Abstract
The cDNA clones coding for a form of soluble starch synthase from developing rice seeds have been isolated using a synthetic oligonucleotide as a probe. The deduced amino acid sequence of this enzyme includes a Lys-Xaa-Gly-Gly consensus sequence for the substrate-binding site of glycogen synthase. The precursor of the enzyme contains 626 amino acids, including a 113-residue transit peptide at the amino-terminus. The mature form of soluble starch synthase shares a significant, but low sequence identity with rice granule-bound starch synthase and Escherichia coli glycogen synthase. However, several regions, including the substrate-binding site, are highly conserved among these three enzymes. Northern blot analysis demonstrates that the gene encoding soluble starch synthase is expressed in both leaves and developing seeds. The cDNA cloning of two major forms of rice starch branching enzyme, RBE1 and RBE3, has been also carried out. The nucleotide sequences indicate that RBE1 and RBE3 are initially synthesized as 820- and 825-residue precursor proteins, including putative 64- and 65-residue transit peptides at the amino-termini, respectively. The consensus sequences of the four regions that form the catalytic sites of amylolytic enzymes are conserved in the central region of the RBE1 and RBE3 sequences. Thus, branching enzyme belongs to a family of amylolytic enzymes. RBE1 shares a noticeable degree of sequence identity with RBE3, especially at the central portion of the protein molecule. However, as compared with RBE1, RBE3 possesses an approximately 70-residue extra sequence at the amino-terminus, and lacks a carboxyl-terminal sequence of almost 50 residues. On the basis of the primary structures of the enzymes participating in starch biosynthesis, the possible functions of these enzymes are discussed.
