Abstract
Arabidopsis thaliana purple acid phosphatase 2 (AtPAP2), which is anchored to the outer membranes of chloroplasts and mitochondria, affects carbon metabolism by modulating the import of some preproteins into chloroplasts and mitochondria. AtPAP9 bears a 72% amino acid sequence identity with AtPAP2, and both proteins carry a hydrophobic motif at their C-termini. Here, we show that AtPAP9 is a tail-anchored protein targeted to the outer membrane of chloroplasts. Yeast two-hybrid and bimolecular fluorescence complementation experiments demonstrated that both AtPAP9 and AtPAP2 bind to a small subunit of rubisco 1B (AtSSU1B) and a number of chloroplast proteins. Chloroplast import assays using [35S]-labeled AtSSU1B showed that like AtPAP2, AtPAP9 also plays a role in AtSSU1B import into chloroplasts. Based on these data, we propose that AtPAP9 and AtPAP2 perform overlapping roles in modulating the import of specific proteins into chloroplasts. Most plant genomes contain only one PAP-like sequence encoding a protein with a hydrophobic motif at the C-terminus. The presence of both AtPAP2 and AtPAP9 in the Arabidopsis genome may have arisen from genome duplication in Brassicaceae. Unlike AtPAP2 overexpression lines, the AtPAP9 overexpression lines did not exhibit early-bolting or high-seed-yield phenotypes. Their differential growth phenotypes could be due to the inability of AtPAP9 to be targeted to mitochondria, as the overexpression of AtPAP2 on mitochondria enhances the capacity of mitochondria to consume reducing equivalents.
Highlights
Purple acid phosphatases (PAPs) are present in yeasts, animals, and plants
Many nucleus-encoded proteins imported into chloroplasts and mitochondria are targeted by their N-terminal transit peptides/presequences, which are recognized by receptor proteins in the translocon of the outer membranes of chloroplasts (TOC) and translocon of the outer membranes of mitochondria (TOM), respectively [12,13]
Previous studies have shown that Arabidopsis thaliana purple acid phosphatase 2 (AtPAP2) is dually targeted to plastids and mitochondria via its C-terminal hydrophobic motif [8,22]
Summary
Purple acid phosphatases (PAPs) are present in yeasts, animals, and plants. These enzymes contain seven conserved metal-coordinating amino acid residues in five invariant blocks—DXG, GDXXY, GNH(D/E), VXXH, and GHXH [1,2]. AtPAP2 is targeted to the outer membranes of both plastids and mitochondria by this unique C-terminal hydrophobic motif [8,9] and plays a role in the import of some nucleus-encoded proteins into chloroplasts and mitochondria [10,11]. Many nucleus-encoded proteins imported into chloroplasts and mitochondria are targeted by their N-terminal transit peptides/presequences, which are recognized by receptor proteins in the translocon of the outer membranes of chloroplasts (TOC) and translocon of the outer membranes of mitochondria (TOM), respectively [12,13].
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