulates the innate immune response of macrophages to LPS, in part, by regulating internalization and down-regulation of the TLR4 receptor complex.

Lsb5p interacts with actin regulators Sla1p and Las17p, ubiquitin and Arf3p to couple actin dynamics to membrane trafficking processes

The importance of coupling the process of endocytosis to factors that regulate actin dynamics has been clearly demonstrated in yeast, and many proteins involved in these mechanisms have been identified. Sla1p is a well-characterized yeast protein that binds both to activators of actin dynamics, Las17p and Pan1p, and to cargo proteins, such as the pheromone receptor Ste2p. Previously, we reported that the Lsb5 protein plays a role in endocytosis in yeast and that it localizes to the plasma membrane. Lsb5p has a similar structure to the GGA [Golgi-localized, gamma-ear-containing, Arf (ADP-ribosylation factor)-binding] family of proteins with an N-terminal VHS [Vps27p (vacuolar protein sorting protein 27), Hrs, Stam] domain and a GAT (GGA and Tom1) domain. It does not, however, contain either a gamma-adaptin ear or a clathrin-binding motif. In the present study, we have further defined its interaction site with both Sla1p and with Las17p, two regulators of actin dynamics. The site of interaction with Sla1p involves the Sla1 HD1 (homology domain 1), which also was shown previously to interact with the pheromone receptor Ste2p. We also demonstrate hitherto unknown interactions between Lsb5p and the active form of the yeast Arf3 protein, and with ubiquitin. Finally, we demonstrate a requirement for Arf3p expression in order to localize Lsb5p to the correct cortical site in cells. Taken together, our data provide further evidence for the role of Lsb5p in membrane-trafficking events at the plasma membrane and also demonstrate for the first time an interaction of Arf3 with the endocytic machinery in yeast.

Actin-dependent regulation of the cardiac Na+/Ca2+exchanger

In the present study, the bovine cardiac Na+/Ca2+exchanger (NCX1.1) was expressed in Chinese hamster ovary cells. The surface distribution of the exchanger protein, externally tagged with the hemagglutinin (HA) epitope, was associated with underlying actin filaments in regions of cell-to-cell contact and also along stress fibers. After we treated cells with cytochalasin D, NCX1.1 protein colocalized with patches of fragmented filamentous actin (F-actin). In contrast, an HA-tagged deletion mutant of NCX1.1 that was missing much of the exchanger's central hydrophilic domain Δ(241–680) did not associate with F-actin. In cells expressing the wild-type exchanger, cytochalasin D inhibited allosteric Ca2+activation of NCX activity as shown by prolongation of the lag phase of low Ca2+uptake after initiation of the reverse (i.e., Ca2+influx) mode of NCX activity. Other agents that perturbed F-actin structure (methyl-β-cyclodextrin, latrunculin B, and jasplakinolide) also increased the duration of the lag phase. In contrast, when reverse-mode activity was initiated after allosteric Ca2+activation, both cytochalasin D and methyl-β-cyclodextrin (Me-β-CD) stimulated NCX activity by ∼70%. The activity of the Δ(241–680) mutant, which does not require allosteric Ca2+activation, was also stimulated by cytochalasin D and Me-β-CD. The increased activity after these treatments appeared to reflect an increased amount of exchanger protein at the cell surface. We conclude that wild-type NCX1.1 associates with the F-actin cytoskeleton, probably through interactions involving the exchanger's central hydrophilic domain, and that this association interferes with allosteric Ca2+activation.

Cell biology of membrane trafficking in human disease

Understanding the molecular and cellular mechanisms underlying membrane traffic pathways is crucial to the treatment and cure of human disease. Various human diseases caused by changes in cellular homeostasis arise through a single gene mutation(s) resulting in compromised membrane trafficking. Many pathogenic agents such as viruses, bacteria, or parasites have evolved mechanisms to subvert the host cell response to infection, or have hijacked cellular mechanisms to proliferate and ensure pathogen survival. Understanding the consequence of genetic mutations or pathogenic infection on membrane traffic has also enabled greater understanding of the interactions between organisms and the surrounding environment. This review focuses on human genetic defects and molecular mechanisms that underlie eukaryote exocytosis and endocytosis and current and future prospects for alleviation of a variety of human diseases.

E-cadherin endocytosis regulates the activity of Rap1: a traffic light GTPase at the crossroads between cadherin and integrin function

The coordinate modulation of cadherin and integrin functions plays an essential role in fundamental physiological and pathological processes, including morphogenesis and cancer. However, the molecular mechanisms underlying the functional crosstalk between cadherins and integrins are still elusive.

