An apparent leucine zipper motif was recognized in the predicted amino acid sequence of porcine kidney renin-binding protein (RnBP) by analysis of the nucleotide sequence of a cDNA encoding the protein (Inoue, H., Fukui, K., Takahashi, S., and Miyake, Y. (1990) J. Biol. Chem. 265, 6556-6561). To evaluate the role of this motif in the formation of an RnBP-renin heterodimer and an RnBP homodimer, a porcine mutant cDNA involving Leu185----Asp and Leu192----Asp substitutions was constructed and expressed in vitro and in Xenopus oocytes. The mutant protein neither binds to renin nor forms the homodimer. The results strongly suggest that the leucine zipper motif in the RnBP molecule mediates the formation of both the RnBP-renin heterodimer and the RnBP homodimer observed previously. The existence of the motif should facilitate elucidation of the role of RnBP in renin metabolism.