DEFINING THE GAMMA-ZEIN DOMAIN NECESSARY FOR PROTEIN BODY FORMATION

The seed storage proteins of maize, zeins, belong to four classes: alpha, beta, gamma and delta, and assemble into insoluble protein bodies in the lumen of the endoplasmic reticulum (ER). Gamma-zeins (gz) are fundamental for protein quality of maize seeds. They are the products of three genes that encode polypeptides of 16, 27 and 50kD. 27kDgz is the most abundant gz, is probably the most ancient zein and forms insoluble protein bodies (PB) also when expressed in transgenic plants in the absence of other zeins. In natural maize PB, 27kDgz is located at the PB periphery, in contact with the surrounding ER membrane, whereas 16kDgz is at the border between 27kDgz and the alpha zein inner core. Most probably, 16kDgz originates from gene duplication followed by deletion after maize allotetraplodization, and is mainly characterized by the loss of part of the N-terminal (Pro-rich) domain. We show that 16kDgz also becomes insoluble and accumulates in the endoplasmic reticulum when expressed alone, but is unable to form regularly assembled PB. In pulse-chase experiments, newly synthesized 27kDgz becomes insoluble more rapidly than 16kDgz.When the N-terminal domains of the two gz are swapped, the 27kDgz Pro-rich domain has a dominant effect on the rate of insolubilization. On the whole, these results indicate that the inability of 16kDgz to form PB is due to the mutations in the Pro-rich domain and that this polypeptide has possibly a role in establishing a stable contact between 27kDgz and alpha zeins.