Agriculture Reference
In-Depth Information
Fig. 7.1 Ubiquitin/26S proteasome system (
UPS
) overview (a) and schematic structure of 26S
proteasome (b). (a) Ubiquitin (
Ub
) is in an ATP-dependent manner activated by E1 enzyme,
transferred to an E2 enzyme and delivered to the target protein due to E3 attachment. Rounds of
ubiquitin conjugation cascades polyubiquitinates the substrate to be recognised by 26S proteasome
complexes, which hydrolyse peptide bonds and releases oligopeptides (consisting of 2-10 resi-
dues) to the cytoplasm; the Ub moieties are reused.
Su
substrate protein,
G
Gly-76 C-terminal
residue of Ub,
K
Lys residue of the target protein,
S
thiolester bond,
DUBs
deubiquitinating
enzymes. (b) One or two 19S regulatory complexes (
RP
) composed of base and lid subcomplexes
cap the 20S core complex (
CP
) formed by two external and two internal rings of seven
α
-subunits
and seven
ʲ
-subunits respectively
The proteasome exists in multiple forms, and contains two major assemblies, the
28-subunit core particle (CP, also known as the 20S core) and the regulatory subunit
(RP, also known as the 19S complex and PA700) consisting of 19 subunits in yeast,
and probably the same number in other eukaryotes (Finley
2009
). The CP is
composed of four rings: two external rings made of seven
α
-subunits (
α
1-7)
flanking two rings of
1-7 (A-G in plants). The CP functions as a
non-specific ATP and Ub-independent protease with the capacity to cleave most
peptide bonds due to peptidylglutamyl, trypsin-like and chymotrypsin-like activi-
ties. One or two RP complexes, each consisting of two parts base and lid, can be
attached to the CP. The base consists of a ring of six related AAA+ATPase subunits
(RPT1-6) and three non-ATPase subunits (RPN1, 2 and 10) and is attached directly
to the CP. The lid contains non-ATPase subunits (RPN3, 5-9 an 11-12). RPN11 is a
metaloprotease, which allows recycling of the substrate recognition tag, ubiquitin.
The RP is responsible for recognition of K48 linked poly(Ub) chains, removal of
ʲ
-subunits (
ʲ
