A cut above the rest: the regulatory function of plant proteases

Proteolytic enzymes are intricately involved in many aspects of plant physiology and development. On the one hand, they are necessary for protein turnover. Degradation of damaged, misfolded and potentially harmful proteins provides free amino acids required for the synthesis of new proteins. Furthermore, the selective breakdown of regulatory proteins by the ubiquitin/proteasome pathway controls key aspects of plant growth, development, and defense. Proteases are, on the other hand, also responsible for the post-translational modification of proteins by limited proteolysis at highly specific sites. Limited proteolysis results in the maturation of enzymes, is necessary for protein assembly and subcellular targeting, and controls the activity of enzymes, regulatory proteins and peptides. Proteases are thus involved in all aspects of the plant life cycle ranging from the mobilization of storage proteins during seed germination to the initiation of cell death and senescence programs. This article reviews recent findings for the major catalytic classes, i.e. the serine, cysteine, aspartic, and metalloproteases, emphasizing the regulatory function of representative enzymes.