Pointed ends, of actin filaments

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. 

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.

Dynein seems to be a complementary partner of kInesIn and myosin In trafficking vesicles and chromosomes. Although an emerging theme Is that dynactin couples a membrane cargo to dynein or kinesin, how microtubule-based movements are coordinated with myosin-based movements remains an unresolved question. Several lines of Investigation suggest that dynein could play a central role In coordination. Because the (+) ends of actin filaments and microtubules tend to point toward the cell periphery, dynein Is the only major class of motors that moves toward the cell Interior. The location of cytoplasmic dynein In the actin-rich cortex Is similar to that of myosin and distinct from that of kinesin. Whether dynactin also couples myosin to membrane trafficking Is unknown. 

A puzzling discovery was that the motor domain of kinesin, which binds primarily to the P subunits of tubulin (Fig. 7-34) and moves toward the fast growing plus end of the microtubule, is located at the N terminus of the kinesin molecule, just as is myosin. However, the Ned and Kar3 motor domains are at the C-terminal ends of their peptide chains and move their "cargos" toward the minus ends of microtubules. Nevertheless, the structures of all the kinesin heads are conserved as are the basic chemical mechanisms. The differences in directional preference are determined by a short length of peptide chain between the motor domain and the neck, which allows quite different geometric arrangements when bound to microtubules. Like Ned, myosin VI motor domains also move "backwards" toward the pointed (minus) ends of actin filaments. " ... 

Pointed-end-capping proteins are acumentin (65 kD), spectrin (220-260 kD), and p-actinin (37 kD). They also regulate the length of actin filaments. 

Figure 4.4. Structure of actin filaments (a) shows the beads on a string appearance of an actin filament. This filament comprises actin monomers, which themselves have a polarity, so that they can only assemble head to tail , as shown in (b) thus, the actin filament is polarised, having a barbed and a pointed end.

Thus, assembly of actin filaments may occur at the pointed ends during phagocytosis, but at the barbed ends during locomotion. 

Figure 2. Scheme for the addition and loss of actin-ATP or actin-ADP from the barbed and pointed ends of an actin filament. The barbed end is the faster growing and more stable end of an actin filament. While the exchange of actin-ADP with ATP to yield actin-ATP and ADP is shown here as a spontaneous process, the actin regulatory protein profilin greatly accelerates the exchange process. Note also that hydrolysis is thought to occur after (and not coincident with) addition of actin-ATP at either end. 

Krieger, I., Kostyukova, A., Yamashita, A., Nitanai, Y., and Maeda, Y. (2002). Crystal structure of the C-terminal half of tropomodulin and structural basis of actin filament pointed-end capping. Biophys.J. 83, 2716-2725. 

Figure 5.13. Assembly of actin filaments. The diagram shows the steps in the transition of actin monomer, G-actin, to actin filaments, F-actin. Monomers are activated by binding calcium and then exchanging calcium for magnesium, leading to nucleus formation. The nucleus consists of a trimer of G-actin with one pointed end and one barbed end. The addition of activated G-actin monomer to the nucleus causes elongation of the barbed end faster than the pointed end.

The protein myosin (Fig. 5-32) interacts specifically with each actin molecule in a filament and, as a result, the filament becomes decorated with a pattern of arrow-heads all pointing the same way. Because of this pattern, one end of the filament is known as the pointed end while the other is called the barbed end. 

Polymerization in the presence of ATP, as opposed to ADP, is more complex but physiologically relevant. The main differences lie in the properties of F-actin (Fig. 5-27). The two ends of the filament now become kinetically distinct. When polymerization is initiated, monomers with ATP bound to them add much more rapidly to the barbed end than to the pointed 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)...

All subunits in an actin filament point toward the same end of the filament. Consequently, a filament exhibits polarity that is, one end differs from the other. By convention, the end at which the ATP-blndlng cleft of the terminal actin subunit is exposed to the surrounding solution is designated the (—) end. At the opposite end, the (+) end, the cleft contacts the neighboring actin subunit and is not exposed (see Figure 19-3c). 

Without the atomic resolution afforded by x-ray crystallography, the cleft in an actin subunit and therefore the polarity of a filament are not detectable. However, the polarity of actin filaments can be demonstrated by electron microscopy in decoration experiments, which exploit the ability of myosin to bind specifically to actin filaments. In this type of experiment, an excess of myosin SI, the globular head domain of myosin, is mixed with actin filaments and binding is permitted to take place. Myosin attaches to the sides of a filament with a slight tilt. When all the actin subunits are bound by myosin, the filament appears coated ( decorated ) with arrowheads that all point toward one end of the filament (Figure 19-4). Because... 

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