Polymerization actinically activated

Blood platelets are key players in the blood-clotting mechanism. These tiny fragments of cytoplasm are shed into the circulation from the surface of megakaryocytes located in the bone marrow. When the lining of a blood vessel is injured, activated platelets release clotting factors, adhere to each other and to damaged surfaces, and send out numerous filopodia. The shape changes that occur in activated platelets are the result of actin polymerization. Before activation, there are no microfilaments because profilin binds to G-actin and prevents its polymerization. After activation, profilin dissociates from G-actin, and bundles and networks of F-actin filaments rapidly appear within the platelet. 

Brock MA and Chrest F [1993] Differential regulation of actin polymerization following activation of resting T lymphocytes from young and aged mice. J Cellular Phys 157 367-378... 

Lee, S., Park, J. and Lee, Y., 2003, Phosphatidic acid induces actin polymerization by activating protein kinases in soybean cells. Mol. Cell 15 313-319. 

Fig. 1. Toxin-catalyzed ADP-ribosylation inhibits nucleation activity of the gelsolin-octin complex. In the presence of Ca, gelsotin forms a 1 1 and a 1 2 complex with octin monomers at the so-called EGTA-resistant (a) and Ca -sensitive (b) binding site, respectively. Gelsolin-actin complexes act as nuclei for actin polymerization. Actin bound to both sites (a, b) can be ADP-ribosylated. Whereas ADP-ribosylation of actin bound to the EGTA-resistant site has no effect on nucleation, ADP-ribosylation of actin bound to the Ca " -sensitive site inhibits nucleation activity of the gelsolin-actin complex...

An additional indirect assay of neutrophil motility is the actin polymerization assay. Chemoattractants induce a rapid, transient actin polymerization response in neutrophils (reviewed in [289, 391]), and since actin polymerization is required for migration to occur, it is often used as an indication of chemotactic capability. Several approaches have been developed to measure this response (reviewed in [289, 391]) (Table 2). Most commonly, cells in suspension are mixed with a putative chemoattractant then fixed and stained with a fluorescent phaUotoxin that binds to polymerized actin, but not monomeric actin. The bound fluorescence is quantified using flow cytometry or spectrofluorometry. The actin polymerization response to many neutrophil chemoattractants is rapid, reaching a maximum within 10 s of stimulus addition at 37° C. As with the cell polarization assay, no information about the migratory properties of the cells is obtained with the actin polymerization assay, only the likelihood of chemotactic activity is assessed. Like the cell polarization assay, the actin polymerization assay is useful for initial screening of chemoattractants, but must be followed up with an actual measurement of motility. 

Complexation of the initiator and/or modification with cocatalysts or activators affords greater polymerization activity (11). Many of the patented processes for commercially available polymers such as poly(MVE) employ BE etherate (12), although vinyl ethers can be polymerized with a variety of acidic compounds, even those unable to initiate other cationic polymerizations of less reactive monomers such as isobutene. Examples are protonic acids (13), Ziegler-Natta catalysts (14), and actinic radiation (15,16). 

Just as myosins are able to move along microfilaments, there are motor proteins that move along microtubules. Microtubules, like microfilaments, are polar polymeric assemblies, but unlike actin-myosin interactions, microtubule-based motors exist that move along microtubules in either direction. A constant traffic of vesicles and organelles is visible in cultured cells especially using time-lapse photography. The larger part of this movement takes place on micrombules and is stimulated by phorbol ester (an activator of protein kinase C), and over-expression of N-J aj oncoprotein (Alexandrova et al., 1993). 

The cellular/molecular mechanism of action for these cyclic peptide toxins is now an area of active research in several laboratories. These peptides cause striking ultrastructural changes in isolated hepatocytes (95) including a decrease in the polymerization of actin. This effect on the cells cytoskeletal system continues to be investigated and recent work indirectly supports the idea that these toxins interact with the cells cytoskeletal system (86,96). Why there is a specificity of these toxins for liver cells is not clear although it has been suggested that the bile uptake system may be at least partly responsible for penetration of the toxin into the cell (92). 

Two of the cytoskeletal components, the actin filaments and the microtubules have been studied with molecular rotors. The main component of the actin filaments is the actin protein, a 44 kD molecule found in two forms within the cell the monomeric globulin form (G-actin) and the filament form (F-actin). Actin binds with ATP to form the microfilaments that are responsible for cell shape and motility. The rate of polymerization from the monomeric form plays a vital role in cell movement and signaling. Actin filaments form the cortical mesh that is the basis of the cytoskeleton. The cytoskeleton has an active relationship with the plasma membrane. Functional proteins found in both structures... 

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