Purification of a calcium-sensitive actin gelation protein from Acanthamoeba.

A new calcium-sensitive actin filament cross-linking protein has been purified from Acanthamoeba. By gel electrophoresis in sodium dodecyl sulfate, the apparent subunit molecular weight varies depending on the concentration of sulfhydryl reducing agent in the sample. The major band is 60,000 in 10% beta-mercaptoethanol, 85,000 in 1% mercaptoethanol, an 90,000 without beta-mercaptoethanol. By electron microscopy, the molecule is a rod about 3 nm wide and 55 nm long with a 5.5-nm globular region at one end, which accounts for its large Stokes radius of 8.5 nm. At low concentrations, the gelation protein cross-links actin filaments to form a solid gel. This gelation reaction is inhibited by micromolar concentrations of Ca2+, by cytochalasin B, and by capping protein and is promoted by ATP, MgCl2, and KCl.