Monocot regulatory protein Opaque-2 is localized in the nucleus of maize endosperm and transformed tobacco plants.

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Abstract
Protein targeting to the nucleus has been studied extensively in animal and yeast systems; however, nothing is known about nuclear targeting in plants. The Opaque-2 (O2) gene produces a regulatory protein that is responsible for inducing transcription of the alpha-zein class of storage proteins in maize kernels. The cloned O2 gene encodes a protein that contains a leucine zipper DNA binding domain that can interact with zein gene promoters. We have used immunolocalization to show that the O2 protein is present in nuclei in the maize endosperm tissues known to produce alpha-zeins. In addition, neither embryo tissue from wild-type kernels nor endosperm from kernels harboring a null o2 allele contain the O2 protein. Analysis of a transposable, element-induced o2 allele, o2-m20, revealed that sectors of endosperm cells contained the nuclear-localized O2 protein, indicating excision of the transposable element. To study further the nuclear transport of the O2 protein, we have transformed this gene, under the control of a constitutive promoter, into tobacco. Plants were shown to have detectable levels of steady-state O2 mRNA and O2 protein. Immunolocalization of O2 protein in transformed tobacco plants indicated that the O2 protein was transported into tobacco nuclei. Therefore, we have developed a system to study nuclear targeting in plants and have established that the nuclear transport machinery is similar in monocots and dicots.