Differential activity of glucan phosphatase starch EXcess4 orthologs from agronomic crops

Abstract

The glucan phosphatase Starch EXcess4 (SEX4) plays an essential role in regulating starch degradation through reversible phosphorylation. However, starch granule properties and phosphorylation levels vary widely between different organisms. We biochemically characterized SEX4 from agronomically relevant plants and found that SEX4 orthologs display differential glucan phosphatase activity. SEX4 from cereal crops displayed higher dephosphorylation rates than SEX4 from storage tubers. Intriguingly, these rates were found to be inversely related to glucan substrate binding. To determine the effects of this difference, the ability of SEX4 orthologs to enhance in vitro starch degradation by the β-amylase BAM3 was measured. An inverse relationship was observed between SEX4 ortholog starch binding affinity and its ability to enhance BAM3-mediated glucan degradation. Collectively, our findings reveal a direct correlation between the dephosphorylation rates of SEX4 orthologs and their ability to enhance in vitro starch degradation. These data provide significant insights into the differential activity of SEX4 from different organisms, corresponding to their distinct biological roles and providing the basis for utilizing their specific properties for industrial and biotechnological applications.