Progressive aggregation of 16 kDa gamma-zein during seed maturation in transgenic arabidopsis thaliana

Prolamins constitute a unique class of seed storage proteins, present only in grasses. In the lumen of the endoplasmic reticulum (ER), prolamins form large, insoluble heteropolymers termed protein bodies (PB). In transgenic Arabidopsis (Arabidopsis thaliana) leaves, the major maize (Zea mays) prolamin, 27 kDa gamma-zein (27gz), assembles into insoluble disulfide-linked polymers, as in maize endosperm, forming homotypic PB. The 16 kDa gamma-zein (16gz), evolved from 27gz, instead forms disulfide-bonded dispersed electron-dense threads that enlarge the ER lumen without assembling into PB. We have investigated whether the peculiar features of 16gz are also maintained during transgenic seed development. We show that 16gz progressively changes its electron microscopy appearance during transgenic Arabidopsis embryo maturation, from dispersed threads to PB-like, compact structures. In mature seeds, 16gz and 27gz PBs appear very similar. However, when mature embryos are treated with a reducing agent, 27gz is fully solubilized, as expected, whereas 16gz remains largely insoluble also in reducing conditions and drives insolubilization of the ER chaperone BiP. These results indicate that 16gz expressed in the absence of the other zein partners forms aggregates in a storage tissue, strongly supporting the view that 16gz behaves as the unassembled subunit of a large heteropolymer, the PB, and could have evolved successfully only following the emergence of the much more structurally self-sufficient 27gz.