Jm. Huibregtse et al., A FAMILY OF PROTEINS STRUCTURALLY AND FUNCTIONALLY RELATED TO THE E6-AP UBIQUITIN PROTEIN LIGASE, Proceedings of the National Academy of Sciences of the United Statesof America, 92(7), 1995, pp. 2563-2567
E6-AP is a 100-kDa cellular protein that interacts with the E6 protein
of the cancer-associated human papillomavirus types 16 and 18. The E6
/E6-AP complex binds to and targets the p53 tumor-suppressor protein f
or ubiquitin-mediated proteolysis. E6-AP is an E3 ubiquitin-protein li
gase which accepts ubiquitin from an E2 ubiquitin-conjugating enzyme i
n the form of a thioester and then directly transfers the ubiquitin to
targeted substrates, The amino acid sequence of E6-AP shows similarit
y to a number of protein sequences over an approximate to 350-aa regio
n corresponding to the carboxyl termini of both E6-AP and the E6-AP-re
lated proteins. Of particular note is a conserved cysteine residue wit
hin the last 32-34 aa, which in E6-AP is likely to be the site of ubiq
uitin thioester formation. Two of the E6-AP-related proteins, a rat 10
0-kDa protein and a yeast 95-kDa protein (RSP5), both of previously un
known function, are shown here to form thioesters with ubiquitin. Muta
tion of the conserved cysteine residue of these proteins destroys thei
r ability to accept ubiquitin. These data strongly suggest that the ra
t 100-kDa protein and RSP5, as well as the other E6-AP-related protein
s, belong to a class of functionally related E3 ubiquitin-protein liga
ses, defined by a domain homologous to the E6-AP carboxyl terminus (he
ct domain).