Ca2+, Mg2+-dependent endonuclease and ADP-ribosylation

Area of Science:

• Biochemistry
• Molecular Biology
• Enzymology

Background:

• Ca2+, Mg2+-dependent endonuclease plays a role in cellular processes.
• ADP-ribosylation is a post-translational modification with diverse regulatory functions.
• The specific mechanism of endonuclease inhibition by ADP-ribosylation was not fully understood.

Purpose of the Study:

• To elucidate the molecular mechanism by which ADP-ribosylation inhibits Ca2+, Mg2+-dependent endonuclease.
• To identify the key components involved in this inhibition process.
• To investigate the role of poly(ADP-ribose) chain length and electrostatic interactions.

Main Methods:

• Purification of bull seminal plasma Ca2+, Mg2+-dependent endonuclease.
• Enzyme activity assays in the presence of poly(ADP-ribose) polymerase, NAD+, DNA, and Mg2+.
• Sephadex G-100 column chromatography and SDS-polyacrylamide gel electrophoresis to analyze reaction products.
• Mild alkaline treatment to assess reversibility of inhibition.

Main Results:

• Ca2+, Mg2+-dependent endonuclease activity was significantly suppressed by preincubation with NAD+, poly-(ADP-ribose) polymerase, DNA, and Mg2+.
• ADP-ribosylation of the endonuclease was confirmed, with longer poly(ADP-ribose) chains causing greater inhibition.
• Inhibition was reversed by alkaline treatment, indicating hydrolysis of the enzyme-polymer linkage.
• Histone ADP-ribosylation did not significantly affect their endonuclease-stimulating ability.

Conclusions:

• ADP-ribosylation inhibits Ca2+, Mg2+-dependent endonuclease activity through the covalent attachment of poly(ADP-ribose).
• The extent of inhibition correlates with the length of the ADP-ribose polymer.
• Electrostatic repulsion between DNA and the attached poly(ADP-ribose) is proposed as the primary inhibitory mechanism.
• The inhibition is reversible, suggesting a regulatory role for ADP-ribosylation in endonuclease function.