Complete Amino Acid Sequence of Ubiquitin from the Higher Plant Avena sativa

Ubiquitin is a 76 amino acid, eukaryotic polypeptide with several important regulatory functions that arise from its ability to become covalently ligated to cytoplasmic and nuclear proteins. The amino acid sequence of ubiquitin is remarkably conserved, being identical for all animal forms analyzed to date. Here, we present the complete amino acid sequence of ubiquitin isolated from a higher plant, oats (Avena sativa L.). This sequence was determined by repetitive Edman degradation of the intact molecule and of peptides derived by proteolytic digestion with trypsin or Staphylococcus aureus V8 protease. Comparison of this sequence with that obtained for animal ubiquitins indicates that the two forms are homologous but not identical. Like the animal form, oat ubiquitin contains 76 amino acid residues, no tryptophan or cysteine, and a carboxyl terminus of Leu-Arg-Gly-Gly. However, oat ubiquitin contains three amino acid substitutions at positions 19, 24, and 57.