ISOLATION AND CHARACTERIZATION OF MULTIPLE FORMS OF FRIABILIN

The control of endosperm texture in wheat grain is poorly understood at the molecular level. This study reports the isolation and partial characterization of multiple forms of friabilin, a starch granule protein associated with endosperm softness. Starch granule proteins were isolated 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. starch granule proteins of ca. M(r) 15k, which have been considered previously to be the single protein, friabilin. Based on selective extraction from starch, a fourth component may also be present. One of these four components may be an alpha-amylase inhibitor reported previously. N-terminal amino acid sequence data suggest that major components of the M(r) 15k complex may be similar to Triton X-114-extractable proteins, 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 the operational use of friabilin as a discrete, single protein associated with endosperm texture, and possibly involved in texture control, must be re-evaluated. Nevertheless, friabilin remains a useful, though enigmatic, biochemical marker for grain softness.