β-Zein protein bodies sequester and protect the 18-kDa δ-zein protein from degradation

Abstract

The 18-kDa δ-class zein protein is characterized by having the highest mol% methionine content of any of the known zeins. Sequencing of the 18-kDa zein cDNA revealed an error in the published nucleotide sequence, which changes the 18-kDa amino acid sequence from residues 25 to 38. These changes result in a higher degree of homology between the 10- and 18-kDa zeins than previously reported. Expression of the 18-kDa zein gene under the control of the CaMV 35S promoter results in stable transcript and protein accumulation in leaf tissues of transgenic tobacco. Co-expression of the 15- and 18-kDa zein genes followed by immunolocalization revealed that both proteins accumulate in the same protein bodies. Furthermore, there is a 16-fold increase in accumulation of the 18-kDa zein protein in co-expressing tobacco plants. Transgenic tobacco leaves co-expressing the 15- and 18-kDa zein genes, or expressing only the 18-kDa zein gene, accumulate the 18-kDa zein transcript to equal levels. This suggests that the 15-kDa zein protein stabilizes the 18-kDa zein protein when sequestered in protein bodies. The possibility of 18-kDa zein protein stabilization by the 15-kDa zein protein is further evidenced by L-[ 35S] methionine pulse-chase experiments. A dramatic decrease in the rate of 18-kDa protein degradation in transgenic tobacco leaves co-expressing the 15- and 18-kDa zein genes was observed when compared with 18-kDa protein degradation in transgenic tobacco leaves expressing only the 18-kDa zein gene.