Endonuclease activity in lipocalins.

Area of Science:

• Biochemistry
• Molecular Biology
• Enzymology

Background:

• Lipocalins are a diverse protein superfamily with various functions.
• Some lipocalins share conserved amino acid sequences with known nucleases.
• The phosphodiester bond cleavage domain of Serratia marcescens nuclease is a well-characterized enzyme active site.

Purpose of the Study:

• To investigate the nuclease activity of lipocalins.
• To identify conserved motifs responsible for endonuclease activity in lipocalins.
• To determine the role of specific amino acids in the catalytic mechanism.

Main Methods:

• Comparative sequence analysis of lipocalins and known nucleases.
• Enzyme activity assays to measure DNA-nicking and endonuclease activity.
• Site-directed mutagenesis to probe the function of key amino acid residues.

Main Results:

• Tear lipocalin exhibits Mg(2+)/Mn(2+)-dependent endonuclease activity.
• Beta-lactoglobulin shows significant nuclease activity, retaining function with only a key glutamic acid residue.
• Retinol-binding protein lacks detectable nuclease activity.
• Mutating a specific glutamic acid in tear lipocalin significantly reduced DNA-nicking activity, while mutating a histidine had no effect.
• Activity was dependent on divalent cations and inhibited by high salt concentrations.

Conclusions:

• Certain lipocalins possess inherent non-specific endonuclease activity.
• A conserved glutamic acid residue within the LEDFXR motif is critical for this activity.
• Lipocalin nuclease activity shares characteristics with magnesium-dependent nucleases.