The determination of the Mg2+.ATP dissociation constant by competition with Eu3+ ion using laser-induced Eu3+ ion luminescence spectroscopy

ATP competes with PIP2 for binding to gelsolin

Gelsolin is a severing and capping protein that targets filamentous actin and regulates filament lengths near plasma membranes, contributing to cell movement and plasma membrane morphology. Gelsolin binds to the plasma membrane via phosphatidylinositol 4,5-bisphosphate (PIP2) in a state that cannot cap F-actin, and gelsolin-capped actin filaments are uncapped by PIP2 leading to filament elongation. The process by which gelsolin is removed from PIP2 at the plasma membrane is currently unknown. Gelsolin also binds ATP with unknown function. Here we characterize the role of ATP on PIP2-gelsolin complex dynamics. Fluorophore-labeled PIP2 and ATP were used to study their interactions with gelsolin using steady-state fluorescence anisotropy, and Alexa488-labeled gelsolin was utilized to reconstitute the regulation of gelsolin binding to PIP2-containing phospholipid vesicles by ATP. Under physiological salt conditions ATP competes with PIP2 for binding to gelsolin, while calcium causes the release of ATP from gelsolin. These data suggest a cycle for gelsolin activity. Firstly, calcium activates ATP-bound gelsolin allowing it to sever and cap F-actin. Secondly, PIP2-binding removes the gelsolin cap from F-actin at low calcium levels, leading to filament elongation. Finally, ATP competes with PIP2 to release the calcium-free ATP-bound gelsolin, allowing it to undergo a further round of severing.

A continuous spectrophotometric assay for the activation of plant NAD kinase by calmodulin, calcium(II), and europium(III) ions

A continuous spectrophotometric assay has been developed to quantify the calmodulin, calcium(II) ion, and europium(III) ion dependence of the activation of NAD kinase from pea seedlings. Experimental enzyme activation data are compared with the theoretical curves for the binding of calcium(II) ions to the individual calcium binding sites of calmodulin. These results indicate that the binding of three calcium(II) ions is necessary for activation of plant NAD kinase. Further studies demonstrate that europium(III) ions can replace calcium(II) ions in calmodulin with retention of its ability to activate NAD kinase.

Lanthanide ion luminescence as a probe of DNA structure. 2. Non- guanine-containing oligomers and nucleotides

Oligo(dC)8, oligo(dA)8, and oligo(dT)8 as well as d-CMP, d-AMP, and d-TMP, when complexed to Eu(3+), possess two classes of Eu(3+) binding environment. The binding environments consist of two classes, tight sites which coordinate two H2O molecules, and weaker sites which coordinate six or seven, analogous to the previously studied guanine-containing molecules. It is inferred that the tight class of Eu(3+) ion site observed with these oligomers and nucleotides corresponds to dimeric or polymeric structures. Comparison of the results for the guanine and non-guanine containing oligomers suggests that Eu(3+) possibly coordinates base nitrogen atoms in the former and in an outer sphere mode (hydrogen bonding via the H2O molecules coordinated to Eu(3+)) in the species examined here.

Lanthanide ion luminescence as a probe of DNA structure. 1. Guanine-containing oligomers and nucleotides

Laser-induced Eu(3+) luminescence spectroscopy is used to probe the interaction of Eu(3+) ion with guanine-containing nucleotides and single-stranded oligomers. By using time-resolved and non-time-resolved Eu(3+) luminescence techniques, two classes of Eu(3+) binding site are observed in oligo(dG)10, oligo(dG)8, oligo(dG)6, oligo(dG)4, and d-GMP. One class of site binds Eu(3+) ions more strongly than the other. Since the "tight" class of bound Eu(3+) ions have two coordinated water molecules, it is inferred that six or seven atoms from the oligomers are coordinating the Eu(3+). The "weaker" class of Eu(3+) ion sites involve the coordination of six or seven water molecules and therefore, are coordinated by one or two atoms from the oligomer. The tight class of Eu(3+) binding site is attributed to an interstrand association of Eu(3+) with the oligomers forming dimeric or polymeric structures. The dissociation constants (Kd) for the 1:1 complexes Eu(d-GMP)+ and Eu(d-GTP)- have been determined as well as the Kd for the dimerization reaction of Eu(d-GMP)+. The Tb(3+) luminescence enhancement properties of these molecules are also examined in relation to their EU(3+) binding characteristics.