Zn(2+) rather than Ca(2+) or Mg(2+) used as a cofactor in non-muscular actin from the oyster to control protein polymerization.

The major cytoskeletal protein of most cells is actin, which polymerizes to form actin filaments (F-actin). Each actin monomer (G-actin) contains a divalent alkaline earth metal ion (in vivo Mg(2+); in vitro usually Ca(2+)) as a cofactor that is crucial for protein polymerization. Prior to this study, however, whether or not other types of metal ions can play the same role as Mg(2+) or Ca(2+) in actins remains unknown. A new actin from the gills of oyster (AGO) was prepared and characterized by protein purification techniques, SDS- and native-PAGE, and LC-MS\MS for the first time. The property of this protein was studied by CD, fluorescence and UV/vis spectroscopy, laser light scattering, and TEM. AGO is a monomer with a MW of ~42kDa. AGO is unique among all known actins in that Zn(2+) is only a naturally binding metal in the protein, and that one native AGO molecule binds 8 zinc ions, which can be removed by EDTA treatment at pH7.2. The presence of zinc has a great effect on the secondary and tertiary structure of the protein. Correlated with such effect is that these zinc ions in native AGO facilitate protein polymerization, whereas removal of zinc ions from native AGO results in a loss of such polymerization property. The present work demonstrates that AGO is a novel zinc-binding protein with high capacity, and high selectivity. This work extends an understanding of the function of zinc and actin.