A Regulatory Role for 14-3-3 Proteins in the Starch Biosynthetic Pathway of Zea mays
Starch derived from cereal endosperm is responsible for supporting humanity’s global caloric requirements and is applied in various industrial applications. Semi-crystalline starch granules are comprised of glucose polymers, amylose and amylopectin, which are synthesized by four classes of enzymes including ADP-glucose pyrophosphorylase (AGPase), starch synthases (SS), starch branching enzymes (SBE), and starch debranching enzymes (DBE). Several SS and SBE isoforms form large phosphorylation-dependent heteromeric protein complexes. A class of eukaryotic proteins called “14-3-3” dimerize and interact with phosphorylated clients to regulate enzymatic activity, localization and protein interactions. A series of bioinformatic and biochemical techniques identified GF14-6 as the only detectable 14-3-3 isoform in maize amyloplasts. Recombinant GF14-6 was shown to interact with SSI, SBEIIa and SBEIIb in affinity bait assays. Reciprocal interactions using recombinant SBEIIb demonstrated phosphorylation-dependent interactions with endogenous 14-3-3s. These results point towards a regulatory role for 14-3-3 proteins regulating starch biosynthesis in maize endosperm.