A novel protein-deamidating enzyme was purified to homogeneity from Chryseobacterium proteolyticum and the gene encoding it was cloned. The enzyme is a monomer with a pI of 10.0, a measured Mr of ≈ 20 000 and a calculated Mr of 19 860. Extensive comparison with Streptoverticillium transglutaminase showed that the protein-deamidating enzyme lacked transglutaminase activity in terms of hydroxamate-formation between benzyloxycarbonyl-Gln-Gly and hydroxylamine, or monodansylcadaverine incorporation into casein. The enzyme deamidated the two glutaminyl residues in the oxidized insulin A chain and deamidated both casein and the oxidized insulin B chain with higher catalytic efficiencies (kcat/Km) than with short peptides. The enzyme was active against several proteins, including insoluble wheat gluten, but did not deamidate asparaginyl residues in peptides, free glutamine or other amides. The enzyme was therefore named protein-glutaminase (EC 3.5.1). The gene encoding the protein was cloned and, when expressed in Escherichia coli, the protein product had protein-glutaminase activity and cross-reacted with antiserum raised against the purified enzyme. The protein-glutaminase was shown to be expressed as a prepro-protein with a putative signal peptide of 21 amino acids and a pro-sequence of 114 amino acids. The amino-acid sequence had no obvious homology to any published sequence and is therefore a novel protein-glutaminase.