Abstract
Chloroplasts need to import preproteins and amino acids from the cytosol during their light-induced differentiation. Similarly, chloroplasts have to export organic matter including proteins and amino acids during leaf senescence. Members of the PRAT (preprotein and amino acid transporter) family are candidate transporters for both processes. Here, we defined the role of two small PRAT gene families, At4g26670 and At5g55510 (HP20 subfamily) versus At3g49560 and At5g24650 (HP30 subfamily) during greening of etiolated plants and during leaf senescence. Using a combination of reverse genetics, protein biochemistry and physiological tools, evidence was obtained for a role of chloroplast HP20, HP30 and HP30-2 in protein, but not amino acid, import into chloroplasts. HP20, HP30 and HP30-2 form larger complexes involved in the uptake of transit sequence-less cytosolic precursors. In addition, we identified a fraction of HP30-2 in mitochondria where it served a similar function as found for chloroplasts and operated in the uptake of transit sequence-less cytosolic precursor proteins. By contrast, HP22 was found to act in the export of proteins from chloroplasts during leaf senescence, and thus its role is entirely different from that of its orthologue, HP20. HP22 is part of a unique protein complex in the envelope of senescing chloroplasts that comprises at least 11 proteins and contains with HP65b (At5g55220) a protein that is related to the bacterial trigger factor chaperone. An ortholog of HP65b exists in the cyanobacterium Synechocystis and has previously been implicated in protein secretion. Whereas plants depleted of either HP22 or HP65b or even both were increasingly delayed in leaf senescence and retained much longer stromal chloroplast constituents than wild-type plants, HP22 overexpressors showed premature leaf senescence that was associated with accelerated losses of stromal chloroplast proteins. Together, our results identify the PRAT protein family as a unique system for importing and exporting proteins from chloroplasts.
Highlights
Plastids are hall-mark organelles of plants [1,2,3,4]
Using a reverse genetic approach, in combination with biochemical and physiological tests, we report on HP20, HP30 and HP30-2 as being involved in protein, but not amino acid, import into chloroplasts, and on HP22 as being involved in a, far unrecognized, protein export pathway from chloroplasts that operates during leaf senescence in A. thaliana
HP20, HP30 and HP30-2 were previously reported to function in the uptake of transit sequence-less cytosolic precursors into chloroplasts [37,38,39]
Summary
Plastids are hall-mark organelles of plants [1,2,3,4]. They comprise a family of partially inter-convertible forms, all originating from a simple progenitor called proplastid [1,2].When seedlings develop in the dark and undergo skotomorphogenesis, proplastids differentiate into etioplasts. Plastids are hall-mark organelles of plants [1,2,3,4]. They comprise a family of partially inter-convertible forms, all originating from a simple progenitor called proplastid [1,2]. This developmental step, called photomorphogenesis, is associated with the establishment of the photosynthetic apparatus and involves the synthesis of nucleus-encoded and plastidencoded photosynthetic proteins [2,5,6]. Photosynthetic proteins encoded in the nucleus are synthesized as precursors with NH2 -terminal transit sequences referred to as transit peptides and must be imported post-translationally from the cytosol. Amino acids for the biosynthesis of plastid-encoded proteins must be imported from the cytosol
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