Abstract
Equilibrium dialysis experiments have shown that G-actin preparations can bind up to 9 phosphate ions and 13 vanadate ions per actin monomer with association constants of 3.00 x 10 super(2) M super(-1) and 1.24 x 10 super(2) M super(-1), respectively. Phosphate binding at low ionic strength caused removal of bound Ca super(2+) from G-actin and polymerization of the actin. The phosphate-treated polymeric actin was much more resistant to Pronase digestion than Ca super(2+)-free polymeric actin which did not contain bound phosphate but which was prepared by dialysis against EGT A-containing buffer. Vanadate-treated actin only polymerized to 47% of the extent of polymerization measured for phosphate-treated actin, indicating that vanadate ion is not as effective a promoter of low-ionic strength actin polymerization as phosphate ion.