Rad23 Ubiquitin-associated Domains (UBA) Inhibit 26 S Proteasome-catalyzed Proteolysis by Sequestering Lysine 48-linked Polyubiquitin Chains

Shahri Raasi, Cecile M. Pickart
2003 Journal of Biological Chemistry  
Most substrates of the 26 S proteasome are recognized only following conjugation to a Lys 48 -linked polyubiquitin chain. Rad23 is one member of a family of proteins that possesses an N-terminal ubiquitin-like domain (UbL) and a C-terminal ubiquitin-associated domain(s) (UBA). Recent studies have shown that UbLs interact with 26 S proteasomes, whereas UBAs bind polyubiquitin chains. These biochemical properties suggest that UbL-UBA proteins may shuttle polyubiquitinated substrates to
more » ... . Here we show that contrary to prediction from this model, the effect of human Rad23A on the degradation of polyubiquitinated substrates catalyzed by purified proteasomes is exclusively inhibitory. Strong inhibition is dependent on the presence of both UBAs, independent of the UbL, and can be explained by competition between the UBA domains and the proteasome for binding to substrate-linked polyubiquitin chains. The UBA domains bind Lys 48 -linked polyubiquitin chains in strong preference to Lys 63 or Lys 29 -linked chains, leading to selective inhibition of the assembly and disassembly of Lys 48 -linked chains. These results place constraints on the mechanism(s) by which UbL-UBA proteins promote proteasome-catalyzed proteolysis and reveal new properties of UBA domains.
doi:10.1074/jbc.m212841200 pmid:12643283 fatcat:67w6ig7uabacndubrz56kcwhum