A KUNITZ-TYPE PROTEINASE-INHIBITOR WITH DIFFERENTIAL SOLUBILITY DURING THE DEVELOPMENT OF POTATO (SOLANUM-TUBEROSUM L.) TUBERS

Two potato (Solanum tuberosum L., cv. Irish Cobbler) Kunitz-type prote inase inhibitors (PKPI) were previously described to be present in the soluble fraction of proteins from tubers in the early stages of devel opment. One of them became insoluble in mature tubers, being extractab le from this material in presence of urea. Amino acid sequencing showe d that the soluble and insoluble PKPI were identical to each other. Al so, immunolocalization using the protein A-gold method showed that bot h proteins were present inside the vacuole in free (intravacuolar spac e) and aggregated forms. The density of PKPI in the vacuolar protein a ggregates increased from developing to mature tubers. showing that the soluble-insoluble state of this protein is related to the aggregation levels. Purified PKPI precipitated in vitro, mainly in presence of hi gh calcium concentrations and low FH, but this precipitated form was n ot as stable as aggregates found in vivo. Based on the results obtaine d, a model of PKPI insolubilization in vivo is discussed.