Biosynthesis and its regulation of storage proteins of higher plant tuber crops.

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Cell fractionation studies indicated that the storage proteins of the sweet potato tuberous root and the potato tuber are deposited in vacuoles. Precursors for these proteins were synthesized by membrane-bound polysomes, and the precursor for the sweet potato storage protein sporamin was 4 kd larger than the mature form. By immunological screening of cDNA expression libraries constructed by a novel method employing an E. coli expression vector-primed cDNA synthesis, full-length cDNA clones for sporamin and three different storage proteins of the potato tuber were identified. An improved E. coli expression vector for the construction and identification of full-length cDNAs has also been developed. Nucleotide sequencing of these cDNA clones indicated that precursors for potato storage proteins contain typical signal peptide sequences at their N-termini. The N-terminal extra-sequence of sporamin precursor, however, contained 16 amino acid residues enriched with basic amino acids following the signal peptide segment. In vitro processing studies suggested that the sporamin precursor undergoes two-step processings where the signal peptide is removed co-translationally followed by the post-translational removal of the segment enriched with basic amino acids. The structure and the processing mechanisms of the sporamin precursor resembles to those of the precursors for yeast vacuolar proteins and animal cell lysosomal proteins. When sporamin precursor was expressed in yeast cells using an yeast expression vector, at least part of them were transported into vacuole. We have also succeeded introducing and expressing a chimeric gene in which sporamin cDNA is joined to a strong plant virus promoter in tobacco. The sporamin precursor expressed in tobacco callus cells seems to be processed into mature size.