Ubiquitin-Proteasome System-Mediated Protein Degradation in Defense Signaling - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 407

ATP AMP E2 E1

Ub Ub

Ub E2

E1

E3 ubiquitin
ligase

Protein
substrate

Ub

Ub

E2

Ub

Ub

Protein
substrate

Target
protein

Peptides

Protein
degradation

26S proteasome

Ub - Ubiquitin; E1 - Ubiquitin activating enzyme; E2 - Ubiquitin conjugating enzyme;
E3 - Ubiquitin ligase

Fig. 10.1 Ubiquitin-proteasome pathway

suggesting that this pathway is one of the most elaborate regulatory mechanisms in
plants (Vierstra 2003 ; Serrano et al. 2006 ). Proteasomes are large protein complexes
located in the nucleus and the cytoplasm (Peters et al. 1994 ). The main function of
the proteasome is to degrade unneeded or damaged proteins by proteolysis.
Proteasomes regulate the concentration of particular proteins and degrade misfolded
proteins. Proteins are tagged for degradation by a small protein called ubiquitin. The
ubiquitin protein is 76 amino acids long and was named due to its ubiquitous nature
(Pickart and Eddins 2004 ).
Ubiquitin acts as a covalent molecular tag and its attachment requires three
distinct enzymatic activities. Proteins are targeted for degradation by the proteasome
by covalent modifi cation of a lysine residue that requires the coordinated reactions
of three enzymes. In the fi rst step, a ubiquitin-activating enzyme, E1, activates ubiq-
uitin C-terminal carboxyl group by adenylation, and then forms a thioester bond
with cysteinyl sulfhydryl residue on the E1 protein itself. The ubiquitin-activating
enzyme E1 activates ubiquitin in an ATP-dependent manner (Fig. 10.1 ; Lee et al.
2011 ). The adenylated ubiquitin is then transferred to a cysteine of a second enzyme,
ubiquitin-conjugating enzyme (E2). In the last step, a member of a highly diverse
class of enzymes known as ubiquitin ligases (E3) recognizes the specifi c protein to
be ubiquitinated and catalyzes the transfer of ubiquitin from E2 to this target protein
(Haas et al. 1982 ; Clechanover 1998 ; Pickart and Eddins 2004 ; Smalle and Vierstra
2004 ; Dreher and Callis 2007 ). A target protein must be labeled with at least four
ubiquitin monomers in the form of a polyubiquitin chain before it is recognized by