Maize 16-kD gamma-zein forms very unusual disulfide-bonded polymers in the endoplasmic reticulum: implications for prolamin evolution

In the lumen of the endoplasmic reticulum (ER), prolamin storage proteins of cereal seeds form very large, ordered
heteropolymers termed protein bodies (PBs), which are insoluble unless treated with alcohol or reducing agents. In
maize PBs, 16-kD gamma-zein locates at the interface between a core of alcohol-soluble alpha-zeins and the outermost layer
mainly composed of the reduced-soluble 27-kD gamma-zein. 16-kD gamma-zein originates from 27-kD gamma-zein upon whole-genome
duplication and is mainly characterized by deletions in the N-terminal domain that eliminate most Pro-rich repeats
and part of the Cys residues involved in inter-chain bonds. 27-kD gamma-zein also forms insoluble PBs when expressed
in transgenic vegetative tissues. We show that in Arabidopsis leaves, 16-kD gamma-zein assembles into disulfide-linked
polymers that fail to efficiently become insoluble. Instead of forming PBs, these polymers accumulate as very unusual
threads that markedly enlarge the ER lumen, resembling amyloid-like fibers. Domain-swapping between the two
gamma-zeins indicates that the N-terminal region of 16-kD gamma-zein has a dominant effect in preventing full insolubilization.
Therefore, a newly evolved prolamin has lost the ability to form homotypic PBs, and has acquired a new function in
the assembly of natural, heteropolymeric PBs.