InterPro domain: IPR030079

Dual specificity protein phosphatase 4 (DSP4), also known as STARCH-EXCESS 4 or SEX4 (AT3G52180), is a starch granule-associated phosphoglucan phosphatase involved in the control of starch accumulation [ 1 , 2 , 3 ]. All known plant genomes encode DSP4. Interestingly, DSP4 resembles laforin, the dual specificity phosphatase that regulates glycogen accumulation in animal cells. DSP4 acts as a bridge between light-induced redox changes and protein phosphorylation in the regulation of starch accumulation [ 4 ]. In chestnut stem tissues it has a potential role in starch degradation in response to low temperatures or during winter dormancy [ 5 ]. The protein consists of phosphatase, carbohydrate-binding, and C-terminal domains [ 6 ].

1. Similar protein phosphatases control starch metabolism in plants and glycogen metabolism in mammals. J. Biol. Chem. 281, 11815-8
2. STARCH-EXCESS4 is a laforin-like Phosphoglucan phosphatase required for starch degradation in Arabidopsis thaliana. Plant Cell 21, 334-46
3. The Laforin-like dual-specificity phosphatase SEX4 from Arabidopsis hydrolyzes both C6- and C3-phosphate esters introduced by starch-related dikinases and thereby affects phase transition of alpha-glucans. Plant Physiol. 152, 711-22
4. A redox-regulated chloroplast protein phosphatase binds to starch diurnally and functions in its accumulation. Proc. Natl. Acad. Sci. U.S.A. 103, 9732-7
5. Identification of a homolog of Arabidopsis DSP4 (SEX4) in chestnut: its induction and accumulation in stem amyloplasts during winter or in response to the cold. Plant Cell Environ. 34, 1693-704
6. Structural basis for the glucan phosphatase activity of Starch Excess4. Proc. Natl. Acad. Sci. U.S.A. 107, 15379-84