N-Acetylneuraminic acid (NeuAc) is an important molecule in biological recognition systems. NeuAc is known to be biosynthesized either from UDP-N-acetyl-D-glucosamine by an action of UDP-N-acetyl-D-glucosamine 2-epimerase or from N-acetyl-D-glucosamine by N-acyl-D-glucosamine 2-epimerase (GlcNAc 2-epimerase). However, the physiological function of the GlcNAc 2-epimerase in NeuAc biosynthesis has not been fully evaluated. To clarify the role of GlcNAc 2-epimerase in NeuAc biosynthesis, the enzyme and its gene were isolated from porcine kidney cortex. Escherichia coli cells transformed with the gene expressed the GlcNAc 2-epimerase having the same properties as those of the GlcNAc 2-epimerase from porcine kidney. Sequence analysis indicated that the gene was capable of synthesizing a 46.5-kDa protein (402 amino acids) with a conserved leucine zipper motif. Homology search for the cloned gene revealed that the GlcNAc 2-epimerase was identical with renin-binding protein (RnBP) in porcine kidney (Inoue, H., Fukui, K., Takahashi, S., and Miyake, Y.(1990) J. Biol. Chem. 265, 6556-6561) (identity: 99.6% in nucleotide sequence, 99.0% in amino acid sequence). That GlcNAc 2-epimerase is a RnBP was confirmed by its ability to bind porcine kidney renin and mask its protease activity. These findings provide unequivocal evidence that the enzyme GlcNAc 2-epimerase is a RnBP.