Heterogeneity of zein mRNA and protein in maize

Zein, the prolamine fraction of maize, is localized in the endosperm in membrane-bound structures called protein bodies, which have polyribosomes on their surfaces. These polysomes or the mRNA fraction isolated from them will direct the synthesis of zein-like proteins in vitro. The in vitro products consist primarily of two molecular weight classes but show considerable charge heterogeneity when analyzed by isoelectric focusing. Although the molecular weight classes are very similar for different inbred lines, the isoelectric focusing patterns differ.Results given here suggest that the extensive charge heterogeneity of zein proteins does not result from the presence of a large number of totally distinct mRNAs. Zein proteins synthesized in vitro fall into several families related by sequence homologies in their mRNAs. In Illinois High Protein (IHP) the major zein mRNAs can be classified into three families based on their binding to cloned complimentary DNA copies of IHP zein mRNA. Each of three other lines we have studied (W22, Oh43, and W64A) has zein mRNAs that are related to those of IHP. Among these four lines the molecular weights of the members of a given family are generally similar, but the number of members in a family and their isoelectric points differ.