Gelsolin, Complex with actin

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...

Because of its tendency to polymerize, G-actin has been difficult to crystallize. However, it forms crystalline complexes with several other proteins, e.g., deoxyribonuclease 1, a fragment of gelsolin, and profilin, which block polymerizafion and if has recently been crystallized as the free ADP complex. The fhree-dimensional structure of the actin is nearly the same in all cases. The molecule folds into four domains, the ATP binding site being buried in a deep cleft. The atomic structure (Fig. 7-10) resembles that of hexo-kinase, of glycerol kinase, and of an ATP-binding domain of a chaperonin of fhe Hsp 70 family. As with the kinases, actin can exist in a closed and more open conformations, one of which is seen in the profilin complex. Addition of 1 mM Mg + or 0.1 M KCl to a solution of G-actin leads to spontaneous transformation into filaments of F-actin (Figs. 7-10 and... 

Figure 3. Critical concentration behavior of actin self-assembly. For the top diagram depicting the macroscopic critical concentration curve, one determines the total amount of polymerized actin by methods that measure the sum of addition and release processes occurring at both ends. Examples of such methods are sedimentation, light scattering, fluorescence assays with pyrene-labeled actin, and viscosity measurements. Forthe bottom curves, the polymerization behavior is typically determined by fluorescence assays conducted under conditions where one of the ends is blocked by the presence of molecules such as gelsolin (a barbed-end capping protein) or spectrin-band 4.1 -actin (a complex prepared from erythrocyte membranes, such that only barbed-end growth occurs). Note further that the barbed end (or (+)-end) has a lower critical concentration than the pointed end (or (-)-end). This differential stabilization requires the occurrence of ATP hydrolysis to supply the free energy that drives subunit addition to the (+)-end at the expense of the subunit loss from the (-)-end.

---