title: High affinity binding of divalent cation to actin monomer is much stronger than previously reported.
abstract: Monomeric actin is known to bind tightly one divalent cation per molecule. We have quantitatively reinvestigated the affinity of actin for Ca++ and Mg++ using the fluorescent Ca++ chelator Quin2 to induce and measure the dissociation of Ca++ from Ca-actin, supporting these studies with measurements using 45Ca. We found that the KD for Ca-actin is actually 1.9 +/- 0.7 nM. Kinetic analysis supported this result and demonstrated a dissociation rate constant (k-) of 0.013 s-1 and an association rate constant (k+) of 6.8 X 10(6)M-1 s-1 for Ca-actin. Competitive binding studies indicated that the binding affinity of actin for Ca++ is 5.4 times that for Mg++, yielding a calculated KD for Mg-actin of about 10 nM. Thus, the tight-binding of divalent cations to actin is 3-4 orders of magnitude stronger than previously thought.
mesh_terms: D000199:Actins / Q000378:metabolism*; D000634:Aminoquinolines; D001667:Binding, Competitive; D002118:Calcium / Q000378:metabolism*; D002413:Cations, Divalent / Q000378:metabolism; D055598:Chemical Phenomena; D002621:Chemistry; D005456:Fluorescent Dyes; D008274:Magnesium / Q000378:metabolism*; D008433:Mathematics; D011485:Protein Binding; D011489:Protein Denaturation; D013050:Spectrometry, Fluorescence
chemical_list: D000199:Actins; D000634:Aminoquinolines; D002413:Cations, Divalent; D005456:Fluorescent Dyes; D008274:Magnesium; C035739:Quin2; D002118:Calcium