Proteases and proteolytic cleavage of storage proteins in developing and germinating dicotyledonous seeds

Abstract

Proteolytic cleavage plays an important role in storage protein deposition and reactivation in seeds. Precursor polypeptides are processed by limited proteolysis to mature subunits of reserve proteins in storage tissue cells of developing seeds. Steps of proteolytic processing are closely related to steps in intracellular protein transfer through the endomembrane system and to the deposition in the storage vacuole. In germinating seeds special endopeptidases trigger storage protein breakdown by limited proteolysis. The induced conformation changes of storage proteins open them to attack by additional endo- and exopeptidases which degrade the protein reserves completely. Proteases that catalyse limited cleavage or complete degradation are synthesized as precursors which also undergo stepwise limited proteolysis when they are formed in cotyledons of developing or germinating seeds. In general, this processing transforms enzymatically inactive proenzymes into active proteases. Different compartments participate in the processing steps. Many of the proteases are encoded by small multigene families. Different members of the corresponding protease families seem to act during seed development and germination. Proteolytic processes that contribute to the molecular maturation and to the reactivation of storage proteins in dicotyledonous seeds seem to be controlled by (1) differential expression of members of the protease-encoding gene families; (2) stepwise processing and activation of protease precursor polypeptides; (3) transient differential compartmentation of precursors and mature polypeptides of proteases and storage proteins, respectively; and (4) interacting changes in storage protein structure and protease action. The present knowledge on these processes is reviewed.