ISOLATION AND CHARACTERIZATION OF MULTIPLE FORMS OF FRIABILIN

The control of endosperm texture in wheat grain is poorly understood a t the molecular level. This study reports the isolation and partial ch aracterization of multiple forms of friabilin, a starch granule protei n associated with endosperm softness. Starch granule proteins were iso lated from water-washed wheat starch using sodium dodecyl sulfate (SDS ), NaCl, propan-2-ol or NaCl plus propan-2-ol. SDS polyacrylamide gel electrophoresis under unreduced conditions revealed the presence of at least three components of what we call the M(r) 15k complex, i.e. sta rch granule proteins of ca. M(r) 15k, which have been considered previ ously to be the single protein, friabilin. Based on selective extracti on from starch, a fourth component may also be present. One of these f our components may be an alpha-amylase inhibitor reported previously. N-terminal amino acid sequence data suggest that major components of t he M(r) 15k complex may be similar to Triton X-114-extractable protein s, termed puroindolines, isolated from wheat flour. Members of the M(r ) 15k complex shared similar dehydration-denaturation characteristics based on selective acetone precipitation. The results indicate that th e operational use of friabilin as a discrete, single protein associate d with endosperm texture, and possibly involved in texture control, mu st be re-evaluated. Nevertheless, friabilin remains a useful, though e nigmatic, biochemical marker for grain softness.