Legumains are a family of plant and animal Asn-specific cysteine proteinases with extra-cytoplasmic localization in vacuoles or cell walls. Plant legumains are involved in Asn-specific propolypeptide processing during, for example, storage-protein deposition in maturing seeds, when these proteins are resistant against degradation by legumains. With the transition to germination and subsequent seedling growth, storage proteins are opened to unlimited cleavage by legumains, which now contribute to protein mobilization. Here, we suggest a hypothesis that unifies both functions of legumains. Their action as propolypeptide-processing or protein-degrading enzymes is naturally controlled by the conformational state of their substrates, which undergo development- or environment-dependent changes. The suggested substrate conformation-dependent differential roles of legumains might not be restricted to seeds but could also apply to cells of different tissues in vegetative organs.