An alanine to threonine change in the Wx-D1 protein reduces GBSS I activity in waxy mutant wheat

The Wx-D1 protein (granule-bound starch synthase) of Kanto 107, Tanikei A6099 (low amylose line), and Tanikei A6599-4 (waxy line) has been analyzed by SDS-PAGE, peptide mapping and DNA sequencing. Kanto 107 and Tanikei A6099 have the same amino acid sequences in the mature protein, but amino acid substitution (alanine to threonine) occurs at position 258 in the mature protein in Tanikei A6599-4. A comparison of deduced amino acid sequences of the mature Wx-D1 protein in these lines indicates that point mutation in the Wx-D1 gene of Tanikei A6599-4 is responsible for its waxy character. This mutant waxy wheat does not show a reduction in amylose content identical to other waxy wheats, which probably reduces activity of the GBSS I enzyme but does not to a complete loss of activity. We discuss the function of the mutant Wx-D1 protein in starch synthesis.