Hydrodynamic and optical properties of the wheat germ Em protein

Abstract

The size and shape of the Em protein from wheat embryos have been examined by gel filtration, densitometry, ultracentrifugation, and viscosity. Circular dichroism and fluorescence measurements have also been made. Em has an intrinsic sedimentation coefficient S020,w, of 1.11S and a Stokes radius, Rs, of 28.2 .ANG. (1 .ANG. = 0.1 nm) as determined by high performance liquid chromatography gel filtration on a TSK 2000SW column. The partial specific volume .hivin.v from density measurements is 0.683 mL/g, a much lower than typical value. The molecular weight from sedimentation equilibrium is 11,200, with no indication for protein aggregation. The intrinsic vicosity [n] of Em is 6.02 mL/g. Circulation dichroism shows the molecule to be about 70% random coil. The fluorescence emission spectrum is typical for a tyrosine-containing protein. The hydrodynamic data indicates a poor fit to either a prolate or oblate ellipsoid model; excess hydration or flexibility of the polypeptide chain caused by the rather unusual amino acid composition may be a possible cause. The implications that the low value of .hivin.v, the high value of RS, and the random-coil configuration of the Em protein may have on its ability to bind water and to contribute to the maintenance of a minimal level of hydration compatible with the sustained viability of the "dry" organism are subjects of an extended discussion. Briefly, it is suggested that Em may provide a matrix of bound water which opposes denaturation of proteins in the "desiccated" cytoplasm of dry plant embryos.