A FAMILY OF POTATO GENES THAT ENCODE KUNITZ-TYPE PROTEINASE-INHIBITORS - STRUCTURAL COMPARISONS AND DIFFERENTIAL EXPRESSION

Potato tubers contain a complex group of proteins of 20 to 24 kDa that exhibit homology to Kunitz-type proteinase inhibitors. We isolated th ree cDNAs and two genomic clones that encode members of the potato Kun itz-type proteinase inhibitor (PKPI) family. Comparison of the structu res of these and other cloned genes indicated that genes of the PKPI f amily 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 p roteins that had been separated by column chromatography and subsequen tly gel electrophoresis revealed three different sequences that corres ponded 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 pr opeptide with a highly conserved Asn-Pro-Ile-Xxx-Leu-Pro motif that is identical to the potential vacuolar-sorting determinant in the N-term inal propeptide of a precursor to sporamin of sweet potato. Expression of the PKPI-A and -B genes is differentially regulated: PKPI-A mRNA b ut not PKPI-B mRNA were induced in leaves after wounding or upon treat ment with methyl jasmonate. Nuclear genes for PKPI-A and -B do not con tain introns, and the homology between the two types of gene extends o nly 72 bp upstream from the site of initiation of transcription.