Here, we demonstrate that the small GTPase Rap1, a crucial regulator of the inside-out activation of integrins, is a target for E-cadherin-mediated outside-in signaling. In particular, we show that a strong activation of Rap1 occurs upon adherens junction disassembly that is triggered by E-cadherin internalization and trafficking along the endocytic pathway. By contrast, Rap1 activity is not influenced by integrin outside-in signaling. Furthermore, we demonstrate that the E-cadherin endocytosis-dependent activation of Rap1 is associated with and controlled by an increased Src kinase activity, and is paralleled by the colocalization of Rap1 and E-cadherin at the perinuclear Rab11-positive recycling endosome compartment, and the association of Rap1 with a subset of E-cadherin-catenin complexes that does not contain p120ctn. Conversely, Rap1 activity is suppressed by the formation of E-cadherin-dependent cell-cell junctions as well as by agents that inhibit either Src activity or E-cadherin internalization and intracellular trafficking. Finally, we demonstrate that the E-cadherin endocytosis-dependent activation of Rap1 is associated with and is required for the formation of integrin-based focal adhesions.

Our findings provide the first evidence of an E-cadherin-modulated endosomal signaling pathway involving Rap1, and suggest that cadherins may have a novel modulatory role in integrin adhesive functions by fine-tuning Rap1 activation.

Two plant-viral movement proteins traffic in the endocytic recycling pathway

Many plant viruses exploit a conserved group of proteins known as the triple gene block (TGB) for cell-to-cell movement. Here, we investigated the interaction of two TGB proteins (TGB2 and TGB3) of Potato mop-top virus (PMTV), with components of the secretory and endocytic pathways when expressed as N-terminal fusions to green fluorescent protein or monomeric red fluorescent protein (mRFP). Our studies revealed that fluorophore-labeled TGB2 and TGB3 showed an early association with the endoplasmic reticulum (ER) and colocalized in motile granules that used the ER-actin network for intracellular movement. Both proteins increased the size exclusion limit of plasmodesmata, and TGB3 accumulated at plasmodesmata in the absence of TGB2. TGB3 contains a putative Tyr-based sorting motif, mutations in which abolished ER localization and plasmodesmatal targeting. Later in the expression cycle, both fusion proteins were incorporated into vesicular structures. TGB2 associated with these structures on its own, but TGB3 could not be incorporated into the vesicles in the absence of TGB2. Moreover, in addition to localization to the ER and motile granules, mRFP-TGB3 was incorporated into vesicles when expressed in PMTV-infected epidermal cells, indicating recruitment by virus-expressed TGB2. The TGB fusion protein-containing vesicles were labeled with FM4-64, a marker for plasma membrane internalization and components of the endocytic pathway. TGB2 also colocalized in vesicles with Ara7, a Rab5 ortholog that marks the early endosome. Protein interaction analysis revealed that recombinant TGB2 interacted with a tobacco protein belonging to the highly conserved RME-8 family of J-domain chaperones, shown to be essential for endocytic trafficking in Caenorhabditis elegans and Drosophila melanogaster. Collectively, the data indicate the involvement of the endocytic pathway in viral intracellular movement, the implications of which are discussed.

Beginning and ending an actin filament: control at the barbed end

Dynamic actin filaments contribute to cell migration, organelle movements, memory, and gene regulation. These dynamic processes are often regulated by extracellular and?or cell cycle signals. Regulation targets, not actin itself, but the factors that determine it's dynamic properties. Thus, filament nucleation, rate and duration of elongation, and depolymerization are each controlled with regard to time and?or space. Two mechanisms exist for nucleating filaments de novo, the Arp23 complex and the formins; multiple pathways regulate each. A new filament elongates rapidly but transiently before its barbed end is capped. Rapid capping allows the cell to maintain fine temporal and spatial control over F-actin distribution. Modulation of capping protein activity and its access to barbed ends is emerging as a site of local regulation. Finally, to maintain a steady state filaments must depolymerize. Depolymerization can limit the rate of new filament nucleation and elongation. The activity of ADF?cofilin, which facilitates depolymerization, is also regulated by multiple inputs. This chapter describes (1) mechanism and regulation of new filament formation, (2) mechanism of enhancing elongation at barbed ends, (3) capping proteins and their regulators, and (4) recycling of actin monomers from filamentous actin (F-actin) back to globular actin (G-actin).