Starch Granule Associated Proteins in Barley and Wheat

Abstract

Grain hardness is an important quality characteristic in wheat and barley. In wheat, it has been shown that softness and hardness are associated with the presence or absence, respectively, of an M r 15 000 protein (called friabilin) on the surface of water-washed starch granules. In order to avoid the redistribution during water washing we have isolated starch granules from hard and soft varieties of barley and wheat using a dry sieving procedure. This showed higher levels of friabilin associated with the starch granules of soft than hard wheats, but similar levels in the two types of barley. The friabilin also differed in distribution and extractability between the hard and soft varieties of the two cereals. In soft varieties the protein was associated with the granule surface and was substantially removed by repeated washing with water. In contrast, friabilin was mainly associated with matrix protein in hard varieties and was readily removed by water washing in wheat but not barley. This difference may relate to the binding properties of the matrix proteins in the two species. Even after repeated washing friabilin was still detected in all the granule preparations using immunofluoresence and western blotting, indicating that a small proportion of the total is tightly bound to the granule.