Ubiquitin is the ATP-dependent proteolysis factor I of rabbit reticulocytes.

A small heat-stable polypeptide, ATP-dependent proteolysis factor 1 (APF-1), is an essential component of the ATP-dependent proteolytic system of rabbit reticulocytes (Ciechanover, A., Hod, Y., and Hershko. A. (1978) Biochem. Biophys. Res Commun. 81, 1100-1105). The following evidence supports the view that APF-1 is ubiquitin, a highly conserved heat-stable polypeptide found universally in nature: 1) APF-1 and ubiquitin (generously given by G. Goldstein) yield co-migrating bands on five polyacrylamide gel electrophoresis systems and in isoelectric focusing; 2) amino acid analysis shows excellent agreement between the two proteins; 3) APF-1 and ubiquitin give similar specific activity, in activating the ATP-dependent proteolysis system; 4) 125I-APF-1 and 125I-ubiquitin form electrophoretically identical covalent conjugates with endogenous reticulocyte proteins. Recently, such conjugates have been proposed as the active intermediates in ATP-dependent proteolysis (Ciechanover, A., Heller, H., Hershko, A., Haas, A.L., and Rose, I.A. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 1783-1786). Thus, ubiquitin is an essential component of the ATP-dependent system in reticulocytes and a similar role in degradation and proteolytic processing in other cells is likely.