Identification and characterization of granule bound starch synthase (GBSS-I) from common buckwheat (Fagopyrum esculentum Moench)

Starch grains present in the endosperm of grains of common buckwheat (Fagopyrum esculentum Moench) show a monomodal distribution with size ranging from 4 to 10 mu m. SDS-PAGE analysis of starch granule bound proteins revealed the presence of a single band corresponding to molecular mass of 59.7 kDa. The protein is localized within the central core of the starch grains. Antisera raised against the 59.7 kDa protein cross reacted with the 61 kDa GBSS-I from endosperm starches of maize and the 60 kDa GBSS-I from endosperm starches of rice and wheat, thereby indicating serological homology between the 59.7 kDa buckwheat starch granule bound protein and GBSS-I of wheat, maize and rice. 2D-PAGE of starch granule bound proteins of common buckwheat resolved the fraction into 7 spots with pI ranging from 5.2 to 5.6. N-terminal amino acid sequence for 25 residues of two immunoreactive proteins separated by 2D PAGE showed 94 % homology with N-terminal amino acid sequence of GBSS-I from Hordeum vulgare, Triticum spp. and Phaseolus vulgaris. Even though analysis of the sequence alignment revealed a clear diversification into monocotyledonous and dicotyledonous groups, the protein from buckwheat showed similarities with GBSSI from both dicots as well as monocots. As is the case with dicots, the sequence of GBSS-I from buckwheat has valine as the 11th residue. GBSS-I from majority of monocots has methionine at this position. The sequence also showed similarities with monocots with valine at P'5 from the N-terminus. GBSS-I from majority of dicots has isoleucine at this position. The significance of these substitution remains to be ascertained.