Plastid proteases

Plastids form a major proteinaceous compartment in plant and algal cells. For example, chloroplasts contain 2000 to 2500 proteins [1] and account for 75% to 80% of the total nitrogen in a leaf [2]. Also, the dry weight of a chloroplast is 50% to 60% protein [3]. It is well known that the concentrations of apparently all proteins in plant cells result from both synthesis and degradation of the individual proteins [4]. Thus, plastid proteins, like those in other compartments of eukaryotic cells, are continually synthesized and subsequently degraded by a variety of proteases. Also, many plastid proteins are first synthesized as precursor molecules that are then processed to mature forms by proteases in the organelle (e.g., Ref. [5]).