A nuclear protein associated with actively transcribed nucleosomes exhibits Zn2+ -binding activity

Characterization of Zn(2+)-binding nuclear proteins present in the myocardium

Nonhistone nuclear proteins were isolated from 3-5 day old neonatal as well as 3 month-old adult myocardium. The nuclear proteins were separated and analyzed by two-dimensional polyacrylamide gel electrophoresis. Using a blot transfer technique equilibrated with 65Zn2+, at least four polypeptides exhibited Zn(2+)-binding activity over the spectrum of nonhistone nuclear proteins. A protein with a molecular weight of 68kDa pI7.8, which has been characterized for its involvement in nucleosome structure, consistently binds Zn2+ in both the neonatal and adult myocardium. This nuclear protein has now been further characterized by partial amino acid microsequencing. It was found that this novel polypeptide is distinct from the pore-complex lamina proteins. Three other polypeptides with M tau 90kDa, pI7.8, M tau 68kDa, pI6.5 and M tau 35kDa, pI7.5 exhibited increased Zn(2+)-binding activity in neonatal myocardium as compared to adult myocardium. Together with results from our previous studies, this study provides the first evidence implicating Zn(++)-binding nuclear proteins in the processes of growth and differentiation of myocardial development.

Detection of two Zn-finger proteins of Xenopus laevis, TFIIIA, and p43, by probing western blots of ovary cytosol with 65Zn2+, 63Ni2+, or 109Cd2+

Two Zn-finger proteins, TFIIIA (a constituent of 7S RNP particles) and p43 (a constituent of 42S RNP particles), were detected in ovary extracts of juvenile Xenopus laevis females by in vitro binding of radiolabeled divalent metals. Proteins fractionated by SDS-PAGE (sodium dodecylsulfate-polyacrylamide gel electrophoresis) were transferred by Western blotting onto nitrocellulose membranes, probed with 65Zn2+, 63Ni2+, or 109Cd2+, and visualized by autoradiography. Detection limits for TFIIIA were approx 0.07 micrograms/well by 109Cd(2+)-probing, 0.13 micrograms/well by 65Zn(2+)-probing, and 0.26 mu/well by 63Ni(2+)-probing. Protein p43 was more clearly visualized by probing with 63Ni2+ than with 65Zn2+ or 109Cd2+. After purified TFIIIA was cleaved with cyanogen bromide, 65Zn2+, 109Cd2+, and 63Ni2+ distinctly labeled the 22 kDa middle fragment; 65Zn2+ and 109Cd2+ also labeled the 11 kDa N-terminal fragment, but did not label the 13 kDa C-terminal fragment. These results are consistent with the notion that the radioligands were bound to finger-loop domains of TFIIIA, which occur in the middle and N-terminal fragments. Based on the abilities of nonradioactive metal ions to compete with 65Zn2+ for binding to TFIIIA on Western blots, the relative affinities of the metals for TFIIIA were ranked as follows: Zn2+ = Cu2+ greater than or equal to Hg2+ greater than Cd2+ greater than Co2+ greater than or equal to Ni2+. Even at a 1000-fold molar excess, Mn2+ did not compete with 65Zn2+ for binding to TFIIIA. Probing Western blots with the radiolabeled metal ions greatly facilitates the detection, isolation, and quantitation of TFIIIA and p43.

Interaction of Zn2+ with the bovine-heart mitochondrial bc1 complex

A study is presented of the effect of Zn2+ on the enzymatic properties of the bovine-heart cytochrome-bc1 complex. Micromolar concentrations of Zn2+ reversibly inhibit the cytochrome-c reductase activity of either the cholate-solubilized or liposome-reconstituted complex. Kinetic analysis of the redox reactions of the cytochromes indicate that Zn2+ affects the activity of the complex at the quinol oxidation site. The following have been determined: (a) Zn2+ inhibits the pre-steady-state reduction of cytochrome c1 by duroquinol either in the absence or in the presence of antimycin, (b) it does not inhibit the reduction of b cytochromes in the absence of antimycin or in the presence of myxothiazol, (c) it inhibits cytochrome-b reduction in the presence of antimycin. Furthermore Zn2+ inhibits the antimycin-promoted oxidant-induced extrareduction of b cytochromes. Addition of Zn2+ to reduced bc1 complex causes a red shift in the absorption spectrum of cytochrome b566 and a substantial decrease in the signal intensity of the EPR spectrum of the Fe-S protein. This is interpreted as an interaction of Zn2+ with the 2Fe-2S-cluster region of the Fe-S protein, thus giving rise to inhibition of the reductase activity and of the antimycin-insensitive reduction route of b cytochromes. A Scatchard-plot of 65Zn2+ binding to the native isolated complex gave a straight line from which a value of three binding sites and a single dissociation constant of 3 x 10(-6) M can be calculated, which is practically equal to the concentration causing 50% inhibition of electron flow.