Physicochemical and chemical studies on wheat embryo ribosomal proteins
The physical heterogeneity of unfractionated wheat embryo ribsomal proteins, prepared by the glacial acetic acid method of Waller and Harris, has been investigated in 8 M urea −10−3 M dithio-threitol solutions of low pH (4.5). Sedimentation–diffusion measurements resulted in a weight average molecular weight of 29 000 ± 2 500, with no obvious evidence of heterogeneity. High-speed membrane osmometry was employed to establish the number average molecular weight of this system as 24 500 ± 1 000. The disparity in molecular weight averages suggests some size heterogeneity, and statistical analysis based on the two average molecular weights resulted in a calculated range of molecular weights for wheat embryo ribosomal proteins from 15 000 to 34 000 a.m.u. Charge differences, reflecting presumably primary structure differences, also exist among the members of this class, since about 26 different bands were resolved on polyacrylamide gel electrophoresis. The weight intrinsic viscosity of the ribosomal proteins in 8 M urea solutions was established as 0.273 dl/g, a value considerably larger than most globular proteins, suggesting that a major portion of their polypeptide chains are unfolded in this solvent. This conclusion was substantiated by optical rotatory dispersion measurements on this system, which resulted in a dispersion constant, λc, of 213 m μ, a value typical of that of the random coil. Amino acid and N-terminal analyses are also reported for this system, and comparisons of both chemical and physicochemical parameters are made with ribosomal proteins of other sources.