A Regulatory Role for 14-3-3 Proteins in the Starch Biosynthetic Pathway of Zea mays

Abstract

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.