Mature cereal grain endosperm: rapid glass knife sectioning for examination of proteins

Maize opaque10 Encodes a Cereal-Specific Protein That Is Essential for the Proper Distribution of Zeins in Endosperm Protein Bodies

Cereal storage proteins are major nitrogen sources for humans and livestock. Prolamins are the most abundant storage protein in most cereals. They are deposited into protein bodies (PBs) in seed endosperm. The inner structure and the storage mechanism for prolamin PBs is poorly understood. Maize opaque10 (o10) is a classic opaque endosperm mutant with misshapen PBs. Through positional cloning, we found that O10 encodes a novel cereal-specific PB protein. Its middle domain contains a seven-repeat sequence that is responsible for its dimerization. Its C terminus contains a transmembrane motif that is required for its ER localization and PB deposition. A cellular fractionation assay indicated that O10 is initially synthesized in the cytoplasm and then anchored to the ER and eventually deposited in the PB. O10 can interact with 19-kD and 22-kD α-zeins and 16-kD and 50-kD γ-zeins through its N-terminal domain. An immunolocalization assay indicated that O10 co-localizes with 16-kD γ-zein and 22-kD α-zein in PBs, forming a ring-shaped structure at the interface between the α-zein-rich core and the γ-zein-rich peripheral region. The loss of O10 function disrupts this ring-shaped distribution of 22-kD and 16-kD zeins, resulting in misshapen PBs. These results showed that O10, as a newly evolved PB protein, is essential for the ring-shaped distribution of 22-kD and 16-kD zeins and controls PB morphology in maize endosperm.

Through the positional cloning of the maize classic endosperm mutant opaque10 (o10), we identified a novel protein critical for PB morphology. O10 is a fast-evolving cereal-specific gene with recent origin. A thorough characterization of its three functional domains revealed its important functions for storage protein deposition and organization in PBs. O10 determines a ring-shaped layer in PBs through direct interaction with two major storage proteins (22-kD and 16-kD zeins). This newly characterized PB layer maintains a stable spherical morphology for PB. This study advanced our understanding of PB structure and function. Furthermore, this study demonstrated the origin of a new functional gene and the functional evolution of a storage organelle that is highly valuable to humans.