title: Guardians of the actin monomer.
abstract: Actin is a universal force provider in eukaryotic cells. Biological processes harness the pressure generated from actin polymerization through dictating the time, place and direction of filament growth. As such, polymerization is initiated and maintained via tightly controlled filament nucleation and elongation machineries. Biological systems integrate force into their activities through recruiting and activating these machineries. In order that actin function as a common force generating polymerization motor, cells must maintain a pool of active, polymerization-ready monomeric actin, and minimize extemporaneous polymerization. Maintenance of the active monomeric actin pool requires the recycling of actin filaments, through depolymerization, nucleotide exchange and reloading of the polymerization machineries, while the levels of monomers are constantly monitored and supplemented, when needed, via the access of a reserve pool of monomers and through gene expression. Throughout its monomeric life, actin needs to be protected against gratuitous nucleation events. Here, we review the proteins that act as custodians of monomeric actin. We estimate their levels on a tissue scale, and calculate the implied concentrations of each actin complex based on reported binding affinities. These estimations predict that monomeric actin is rarely, if ever, alone. Thus, the guardians keep the volatility of actin in check, so that its explosive power is only released in the controlled environments of the nucleation and polymerization machineries.
authors.affiliation: Institute of Molecular and Cell Biology, A*STAR (Agency of Science, Technology and Research), Biopolis, Singapore 138673, Singapore.
authors.forename: Bo
authors.initials: B
authors.lastname: Xue
authors.forename: Robert C
authors.initials: RC
authors.lastname: Robinson
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism*; D051339:Actin Depolymerizing Factors / Q000378:metabolism; D000199:Actins / Q000235:genetics / Q000378:metabolism*; D020816:Amino Acid Motifs; D000818:Animals; D018797:Cell Cycle Proteins / Q000378:metabolism; D003598:Cytoskeletal Proteins / Q000378:metabolism; D006801:Humans; D051304:Profilins / Q000378:metabolism; D013947:Thymosin / Q000378:metabolism; D051316:Wiskott-Aldrich Syndrome Protein / Q000378:metabolism
chemical_list: D051339:Actin Depolymerizing Factors; D000199:Actins; C489549:CAP1 protein, human; D018797:Cell Cycle Proteins; D003598:Cytoskeletal Proteins; D051304:Profilins; D051316:Wiskott-Aldrich Syndrome Protein; D013947:Thymosin
keywords: Abundance; Actin-binding protein; Binding affinity; Complex; Monomeric actin; Nucleotide exchange