Heterogeneity and expression of the zein storage protein gene family of maize : (zein, maize, multiple gene families, in vitro processing, protein synthesis, cloning, low molecular weight PAGE)

Abstract

The storage protein zein is synthesized by polysomes on the endoplasmic reticulum in certain cells of the maize seed. As zein accumulates within the membrane lumen the cisternae are transformed into vesiculate protein bodies. Zein is comprised of a large family of structurally related polypeptides that fall into two classes of 19,000 and 25,000 daltons. On two-dimensional IEF:SDS polyacrylamide gels, the number and positions of the electromorphs within each class differ greatly from one strain to another. Typically, however, there may be 8-12 spots for each class. Within any strain that has been examined the polypeptides constricting each molecular weight class have a similar amino-terminal sequence, but there are a number of position differences when the sequences are compared between strains. Zeins are made as preproteins that are 1,000 and 2,000 daltons larger than the respective mature light and heavy zein polypeptide class. As the growing preprotein chain is moved through the ER membrane, the leader sequence is removed by an exopeptidase. One glucose residue is added to the mature portion of the molecule at this time. An in vitro system using maize endosperm membranes has been developed to study the synthesis and processing of maize preproteins. Assessment of the products made indicates that the method faithfully reproduces the in vitro processes. Cloned cDNA zein sequences were employed to examine the extent of genetic heterogeneity. Based on restriction analyses and dot hybridizations, eighteen zein clones could be divided into three classes for the light zein class and two for the heavy zein class. The Southern transfer hybridizations showed a very complex picture that suggests that zein is encoded by many genomic sequences which share partial polypeptides but until more is known about the structures of the different zein genes, an estimate of the number of copies in the maize genome by hybridization with cloned sequences cannot be made.FRANCES BURR and BENJAMIN BURR, Department of Biology, Brookhaven National Laboratory, Upton, N.Y