Heterogeneity and Expression of the Zein Storage Protein Gene Family of Maize

摘要

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 22,500 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 to 12 spots for each class. Within any strain that has been examined the polypeptides constituting 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. 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 recapitulates the in vivo processes. Cloned cDNA zein sequences were employed to examine the extent of genetic heterogeneity. Based on restriction analyses and dot hydridizations, eighteen zein clones could be divided into three classes for the light zein class and two for the heavy zein class. The Southern transfer hydridizations showed a very complex picture that suggests that zein is encoded by many genomic sequences which share partial homologies. Many more sequences are recognized than there are polypeptides. Possibly some of these represent relic sequences and pseudogenes 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 hydridization with cloned sequences cannot be made. (ERA citation 06:035485)