PROTEINS ASSOCIATED WITH THE SURFACE OF WHEAT-STARCH GRANULES PURIFIED BY CENTRIFUGING THROUGH CESIUM-CHLORIDE

Purified starch was isolated by centrifuging crude starch washed from macerated whole wheat, or an aqueous flour-water suspension, through 80% (w/v) caesium chloride. Granule surface proteins were extracted with 2% sodium dodecylsulphate (SDS) at 20 °C and integral proteins were recovered by extracting with SDS at 50 °C. Integral proteins were qualitatively and quantitatively constant in the starches examined, and provided a convenient internal standard for electrophoresis when total (surface+integral) proteins were extracted together at 50 °C rather than consecutively. In typical starch samples total nitrogen content (35-40 mg N/100 g) was comprised of 3-10 mg N in surface proteins, 13-15 mg N in integral proteins and the 14-20 mg N in lysophospholipids. Electrophoresis on SDS-polyacrylamide gradient (10–20 %) gels showed that purification removed most surface proteins other than the 15 kDa protein friabilin, associated with soft endosperm texture, which was not readily desorbed by the caesium chloride. In starches from soft wheats the small B-granules had more friabilin than the large A-granules, roughly in proportion to their surface area. The method is suitable for screening small samples of wheat for friabilin on its starch and (by association) soft or hard endosperm texture. © 1990, Academic Press Limited. All rights reserved.