actin

One of the protein components into which actomyosin can be split; it can exist in a fibrous form (F-a.) or a globular form (G-a.).

- F-a. the association of G-a. subunits into a fibrous (F) protein caused by an increase in salt concentration; the conversion of G-a. to F-a. is catalyzed by small concentrations of magnesium ion, is reversible, and is accompanied by the conversion of the bound ATP molecule to ADP and the conversion of one reactive -thiol group to an unreactive form.

- G-a. the globular (G) subunits of the a. molecule, having a molecular weight 42 kd and containing one molecule of ATP; it is soluble in dilute salt, polymerizing to F-a. when the ionic strength is increased.

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ac·tin 'ak-tən n a protein found esp. in microfilaments (as those comprising myofibrils) and active in muscular contraction, cellular movement, and maintenance of cell shape see F-ACTIN, G-ACTIN

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n.

a protein, found in muscle, that plays an important role in the process of contraction. See striated muscle.

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ac·tin (akґtin) a structural protein present in all eukaryotic cells, important both as a component of the cytoskeleton and for its role in cell motility. In the absence of salt, it exists as a 42-kD globular monomer (G-actin); in the presence of potassium chloride and adenosine triphosphate it polymerizes, forming long fibers (F-actin). It binds over 60 families of cytoplasmic proteins, and genetic defects in the actin system cause a wide variety of diseases, including muscular dystrophy, hemolytic anemia, and cardiomyopathies. In combination with myosin it is responsible for muscular contraction and for the pinching off of daughter cells in cytokinesis. See Plate 32 and see also actomyosin.