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

Davide Mainieri,Eva Stöger,Bhakti Prinsi,Luca Espen,Dario Maffi,Emanuela Pedrazzini,Franco Faoro,Marc Tschofen,Claudia A Marrano,Alessandro Vitale

doi:10.1093/jxb/ery287

Abstract

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 γ-zein locates at the interface between a core of alcohol-soluble α-zeins and the outermost layer mainly composed of the reduced-soluble 27-kD γ-zein. 16-kD γ-zein originates from 27-kD γ-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 γ-zein also forms insoluble PBs when expressed in transgenic vegetative tissues. We show that in Arabidopsis leaves, 16-kD γ-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 γ-zeins indicates that the N-terminal region of 16-kD γ-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.

Highlights

Prolamins are present only in the seeds of grasses, where they are usually the main proteins, constituting the major global source of food protein (Shewry and Halford, 2002)
Results ipt A proportion of 16 z present in maize protein bodies (PBs) is solubilized by alcohol scr 16 z can be efficiently solubilized from maize endosperm by 70% ethanol supplied with u reducing agent (Kim et al, 2006), but its solubility in each of these agents is less clear
Mutations and insertions in the much older seed storage proteins of the 2S albumin class have been the first events originating prolamins (Pedrazzini et al, 2016; Xu and Messing, 2009; Gu et al, 2010). This has led to the assembly in PBs and a change in the subcellular compartment of t permanent accumulation, from the vacuole to the endoplasmic reticulum (ER), in rice and panicoid cereals such as maize, sorghum and millet (Lending and Larkins, 1989; Shewry and Halford, 2002; ip Saito et al, 2012). cr 16 z originated upon maize WGD (Xu and Messing, 2008) and is mainly characterized by s deletions in the N-terminal region of 27 z, the most ancient -zein

Summary

Introduction

Prolamins are present only in the seeds of grasses, where they are usually the main proteins, constituting the major global source of food protein (Shewry and Halford, 2002). 27 z expressed in vegetative c tissues of transgenic plants forms homotypic PBs, indicating that no specific features of the A maize endosperm ER are necessary to form a PB (Geli et al, 1994).

Results

Conclusion