Nuclear shield: a multi-enzyme task-force for nucleus protection

Background

In eukaryotic cells the nuclear envelope isolates and protects DNA frommolecules that could damage its structure or interfere with its processing.Moreover, selected protection enzymes and vitamins act as efficientguardians against toxic compounds both in the nucleoplasm and in thecytosol. The observation that a cytosolic detoxifying and antioxidant enzymei.e. glutathione transferase is accumulated in theperinuclear region of the rat hepatocytes suggests that other unrecognizedmodalities of nuclear protection may exist. Here we show evidence for theexistence of a safeguard enzyme machinery formed by an hyper-crowding ofcationic enzymes and proteins encompassing the nuclear membrane and promotedby electrostatic interactions.

Methodology/Principal Findings

Electron spectroscopic imaging, zeta potential measurements,isoelectrofocusing, comet assay and mass spectrometry have been used tocharacterize this surprising structure that is present in the cells of allrat tissues examined (liver, kidney, heart, lung and brain), and thatbehaves as a “nuclear shield”. In hepatocytes, thishyper-crowding structure is about 300 nm thick, it is mainly formed bycationic enzymes and the local concentration of key protection enzymes, suchas glutathione transferase, catalase and glutathione peroxidase is up toseven times higher than in the cytosol. The catalytic activity of theseenzymes, when packed in the shield, is not modified and their relativeconcentrations vary remarkably in different tissues. Removal of thisprotective shield renders chromosomes more sensitive to damage by oxidativestress. Specific nuclear proteins anchored to the outer nuclear envelope arelikely involved in the shield formation and stabilization.

Conclusions/Significance

The characterization of this previously unrecognized nuclear shield indifferent tissues opens a new interesting scenario for physiological andprotection processes in eukaryotic cells. Selection and accumulation ofprotection enzymes near sensitive targets represents a new safeguardmodality which deeply differs from the adaptive response which is based onexpression of specific enzymes.