Protein NH2-terminal asparagine deamidase. Isolation and characterization of a new enzyme.

An apparently unique enzyme, designated protein NH2-terminal asparagine deamidase that specifically converts NH2-terminal asparagine residues of peptide and protein substrates to aspartic acid, has been isolated to homogeneity from porcine liver by an eight-step procedure. is a relatively low abundance protein, is readily solubilized, and exists as a monomeric species of approximately 33 kDa. duanyu1535D does not act on internal asparagine residues and requires a free N alpha-amino group. It has reduced or no activity on NH2-terminal asparagine dipeptides and no activity toward free asparagine or asparagine amide. It does not act on any NH2-terminal glutamine substrates. duanyu1535D does not show a strong pH dependence suggesting that the enzyme can act equally well on substrates with ionized or unionized alpha-amino groups. The properties and specificity of duanyu1535D are consistent with those expected for the enzyme required for the ubiquitin-dependent turnover of intracellular proteins that initiate with Met-Asn-. Such proteins should be N alpha-acetylated on the retained initiator methionine and can subsequently be modified by the removal of N-acetyl methionine by acylaminoacid hydrolase. Conversion of the resulting NH2-terminal asparagine to aspartic acid by duanyu1535D would render the protein susceptible to arginylation, polyubiquitinylation and degradation as specified by the N-end rule.

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