by Giovanie Adams

Eukaryotic cells have systems for maintaining their shape and all their movement, and for the transport of molecules within them. Intra-cellular networks of fibers assemble from actin proteins, and are an important part of these systems. This network of actin filaments maintains cell shape by forming a support structure—an important component for cell motility—and provides the paths for the transport of molecules within the cell. The intra-cellular traffic of molecules along these paths is also necessary for cell fission (the division of one parent cell into two daughter cells). Actin filaments are also a major component of the muscle fibers of animals and are essential for the contractile apparatus of muscles.1

Actin filaments are made up of two long, twisted chains consisting chiefly of tens to thousands of monomeric or globular actin proteins. The assembly and disassembly of actin filaments is controlled at each step by sets of actin-binding proteins.1 The initial assembly of a short actin filament is a rate-limiting step in filament assembly,2 and a subset of actin-binding proteins has been designed to overcome the relative instability of these short filaments.3 This process of assembling short filaments of two or three subunits from monomeric actin is called actin nucleation.

The spire family of proteins is a family of actin-binding proteins that nucleates a pool of actin monomers and prepares the product of nucleation for elongation.4 Spire overcomes the relative instability of short filaments made up of actin dimers and trimers, by helping to overcome the kinetic barrier to nucleation.2 Spire also attaches the newly synthesized actin filament to a membrane and aligns the filament to a nano-scale machine, a dimer of formin proteins.

The formin dimer then proceeds to extend the filament by adding actin monomers, assembling long double-helical-twisted actin filaments. This complicated mechanism, of actin nucleation and preparation of the nucleated product for elongation by spire, suggests the work of an intelligent designer who engineered it in an incredibly intricate manner….

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