A family of potato genes that encode Kunitz-type proteinase inhibitors: structural comparisons and differential expression

Abstract

Potato tubers contain a complex group of proteins of 20 to 24 kDa that exhibit homology to Kunitz-type proteinase inhibitors. We isolated three cDNAs and two genomic clones that encode members of the potato Kunitz-type proteinase inhibitor (PKPI) family. Comparison of the structures of these and other cloned genes indicated that genes of the PKPI family can be classified into three major homology groups, namely, A, B and C. The PKPI-A and -B genes exhibit higher homology to one another than to the PKPI-C genes. Determination of the N-terminal amino acid sequences of 18 polypeptides from the complex group of 20- to 24-kDa proteins that had been separated by column chromatography and subsequently gel electrophoresis revealed three different sequences that corresponded to PKPI-A, -B, and -C. PKPI-A genes include those coding for a cathepsin D inhibitor, while PKPI-B and -C genes include those coding for trypsin and/or chymotrypsin inhibitors and a subtilisin inhibitor. Precursors to PKPIs are synthesized with an N-terminal extra peptide that appears to contain, in addition to the signal peptide, a short propeptide with a highly conserved Asn-Pro-Ile-Xxx-Leu-Pro motif that is identical to the potential vacuolar-sorting determinant in the N-terminal propeptide of a precursor to sporamin of sweet potato. Expression of the PKPI-A and -B genes is differentially regulated: PKPI-A mRNA but not PKPI-B mRNA were induced in leaves after wounding or upon treatment with methyl jasmonate. Nuclear genes for PKPI-A and -B do not contain introns, and the homology between the two types of gene extends only 72 bp upstream from the site of initiation of transcription.