PASS Scripta Varia 21 - Pontifical Academy of Sciences

AARON CIECHANOVER
Figure 4: Multiple molecules of APF-1/Ubiquitin are conjugated to the proteolytic substrate, probably
signalling it for degradation. To interpret the data described in the experiment depicted in
Figure 2 and to test the hypothesis that APF-1 is conjugated to the target proteolytic substrate,
125
I-APF-1/ubiquitin was incubated along with crude Fraction II (Figure 3 and text) in the absence
(lane 1) or presence (lanes 2-5) of ATP and in the absence (lanes 1,2) or presence (lanes 3-5) of
increasing concentrations of unlabelled lysozyme. Reaction mixtures resolved in lanes 6 and 7
were incubated in the absence (lane 6) or presence (lane 7) of ATP, and included unlabelled APF-
1/ubiquitin and 125 I-labelled lysozyme. C1-C6 denote specific APF-1/ubiquitin-lysozyme adducts
in which the number of APF-1/ubiquitin moieties bound to the lysozyme moiety of the adduct is
increasing, probably from 1 to 6. Reactions mixtures were resolved via sodium dodecyl sulfatepolyacrylamide
gel electrophoresis (SDS-PAGE) and visualized following exposure to an X-ray film
(autoradiography) (with permission from Proceedings of the National Academy of the USA; published
originally in Ref. 40).
An important development in the ubiquitin research field was the discovery
that a single ubiquitin moiety can be covalently conjugated to histones,
particularly to histones H2A and H2B. While the function of these
adducts has remained elusive until recently, their structure was unravelled in
the mid 1970s. The structure of the ubiquitin conjugate of H2A (uH2A; was
also designated protein A24) was deciphered by Goldknopf and Busch
(47,48) and by Hunt and Dayhoff (49) who found that the two proteins are
linked through a fork-like, branched isopeptide bond between the carboxy-
300 The Scientific Legacy of the 20 th Century