Abstract
Arabidopsis thaliana ANKYRIN REPEAT-CONTAINING PROTEIN 2A (AKR2A) interacts with peroxisomal membrane-bound ASCORBATE PEROXIDASE3 (APX3). This interaction involves the C-terminal sequence of APX3 (i.e., a transmembrane domain plus a few basic amino acid residues). The specificity of the AKR2A-APX3 interaction suggests that AKR2A may function as a molecular chaperone for APX3 because binding of AKR2A to the transmembrane domain can prevent APX3 from forming aggregates after translation. Analysis of three akr2a mutants indicates that these mutant plants have reduced steady state levels of APX3. Reduced expression of AKR2A using RNA interference also leads to reduced steady state levels of APX3 and reduced targeting of APX3 to peroxisomes in plant cells. Since AKR2A also binds specifically to the chloroplast OUTER ENVELOPE PROTEIN7 (OEP7) and is required for the biogenesis of OEP7, AKR2A may serve as a molecular chaperone for OEP7 as well. The pleiotropic phenotype of akr2a mutants indicates that AKR2A plays many important roles in plant cellular metabolism and is essential for plant growth and development.
Highlights
The successful targeting of membrane proteins to their destinations is pivotal to the functions of these proteins in eukaryotic cells (Blobel, 2000)
We previously found that purified AKR2 protein could protect ASCORBATE PEROXIDASE3 (APX3) activity in vitro (Yan et al, 2002) and that AKR2 could bind to the mPTS of APX3 but not to PMP22, suggesting a possible role for AKR2 as a molecular chaperone for group I peroxisomal membrane proteins (PMPs)
It is clear that the ANKYRIN REPEAT-CONTAINING PROTEIN 2A (AKR2A) binding site in APX3 is the transmembrane domain plus the last seven amino acid residues, whereas the region in AKR2A involved in binding to APX3 is located between residues 1 and 153 (Figure 1A), and ankyrin repeats are not involved in the AKR2A–APX3 interaction
Summary
The successful targeting of membrane proteins to their destinations is pivotal to the functions of these proteins in eukaryotic cells (Blobel, 2000). The mechanism by which proteins are cotranslationally inserted into the ER membrane is better understood (Blobel, 2000; Rapoport et al, 2004) than is the mechanism by which proteins are first synthesized on free ribosomes before being inserted into their specific membranes (Borgese et al, 2003). One of the GF14l-interacting proteins is AKR2, which contains four ankyrin repeats at the C-terminal side and a PEST domain at the N-terminal end (Yan et al, 2002). It was found that AKR2 interacts with another GF14l-interacting protein, the peroxisomal membrane-bound ASCORBATE PEROXIDASE3 (APX3), even in the absence of GF14l (Yan et al, 2002). APX3 is involved in H2O2 scavenging in plant antioxidant metabolism (Zhang et al, 1997), and the
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