Protein structure actin polymerization

Actin is a 42 kDa bent dumbbell-shaped globular monomer which is found in most eukaryotic cells. It is the primary protein of the thin (or actin) filaments. Also, by mass or molarity, actin is the largest constituent of the contractile apparatus, actually reaching millimolar concentrations. Actins from different sources seem to be more similar than myosins from the same sources. Actin binds ATP which is hydrolyzed to ADP, if the monomeric actin polymerizes. The backbone structure of the actin filament is a helix formed by two linear strands of polymerized actins like two strings of actin beads entwined. 

Monomeric G-actin (43 kDa G, globular) makes up 25% of muscle protein by weight. At physiologic ionic strength and in the presence of Mg, G-actin polymerizes noncovalently to form an insoluble double helical filament called F-actin (Figure 49-3). The F-actin fiber is 6-7 nm thick and has a pitch or repeating structure every 35.5 nm. 

Several mathematical theories concerning iri vitro nucleation and polymerization of cytoskeletal proteins into linear polymeric structures have been developed. Particularly notable is the work of Oosawa and collaborators(9) on the rate limiting cooperative nucleation of actin monomers, and that of Wegner(lO) on the asymmetric ATP-driven addition and dissolution... 

The principal cytoskeletal proteins in non-muscle cells are actin, tubulin, and the components of intermediate filaments. Actin can exist either as monomers ( G-actin ) or polymerized into 70 A diameter double filament ( F-actin ). Polymerized actin usually is localized at the margins of the cells, linked by other proteins to the cell membrane. In contrast, tubulin forms hollow filaments, approximately 250 A in diameter, that are distributed within a cell in association, generally, with cell organelles. Stabilized microtubule structures are found in the flagella and cilia of eucaryotic cells however, in other instances - examples being the mitotic apparatus and the cytoskeletal elements arising in directed cell locomotion - the microtubules are temporal entities. Intermediate filaments, which are composed of keratin-like proteins, are approximately 100 A thick and form stable structural elements that impart rigidity, for example, to nerve axons and epithelial cells. 

Theoretical approaches to structural biophysics, like the theories of transport and reaction kinetics explored in other chapters of this book, are grounded in physical chemistry concepts. Here we explore a few problems in molecular structural dynamics using those concepts. The first two systems presented, helix-coil transitions and actin polymerization, introduce classic theories. The material in the remainder of the chapter arises from the study of macromolecular interactions and is motivated by current research aimed at uncovering and understanding how large numbers of proteins (hundreds to thousands) interact in cells [7],... 

How are actin filaments formed Like many biological structures, actin filaments self-assemble that is, under appropriate conditions, actin monomers will come together to form well-structured, polar filaments. The aggregation of the first two or three monomers to form a filament is highly unfavorable. Thus, specialized protein complexes, including one called Arp2/3, serve as nuclei for actin assembly in cells. Once such a filament nucleus exists, the addition of subunits is more favorable. Let us consider the polymerization reaction in more detail. We designate an actin filament with n subunits A . This filament can bind an additional actin monomer, A, to form A. +. ... 

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... 

The work that follows pertains primarily to actin networks. Many proteins within a cell are known to associate with actin. Among these are molecules which can initiate or terminate polymerization, intercalate with and cut chains, crosslink or bundle filaments, or induce network contraction (i.e., myosin) (A,11,12). The central concern of this paper is an exploration of the way that such molecular species interact to form complex networks. Ultimately we wish to elucidate the biophysical linkages between molecular properties and cellular function (like locomotion and shape differentiation) in which cytoskeletal structures are essential attributes. Here, however, we examine the iri vitro formation of cytoplasmic gels, with an emphasis on delineating quantitative assays for network constituents. Specific attention is given to gel volume assays, determinations of gelation times, and elasticity measurements. 

True self-assembly is observed in the formation of many oligomeric proteins. Indeed, Friedman and Beychok reviewed efforts to define the subunit assembly and reconstitution pathways in multisubunit proteins, and all of the several dozen examples cited in their review represent true self-assembly. Polymeric species are also formed by true self-assembly, and the G-actin to F-actin transition is an excellent example. By contrast, there are strong indications that ribosomal RNA species play a central role in specifying the pathway to and the structure of ribosome particles. And it is interesting to note that the assembly of the tobacco mosaic virus (TMV) appears to be a two-step hybrid mechanism the coat protein subunits first combine to form 34-subunit disks by true self-assembly from monomeric and trimeric com-... 

ACTIN ASSEMBLY KINETICS MICROTUBULE ASSEMBLY KINETICS PROTEIN POLYMERIZATION KINETICS NUCLEIC ACID RENATURATION KINETICS Nucleic acid structure,... 

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