(Model of action proposed by Hershkoet al, 1980) 2 Protein K (APF)n Protein n APF n ATP n APF.x Amino acids Courtesy of A Hershko. Used with permission of the author. Source: Figure 6 in Hershko et al. "Proposed role of ATP in protein breakdown conjugation of protein with multiple chains of the polypeptide of ATP-dependent proteolysis PNAS1980Apml:77(4):1783-1786 Several molecules of APF-1 linked to E-amino groups of the protein substrate by an APF-1-protein amide synthetase (step1) Proteins ligated to several APF-1 are broken down by a specific protease that recognizes such conjugates(step 3). The protein is broken down to free amino acids and to APF-1 still linked by isopeptide linkage to a lysine or a small peptide, APF-1-X. Finally, free APF-1 would be released for re-use by specific amidase-isopeptidase (step 4). a hypothetical correcting isopeptidase would release free APF-1 and substrate protein from erroneous ligations(editing activity: step 2) g Short after Hershko's model proposal, Keith Wilkinson and Arthur Haas(pot-doctoral fellows in Irwin Rose's lab)indicated that APF-1 was indeed UBIQUITIN(Model of action proposed by Hershkoet al., 1980) Courtesy of A. Hershko. Used with permission of the author. Source: Figure 6 in Hershko et al. "Proposed role of ATP in protein breakdown: conjugation of protein with multiple chains of the polypeptide of ATP-dependent proteolysis." PNAS 1980 April; 77(4): 1783–1786. Several molecules of APF-1 linked to ε-amino groups of the protein substrate by an APF-1-protein amide synthetase (step1). Proteins ligated to several APF-1 are broken down by a specific protease that recognizes such conjugates (step 3). The protein is broken down to free amino acids and to APF-1 still linked by isopeptide linkage to a lysine or a small peptide, APF-1-X. Finally, free APF-1 would be released for re-use by specific amidase-isopeptidase (step 4). A hypothetical ‘correcting’ isopeptidase would release free APF-1 and substrate protein from erroneous ligations (editing activity; step 2). ™ Short after Hershko’s model proposal, Keith Wilkinson and Arthur Haas (pot-doctoral fellows in Irwin Rose’s lab) indicated that APF-1 was indeed UBIQUITIN