Endonuclease activity in lipocalins

Several lipocalins contain conserved amino acid sequences similar to the ph osphodiester bond cleavage domain of sugar nonspecific magnesium-dependent nucleases of the Serratia marcescens type. His-89 and Glu-127 of the S. mar cescens endonuclease are believed to have a role in the active catalytic si te by the attack of a water molecule at the phosphorus atom of the bridging phosphate. Tear lipocalin contains both amino acids in analogous regions, and is active as a nuclease. Two forms of beta-lactoglobulin contain only G lu-134 (analogous to Glu-127 of the Serratia nuclease) yet retain nuclease activity equal to or greater than that of tear lipocalin. However, retinol- binding protein lacks both of these motifs and shows no detectable activity . DNA-nicking activity is decreased by 80% in the mutant of tear lipocalin that replaces Glu-128 but is unchanged by mutations of His-84. The endonucl ease activity of tear lipocalin is dependent on the bivalent cations Mg2+ o r Mn2+ but is decreased at high concentrations of NaCl. These findings indi cate that some lipocalins have non-specific endonuclease activity similar i n characteristics to the Mg2+-dependent nucleases and related to the conser ved sequence LEDFXR (where 'X' denotes 'any other residue'), in which the g lutamic residue seems to be important for activity.