A Defective Signal Peptide in a 19-kD α-Zein Protein Causes the Unfolded Protein Response and an Opaque Endosperm Phenotype in the Maize $De^{\ast}$-B30 Mutant

Defective endosperm ($De^{\ast}$)-B30 is a dominant maize (Zea mays) mutation that depresses zein synthesis in the developing endosperm. The mutant kernels have an opaque, starchy phenotype, malformed zein protein bodies, and highly increased levels of binding protein and other chaperone proteins in the endosperm. Immunoblotting revealed a novel α-zein protein in $De^{\ast}$-B30 that migrates between the 22- and 19-kD α-zein bands. Because the $De^{\ast}$-B30 mutation maps in a cluster of 19-kD α-zein genes, we characterized cDNA clones encoding these proteins from a developing endosperm library. This led to the identification of a 19-kD α-zein cDNA in which proline replaces serine at the 15th position of the signal peptide. Although the corresponding gene does not appear to be highly expressed in $De^{\ast}$-B30, it was found to be tightly linked with the mutant phenotype in a segregating F2 population. Furthermore, when the protein was synthesized in yeast cells, the signal peptide appeared to be less efficiently processed than when serine replaced proline. To test whether this gene is responsible for the $De^{\ast}$-B30 mutation, transgenic maize plants expressing this sequence were created. T1 seeds originating from the transformants manifested an opaque kernel phenotype with enhanced levels of binding protein in the endosperm, similar to $De^{\ast}$-B30. These results are consistent with the hypothesis that the $De^{\ast}$-B30 mutation causes a defective signal peptide in a 19-kD α-zein protein.