Glycinin A5A4B3 mRNA: cDNA cloning and nucleotide sequencing of a splitting storage protein subunit of soybean

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Abstract
The complete nucleotide sequence of a cloned cDNA, designated pGA5A4B3822, corresponding to glycinin A5A4B3 mRNA was determined. Analysis of the cDNA insert revealed that it contained 1899 nucleotides of mRNA sequence with a 5′‐terminal non‐translated region of 31 nucleotides, a signal peptide region corresponding to 23 amino acids, an acidic subunit region (A5) corresponding to 97 amino acids, an acidic subunit region (A4) corresponding to 257 amino acids followed by a basic subunit region (B3) corresponding to 185 amino acids, and a 3′‐terminal non‐translated region of 182 nucleotides. These results show that the glycinin A4 subunit, which is not found to be linked to a basic subunit via a disulfide bond, is synthesized as a full‐sized precursor, i.e. the A5A4B3 subunit complex, from a single mRNA, followed by post‐translational processing to generate an intermediary subunit complex (A5‐B3), covalently linked by a disulfide bond, and the mature A4 subunit, which may associate with the above subunit complex by non‐covalent interactions. From the results obtained by the Chou‐Fasman rules we speculated that the two post‐translational cleavage sites of this subunit precursor might be processed by the same proteolytic enzyme.