Barbed end of actin

Gelsolin blocks barbed ends of actin filaments 84 Ca2+,PIP2... 

Southwick, F.S., and MJ. DiNubile. 1986. Rabbit alveolar macrophages contain a Ca2+-sensitive, 41,000-dalton protein which reversibly blocks the barbed ends of actin filaments but does not sever them. J Biol Chem. 261 14191—5. 

Huang, S.J., Blanchoin, L., Kovar, D.R., and Staiger, C.J., 2003, Arabidopsis capping protein (AtCP) is a heterodimer that regulates assembly at the barbed ends of actin filaments. J. Biol. Chem. 278 44832 14842. 

How does this cycle apply to muscle contraction Myosin molecules self-assemble into thick bipolar structures with the myosin heads protruding at both ends of a bare region in the center (Figure 34.19). Approximately 500 head domains line the surface of each thick filament. These domains are paired in myosin dimers, but the two heads within each dimer act independently. Actin filaments associate with each head-rich region, with the barbed ends of actin toward the Z-line. In the presence of normal levels of ATP, most of the myosin heads are detached from actin. Each head can independently hydrolyze ATP, bind to actin, release Pj, and undergo its power stroke. Because few other heads are... 

Vandekerckhove J, Weber K (1978) At least six different actins are expressed in a higher mammal An analysis based on the amino acid sequence of the amino-terminal tryptic peptide. In J. Mol. Biol. 126 783-802 Vandekerckhove J, Weber K (1979) The complete amino acid sequence of actins from bovine aorta, bovine heart, bovine fast skeletal muscle and rabbit slow skeletal muscle. In Differentiation 14 123-33 Wegner A, Aktories K (1988) ADP-ribosylated actin caps the barbed ends of actin filaments. In J. Biol. Chem. 263 13739-42... 

Bailly, M., Macaluso, F., Gammer, M. et al. (1999). Relationship between Arp2/3 complex and the barbed ends of actin filaments at the leading edge of carcinoma cells after epidermal growth factor stimulation./. Cell Biol. 145, 331-345. 

In the above tentative scenario on the vesicle shape change, the elastic interaction between the bundle of actin filaments and the lipid membrane was assumed to play a role. Therefore, it was of interest to see what kind of change occurs to the vesicle shape, if the bundle structure is altered by some means. Cytochalasin D (CD) was chosen for this purpose. CD is a fungal metabolite which enters the cell and dismpts the actin filament therein, when it is added to the medium surrounding the cell [28]. This drug has been widely used to investigate the role of actin in the cell [29]. The mechanism of the drug action is twofold one is to bind to the barbed end of actin filaments and inhibit polymerization, the other is to sever (cut up) the filaments [30]. 

Cytochalasins B and D are used as tools to study F-actin. Cytochalasins bind to the barbed end of F-actin and block the addition as well as dissociation of G-actin at that end. When applied to cultured cells micromolar concentrations of cytochalasins remove stress fibres and other F-actin structures. 

The cy tochalasins A, B, C, D, E, and H are found in various species of mould. Mainly cytochalasin B and D are used as experimental tools. Cytochalasin D is 10 times more potent, acting at concentrations between 2 and 35 nM in cell-free systems. Cy tochalasins bind to the barbed end of F-actin and block the addition as well as dissociation of G-actin at that end. At micromolar concentrations, cytochalasin D can bind to G-actin and actin dimers and thus block additional polymerization. When applied to cultured cells, micromolar concentrations of cytochalasins remove stress fibres and other F-actin structures. 

Figure 2 The actin-ADP-ribosylating toxins, (a) Molecular mode of action. The actin-ADP-ribosylating toxins covalently transfer an ADP-ribose moiety from NAD+ onto Arg177 of G-actin in the cytosol of targeted cells. Mono-ADP-ribosylated G-actin acts as a capping protein and inhibits the assembly of nonmodified actin into filaments. Thus, actin polymerization is blocked at the fast-growing ends of actin filaments (plus or barbed ends) but not at the slow growing ends (minus or pointed ends). This effect ultimately increases the critical concentration necessary for actin polymerization and tends to depolymerize F-actin. Finally, all actin within an intoxicated cell becomes trapped as ADP-ribosylated G-actin.

Certain proteins can specifically bind to either the barbed or pointed ends of actin filaments. Capping the filaments in this way prevents, or retards, any further elongation or depolymerization of monomers from that end. 

Fig. 2. Model of the cytopathic effects of actin ADP-ribosylating toxins. The activated binding component of C. botulinum C2 toxin binds to a receptor of the eukaryotic cell. This induces a binding site for the enzyme component (C2I), Most likely, C2I enters the cell by endocytosis and subsequent translocation. In the cell, G-actin is ADP-ribosylated, which inhibits its polymerization and traps actin in the monomeric form. ADP-ribosylated actin binds in a capping protein-like manner to the barbed ends of filaments to inhibit further polymerization at the fast-growing end of F-acfin. The foxin has no effects on the pointed end of filaments where actin depolymerization takes place. Additionally, ADP-ribosylation may affect functions of complexes of acfin wifh binding proteins as examplified in Fig. 1 (From (Aktories, 1990) with permission)...

Misakinolide A (35) also binds simultaneously to two actin subunits with virtually the same affinity as swinholide A (34), but it does not sever actin filaments like swinholide A (34) rather it caps the barbed end of F-actin [78]. The presence of microbial symbionts in T. swinhoei was reported and the production of these natural products such as swinholide A (34) was believed to be responsible for symbiotic cyanobacteria. Studies by transmission electron microscopy, however, suggested that production of these macrolides was attributable to heterotrophic unicellular bacteria rather than cyanobacteria [79]. 

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