Studies on the Structure and Function of Ubiquitin
Ubiquitin is a small (76 amino acid) protein found in all eukaryotes either free or covalently attached to proteins in the nucleus, cytosol, or plasma membrane (Wilkinson, 1986; Busch, 1984; Hershko, 1988). It is the most highly conserved protein yet discovered in eukaryotes, with only three conservative amino acid substitutions separating yeast ubiquitin from the human (Fig. 1). The protein is water soluble, extremely stable, and may be boiled without loss of activity. Ubiquitin is stable to proteases and has not been shown to be glycosylated (Ozkaynak et al., 1984; Vierstra et al., 1986; Schlesinger and Goldstein, 1975). It contains no cysteine and only one histidine at position 68. The crystal structure of human ubiquitin has been solved at 1.8 Å resolution and demonstrates that ubiquitin is a compact molecule with a hydrophobic core containing three and one-half turns of α helix and five strands of β sheet. At the carboxy terminus, four amino acids protrude from the core of the molecule and have considerable freedom of motion (Vijay-Kumar et al., 1987).
KeywordsCarboxy Terminus Paired Helical Filament Proteolytic Pathway Ubiquitin Gene Polyubiquitin Gene
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