Efficient but aberrant cleavage of mitochondrial precursor proteins by the chloroplast stromal processing peptidase

Essentials of Proteolytic Machineries in Chloroplasts

Plastids are unique organelles that can alter their structure and function in response to environmental and developmental stimuli. Chloroplasts are one type of plastid and are the sites for various metabolic processes, including photosynthesis. For optimal photosynthetic activity, the chloroplast proteome must be properly shaped and maintained through regulated proteolysis and protein quality control mechanisms. Enzymatic functions and activities are conferred by protein maturation processes involving consecutive proteolytic reactions. Protein abundances are optimized by the balanced protein synthesis and degradation, which is depending on the metabolic status. Malfunctioning proteins are promptly degraded. Twenty chloroplast proteolytic machineries have been characterized to date. Specifically, processing peptidases and energy-driven processive proteases are the major players in chloroplast proteome biogenesis, remodeling, and maintenance. Recently identified putative proteases are potential regulators of photosynthetic functions. Here we provide an updated, comprehensive overview of chloroplast protein degradation machineries and discuss their importance for photosynthesis. Wherever possible, we also provide structural insights into chloroplast proteases that implement regulated proteolysis of substrate proteins/peptides.

Dual targeting of a mitochondrial protein: the case study of cytochrome c1

As a result of the endosymbiotic gene transfer, the majority of proteins of mitochondria and chloroplasts is encoded in the nucleus and synthesized in the cytosol as precursor molecules carrying N-terminal transit peptides for the transport into the respective target organelle. In most instances, transport takes place into either mitochondria or chloroplasts, although a few examples of dual targeting into both organelles have been described. Here, we show by a combination of three different experimental strategies that also cytochrome c(1) of potato, a component of the respiratory electron transport chain, is imported not only into mitochondria, but also into plastids. In organello import experiments with isolated mitochondria and chloroplasts, which were analyzed in both single and mixed organelle assays, demonstrate that the processing products accumulating after import within the two endosymbiotic organelles are different in size. Dual targeting of cytochrome c(1) is observed also in vivo, after biolistic transformation of leaf epidermal cells with suitable reporter constructions. Finally, Western analyses employing cytochrome c(1)-specific antiserum provide evidence that the protein accumulates in significant amounts in mitochondria and chloroplasts of both pea and spinach. The possible consequences of our findings on the relevance of the dual targeting phenomenon are discussed.

Purification and properties of a novel chloroplast stromal peptidase. Processing of polyphenol oxidase and other imported precursors

Polyphenol oxidases (PPOs) are nuclear-encoded chloroplast proteins that are targeted to the thylakoid lumen by a bipartite presequence. The N-terminal part of this sequence is removed by a stromal processing peptidase (SPP), and the resulting intermediate is translocated across the thylakoid and processed to the mature protein. A 4800-fold-purified SPP processed a PPO precursor (pPPO) at a site identical to that occurring in organelle. The in vitro product of SPP action on pPPO was further processed and translocated by thylakoids. This SPP processed other precursors but was inactive toward those of light-harvesting chlorophyll binding proteins. The enzyme appeared to be a metalloendopeptidase, like previously reported SPPs. However, it differed in substrate specificity, apparent size, and, most significantly, cleavage site of pPPO. Whereas the processing sites of lumen proteins determined so far were relatively distant from the hydrophobic core of the thylakoid targeting domain, pPPO was cleaved immediately before this domain. Cleavage removed the twin arginine motif characteristic of thylakoid targeting domains of lumen proteins, which are translocated by the DeltapH-dependent pathway. The possible significance of these observations to PPO translocation mechanism is discussed. It is suggested that several SPPs may exist in chloroplasts with preferences for different subsets of precursors.

Protein transport via amino-terminal targeting sequences: common themes in diverse systems

Many proteins that are synthesized in the cytoplasm of cells are ultimately found in non-cytoplasmic locations. The correct targeting and transport of proteins must occur across bacterial cell membranes, the endoplasmic reticulum membrane, and those of mitochondria and chloroplasts. One unifying feature among transported proteins in these systems is the requirement for an amino-terminal targeting signal. Although the primary sequence of targeting signals varies substantially, many patterns involving overall properties are shared. A recent surge in the identification of components of the transport apparatus from many different systems has revealed that these are also closely related. In this review we describe some of the key components of different transport systems and highlight these common features.

Peculiar properties of the PsaF photosystem I protein from the green alga Chlamydomonas reinhardtii: presequence independent import of the PsaF protein into both chloroplasts and mitochondria

It has previously been shown that presequences of nuclear-encoded chloroplast proteins from the green alga Chlamydomonas reinhardtii contain a region that may form an amphiphilic alpha-helix, a structure characteristic of mitochondrial presequences. We have tested two precursors of chloroplast proteins (the PsaF and PsaK photosystem I subunits) from C. reinhardtii for the ability to be imported into spinach leaf mitochondria in vitro. Both precursors bound to spinach mitochondria. The PsaF protein was converted into a protease-protected form with high efficiency in a membrane potential-dependent manner, indicating that the protein had been imported, whereas the PsaK protein was not protease protected. The protease protection of PsaF was not inhibited by a synthetic peptide derived from the presequence of the N. plumbaginifolia mitochondrial F1 beta subunit. Furthermore, if the presequence of PsaF was truncated or deleted by in vitro mutagenesis, the protein was still protease-protected with approximately the same efficiency as the full-length precursor. These results indicate that PsaF can be imported by spinach mitochondria in a presequence-independent manner. However, even in the absence of the presequence, this process was membrane potential-dependent. Interestingly, the presequence-truncated PsaF proteins were also protease-protected upon incubation with C. reinhardtii chloroplasts. Our results indicate that the C. reinhardtii chloroplast PsaF protein has peculiar properties and may be imported not only into chloroplasts but also into higher-plant mitochondria. This finding indicates that additional control mechanisms in the cytosol that are independent of the presequence are required to achieve sorting between chloroplasts and mitochondria in vivo.