Introduction
NudT16 is a member of the Nudix superfamily of hydrolases which break a phosphoester bond between the two phosphates in nucleoside diphosphate-linked to moiety X proteins resulting in a nucleoside monophosphate (NMP) and a phosphate linked to moiety X. While NudT16 was initially described as a nuclear RNA and cytoplasmic mRNA decapping enzyme, further studies have shown that it also effectively hydrolyzes inosine diphsophate (IDP) and its hazardous deoxyribose cognate (dIDP) into inosine monophsophate (IMP) and deoxy inosine monophosphate (dIMP), respectively [1]. NudT16 has also been shown to regulate levels of 53BP1, an adaptor protein that recruits other proteins to the site of a DNA breakage, through hydrolytic removal of ADP-ribose from ADP-ribosylated 53BP1 [2].
Structure
NudT16 is a homodimer, consisting of two monomers of the same sequence. A structure on the left shows one monomer in cyan while the other in purple. This dimerization allows for each subunit to have deeper . Each monomer consists of two beta sheets surrounded by alpha-helices, as is typical of Nudix hydrolases. A variation of the catalytically relevant Nudix box characteristic of Nudix hydrolases consisting of 23 highly conserved residues (G1Z2-6E7Z8-14R15E16U17Z18E19E20Z21G22U23 where Z is any amino acid and U is an aliphatic and hydrophobic residue) is present in NudT16 (GARRLELGEALALGSGWRHVCHA). In contrast to the negatively charged pockets lined with glutamate residues where metal ligands bind, the adenosine binding pocket is positively charged. The mouth of the binding site is about 9Å in width. Thirawatananond et. al. investigated whether widening of this mouth would allow proteins conjugated to ADP further into the binding site. Contrary to Nudix ADPRases, HsNudT16 binds adenosine of ADPr and buries it deep in the core, while leaving the non-adenosine ribose exposed to the surface. This orientation allows the exposed ribose to conjugate another protein[3]. Many in the mouth of this binding pocket are also involved in hydrogen bonding, the binding of metal ligands, and also serve to delimit the binding site.
Function
