Synthesis of an unusual alpha-zein protein is correlated with the phenotypic effects of the floury2 mutation in maize.

The soft, starchy endosperm of the maize (Zea mays L.) floury 2 mutant is associated with a reduction in zein mRNA and protein synthesis, unique protein body morphology, and enhanced levels of a 70 kDa protein, that has been shown to be the maize homolog of a chaperonin found in the endoplasmic reticulum. We found an unusual alpha-zein protein of 24 kDa to be consistently associated with the zein fraction from floury 2 mutants. Three additional alpha-zein proteins with molecular weights ranging from ca. 25 to 27 kDa are detected in the storage protein fraction of a high percentage of floury 2 kernels and a low percentage of normal kernels in a genetically segregating population. The four proteins in a genetically segregating population. The four proteins can be distinguished from one another by immunostaining on Western blots. Synthesis of the 24 kDa protein is regulated by Opaque2, since the 24 kDa protein is lacking in the storage protein fraction of opaque2/floury2 double mutants. The synthesis of an abnormal alpha-zein protein in floury2 could explain many features of the mutant, such as the abnormal protein body morphology, induction of the 70 kDa chaperonin, and hypostasis to opaque2 (o2). Although we cannot prove that the accumulation of this protein is responsible for the floury2 phenotype, we were able to detect a restriction fragment length polymorphism (RFLP) linked to the floury2 locus with a 22 kDa alpha-zein probe. We hypothesize that the unique characteristics of the floury2 mutant could be a response to the accumulation of a defective alpha-zein protein which impairs secretory protein synthesis.

The opaque-2 mutation of maize differentially reduces zein gene transcription.

Zeins, the storage proteins of maize seed, are encoded by a large multigene family that is regulated developmentally and expressed in a tissue-specific manner during endosperm development. The synthesis of these proteins is affected by mutations, such as opaque-2, that cause a reduction in the accumulation of zein proteins and mRNAs. We used nuclear run-on transcription assays to analyze the expression of zein genes in developing normal and opaque-2 endosperms and to map the origin of these transcripts with respect to the coding and noncoding regions of the genes. These analyses demonstrate that zein gene expression is regulated transcriptionally and posttranscriptionally in developing endosperm. Transcription of genes encoding alpha-zeins is inhibited significantly in opaque-2 mutants, with expression of those encoding the M(r) 22,000 proteins being almost totally blocked. Other gene subfamilies were affected but to a lesser extent.