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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

A Defective Signal Peptide in a 19-kD -Zein Protein Causes the Unfolded Protein Response and an Opaque Endosperm Phenotype in the Maize De^*-B30 Mutant¹

Department of Plant Sciences, University of Arizona, Tucson, Arizona 85721 (C.S.K., B.G.H., J.K., B.A.L.); Department of Botany, North Carolina State University, Raleigh, North Carolina 27695 (R.S.B.); and Pioneer Hi-Bred International, Johnston, Iowa 50131 (S.Y., R.J.)

Defective endosperm^* (De^*)-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 -zein protein in De^*-B30 that migrates between the 22- and 19-kD -zein bands. Because the De^*-B30 mutation maps in a cluster of 19-kD -zein genes, we characterized cDNA clones encoding these proteins from a developing endosperm library. This led to the identification of a 19-kD -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^*-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^*-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^*-B30. These results are consistent with the hypothesis that the De^*-B30 mutation causes a defective signal peptide in a 19-kD -zein protein.

² Present address: Agricultural Plant Stress Research Center, Chonnam National University, 300 Yongbong-dong, Puk-gu Kwangju 500-757, South Korea.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.103.031310.

^* Corresponding author; e-mail Larkins{at}Ag.Arizona.edu; fax 520-621-3692.

Received August 4, 2003; returned for revision October 3, 2003; accepted October 15, 2003.

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