Abstract
TAP46 is a protein phosphatase2A (PP2A)-associated protein that regulates PP2A activity in Arabidopsis (Arabidopsis thaliana). To study how PP2A is involved in abscisic acid (ABA) signaling in plants, we studied the function of TAP46 in ABA-regulated seed maturation and seedling development. Expression of TAP46 coincides with the action of ABA in developing seeds and during seed germination, and the TAP46 transcript reaches to the highest level in mature seeds. Real-time polymerase chain reaction analysis indicates that external ABA can increase TAP46 transcript level transiently during seed germination. Overexpression of TAP46 increases plant sensitivity to ABA, while tap46 knockdown mutants are less sensitive to ABA during seed germination, suggesting that TAP46 functions positively in ABA signaling. Overexpression of TAP46 also leads to lower PP2A activity, while tap46-1 knockdown mutant displays higher PP2A activity, suggesting that TAP46 negatively regulates PP2A activity in Arabidopsis. Both TAP46 and PP2A interact with the ABA-regulated transcription factor ABA INSENSITIVE5 (ABI5) in vivo, and TAP46's binding to ABI5 can stabilize ABI5. Furthermore, TAP46's binding to the phosphorylated ABI5 may prevent PP2A or PP2A-like protein phosphatases from removing the phosphate from ABI5, thereby maintaining ABI5 in its active form. Overexpression of TAP46 and inhibition of activities of PP2A or PP2A-like protein phosphatases can increase transcript levels of several ABI5-regulated genes, suggesting that TAP46 is a positive factor in the ABA-regulated gene expression in Arabidopsis.
Highlights
TAP46 is a protein phosphatase2A (PP2A)-associated protein that regulates PP2A activity in Arabidopsis (Arabidopsis thaliana)
The TAP46 transcript was up-regulated by abscisic acid (ABA) treatment during germination (Fig. 4), the expression of TAP46 was found mainly during seed maturation (Fig. 5; Supplemental Fig. 3), and the highest transcript level was found in dry seeds (Supplemental Fig. 3), all suggesting that TAP46 plays positive roles in ABAregulated seed maturation and inhibition of germination
Our data here show that TAP46 interacts with ABA INSENSITIVE5 (ABI5) in vivo (Fig. 8C), and in particular, overexpression of TAP46 increases the steady-state levels of phosphorylated ABI5 and free ABI5 (Fig. 8A), suggesting that TAP46-ABI5 interaction stabilizes both forms of ABI5 and likely prevents the action of phosphatases that remove the phosphate group from ABI5
Summary
TAP46 is a protein phosphatase2A (PP2A)-associated protein that regulates PP2A activity in Arabidopsis (Arabidopsis thaliana). Overexpression of TAP46 leads to lower PP2A activity, while tap knockdown mutant displays higher PP2A activity, suggesting that TAP46 negatively regulates PP2A activity in Arabidopsis Both TAP46 and PP2A interact with the ABA-regulated transcription factor ABA INSENSITIVE5 (ABI5) in vivo, and TAP46’s binding to ABI5 can stabilize ABI5. Overexpression of TAP46 and inhibition of activities of PP2A or PP2A-like protein phosphatases can increase transcript levels of several ABI5-regulated genes, suggesting that TAP46 is a positive factor in the ABA-regulated gene expression in Arabidopsis. The other three genes, ABI3, ABI4, and ABI5 encode transcription factors that function in the last step in this ABA signaling pathway, and all three genes play important roles in seed maturation and seedling development (Finkelstein et al, 2002, 2008). Loss-of-function mutants of ABI4 and ABI5 display insensitivity to ABA in seed germination and seedling development (Finkelstein et al, 1998; Finkelstein and Lynch, 2000; Lopez-Molina and Chua, 2000)
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