Most aspartic proteinases (APs) of plant origin are characterized by the presence of plant-specific insertion (PSI) in their primary structure. PSI has been reported to function as signals for both transport of AP molecules from the endoplasmic reticulum (ER) and for their targeting to the vacuole. To determine the functions of the PSIs in soyAP1 and soyAP2 identified in our previous study, we examined their subcellular localization by transient expression of a green fluorescent protein (GFP) fusion protein in the protoplasts of Arabidopsis suspension-cultured cells. Both soyAP1-GFP and soyAP2-GFP were targeted to the vacuote. To confirm the rote of the PSI, we prepared PSI-deleted soyAP1 and soyAP2, and investigated their vacuolar targeting by the same method. While the former deletion mutant was always transported to the vacuote, the latter sometimes remained in the ER and was only sometimes transported to the vacuole. These observations indicated that, in the case of soyAP1, the PSI is not involved in vacuotar targeting, also suggesting that the function of the PSI differs depending on its origin. (c) 2005 Elsevier GmbH. All rights reserved.