The unpolymerised form of actin in non-muscle cells.

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.