Distribution, purification and N-terminal amino acid sequence of sorghum reduced-soluble protein

Abstract

Reduced-soluble protein (RSP), a protein soluble in water and aqueous alcohol plus reducing agent comprising a very high proportion of proline and high levels of cysteine and histidine, has been found in two widely differing types of sorghum in addition to the cultivar from which it was first isolated. There was no difference in the mobility of the RSPs from the three cultivars when subjected to either sodium dodecyl sulphate- or non-ionic detergent urea-polyacrylamide gel electrophoresis. However, sorghum RSP had slightly higher electrophoretic mobility than its maize homologue using the latter technique. Some purification of sorghum RSP was achieved by phosphocellulose chromatography, although a very low yield was obtained owing to interaction between the protein and the phosphocellulose resin. Isoelectric focusing of the chromatographic fractions resolved sorghum RSP into two main and several minor bands, most of which were more basic than the total kafirin bands. The N-terminal primary structure of the heterogeneous sorghum RSP showed no polymorphism, and ten of the first 11 residues were identical with the corresponding sequence of maize RSP. The sorghum homologue also appeared to contain the repeating hexapeptide Pro-Pro-Pro-Val-His-Leu found in maize RSP.