Partial amino acid sequence of the wheat germ Ec protein. Comparison with another protein very rich in half-cystine and glycine: wheat germ agglutinin

Abstract

A wheat germ protein (Ec), the dominant site of cysteine incorporation during early (E) germination of isolated wheat embryos, has been partially sequenced by automated Edman degradation. The sequence of residues 1–59 is not significantly similar to the amino acid sequence known for any other protein, including wheat germ agglutinin which, like Ec, is very rich in half-cystine and glycine. The partial sequence for Ec contains an almost identical pattern of half-cystine residues in segments 6–20 and 35–48, a duplication which includes 10 of the 12 half-cystine residues in the sequence. The partial sequence of Ec may include a large part of the complete sequence, but this remains uncertain because it has not been possible to arrive at a definitve estimate of molecular weight using different physical techniques. Protein Ec can be prepared from a reticulocyte lysate in which cell-free synthesis is programmed by bulk wheat germ mRNA. Determination of the distribution of half-cystine moieties between residues 1 and 20 by Edman degradation of the [35S]cysteine-labeled product of cell-free synthesis shows that it is devoid of an N-terminal extension. Unlike wheat germ agglutinin, Ec does not seem to arise by processing of a conspicuously larger precursor protein. Unlike Ec, another wheat germ protein, Em, the most conspicuous methionine-labeled protein when cell-free protein synthesis is directed by wheat germ mRNA, is refractory to direct sequence analysis by Edman degradation. However, again unlike Ec, uncertainty about the molecular weight of Em, based on its mobility in different sodium dodecyl sulphate – polyacrylamide gel systems, has been resolved by virtue of Em being ideally suited to study by the Yphantis high-speed sedimentation equilibrium method.