title: The unpolymerised form of actin in non-muscle cells.
abstract: Several lines of evidence point to the existence of unpolymerised actin in non-muscle cells. Ultrastructural examination reveals both a variety of actin filament bundles and actin in a controversial organisational state. Arguments are cited that this material, which at least in part is found close to the plasma membrane, represents unpolymerised actin rather than a random array of single actin filaments. The rearrangement of actin filament bundles during the cell cycle, and in response to experimental manipulation, suggests a turnover of filaments by a polymerisation-depolymerisation cycle. Extracts made from non-muscle cells under conditions where muscle actin would polymerise still contain appreciable fractions of monomeric actin. Studies on purified polymerisation-resistant actin from a variety of sources reveal the presence of a small protein which binds specifically to actin and prevents polymerisation. In the last section of the article, we expand the idea that this auxiliary protein is a central control element in the regulated exchange between non-polymerised and polymerised actin in vivo.
mesh_terms: D000199:Actins / Q000096:biosynthesis / Q000502:physiology*; D000818:Animals; D002462:Cell Membrane / Q000502:physiology; D002642:Chick Embryo; D003285:Contractile Proteins / Q000502:physiology*; D003593:Cytoplasm / Q000502:physiology*; D003599:Cytoskeleton / Q000502:physiology* / Q000648:ultrastructure; D003851:Deoxyribonucleases / Q000502:physiology; D006801:Humans; D066298:In Vitro Techniques; D007448:Invertebrates; D014176:Protein Biosynthesis; D013049:Spectrin / Q000502:physiology; D014714:Vertebrates
chemical_list: D000199:Actins; D003285:Contractile Proteins; D013049:Spectrin; D003851:Deoxyribonucleases