The polymerization of actin. A study of the nucleation reaction

Biological Soft Materials

With one or two exceptions, biological materials are "soft", meaning that they combine viscous and elastic elements. This mechanical behavior results from self-assembled supramolecular structures that are stabilized by noncovalent interactions. It is an ongoing and profound challenge to understand the self-organization of biological materials. In many cases, concepts can be imported from soft-matter physics and chemistry, which have traditionally focused on materials such as colloids, polymers, surfactants, and liquid crystals. Using these ideas, it is possible to gain a new perspective on phenomena as diverse as DNA condensation, protein and peptide fibrillization, lipid partitioning in rafts, vesicle fusion and budding, and others, as discussed in this selective review of recent highlights from the literature.

Self-assembly properties of a model RING domain

RING domains act in a variety of essential cellular processes but have no general function ascribed to them. Here, we observe that purified arenaviral protein Z, constituted almost entirely by its RING domain, self-assembles in vitro into spherical structures that resemble functional bodies formed by Z in infected cells. By using a variety of biophysical methods we provide a thermodynamic and kinetic framework for the RING-dependent self-assembly of Z. Assembly appears coupled to substantial conformational reorganization and changes in zinc coordination of site II of the RING. Thus, the rate-limiting nature of conformational reorganization observed in the folding of monomeric proteins can also apply to the assembly of macromolecular scaffolds. These studies describe a unique mechanism of nonfibrillar homogeneous self-assembly and suggest a general function of RINGs in the formation of macromolecular scaffolds that are positioned to integrate biochemical processes in cells.

Kinetic heterogeneity of F-actin polymers. Further evidence that the elongation reaction may occur through condensation of the actin filaments with small aggregates

We have shown that F-actin, polymerized in 50 mM-KCl at 20 degrees C and pH 8.0, can be resolved by centrifugation into two polymer populations, which differ morphologically as well as kinetically. The first population represents about 10% of the overall polymer and is composed of small amorphous aggregates. It rapidly exchanges the bound nucleotide with free ATP in the medium, either directly or through the monomers. The second population is composed of long actin filaments. These are labelled by free ATP in the medium only through condensation with labelled small amorphous aggregates.

The experimental basis of some recent hypotheses on the mechanism of the polymerization of actin: a reappraisal

We have studied the effect of sonication on the fluorescence of N-(1-pyrenyl)iodoacetamide-labeled F-actin as well as of native actin-pyrenyl-actin mixed oligomers in which the subunits were covalently attached to each other by phenylenebismaleimide. In both cases the fluorescence of the solution was largely decreased by sonication. We have found that this effect is due (a) to a 20-30% decrease of the specific fluorescence of the polymers. These results question the validity of the novel mechanism for the polymerization of actin recently proposed (D. Pantaloni et al. (1984) J. Biol. Chem. 259, 6274-6283). In these studies, in fact, the implicit assumption was made that the quenching of the fluorescence of the solution under sonication was due exclusively to the conversion of F-actin into G-actin.

54Mn2+ as a tracer of the polymerization of actin. Intermediate oligomers condense to give F-actin

Mg2+, at submicromolar concentrations, is needed for the nucleation of actin [Maruyama (1981) J. Biol. Chem. 256, 1060-1062]. I show here that Mn2+ fulfils the same function. It binds to oligomers present in the ATP-G-actin solutions with a ratio of 2-3 Mn2+ ions per 100 actin monomers and with an association constant of 0.66 X 10(10) M-1 at pH 8.2 at 25 degrees C. The time course of the binding of Mn2+ to polymerizing actin is not affected by the initial concentration of the protein. Analysis of the distribution of the binding shows that, both in the large oligomeric species and in the polymers, 1 Mn2+ ion is bound for every 14-25 actin monomers, whereas in the smaller oligomeric species 1 Mn2+ ion is bound for every 4 actin monomers. The proposal is made that Mn2+ stabilizes actin nuclei and decreases the concentration of the monomers at the steady state. It is also proposed that, at least in some experimental conditions, the direct condensation of oligomers of intermediate length is an effective mechanism of F-actin formation.

Characterization of the ATP-G-actin aggregates formed at low potassium chloride concentration

The ATP-G-actin aggregates formed by incubation of ATP-G-actin in 7.5 mM-KCl were characterized by electron-microscopical observation, by high-pressure liquid chromatography and by the study of the 1,N6-etheno-ATP-ATP exchange reaction between the free and the actin-bound nucleotide. In 30 mM-KCl the initial rate of the reduced-viscosity increase is found to be directly related to the amount of the aggregates formed in the course of the preincubation in 7.5 mM-KCl.