Arrestins and Protein Ubiquitination - Progress in Molecular Biology and Translational Science - page 174

in the substrate protein and the C-terminal glycine of activated ubiquitin.
This ubiquitin transfer to the substrate protein requires the activity of a third
enzyme called E3 ubiquitin ligase.
According to a recent genomic annotation, humans express two E1s,
approximately 62 E2s, and nearly 600 E3s. 36 Ubiquitin-protein ligases
(E3s) represent the largest and most diverse group of enzymes in the
ubiquitination process. 37 E3 ubiquitin ligases are traditionally categorized
into two main types: (1) HECT ( H omologous to E 6AP C - t erminus) and
(2) RING ( R eally I nteresting N ew G ene) E3 ligases. The RING E3 ligases
are further classified into two subtypes: (1) enzymes represented by single
polypeptides (examples of single-polypeptide RING E3s are the Cbl onco-
gene, seven in absentia homolog (Siah), and murine double minute 2
(Mdm2)); and (2) ligases made up of multiprotein complexes with separate
units for substrate binding, E2 interaction, and ubiquitin transfer (examples
of multimeric RING E3s include the anaphase-promoting complex (APC,
also called the cyclosome), Skp1-Cullin-F-box (SCF), suppressor of cyto-
kine signaling (SOCS), and the recently identified Broad complex,
Tramtrack, and Bric-a-brac (BTB) families). 37-40 Other E3 ubiquitin ligases
that contain variations within the RING domain have also been described
(e.g., U-box, B-box, and PHD containing E3s). 41,42 The two E3 families
differ in the general mechanism of ubiquitin transfer: HECT E3s accept
ubiquitin from E2s and then ligate the ubiquitin onto substrates, while
RING E3s facilitate ubiquitin transfer from E2 enzyme to substrate lysines. 37
Some proteins become modified with a single ubiquitin resulting
in monoubiquitination, which is considered as a tag that promotes
endocytic trafficking of modified proteins (Ref. 43 ; Fig. 7.2 ). Multi-
monoubiquitination is defined as the addition of single ubiquitin moieties
to multiple lysine residues in a target protein. After ubiquitin is conjugated
to a substrate, successive ubiquitin additions can occur on a lysine of the pre-
viously attached ubiquitin, resulting in polyubiquitination ( Fig. 7.2 ).
Ubiquitin has seven lysines at positions 6, 11, 27, 29, 33, 48, and 63 of
its primary amino acid sequence and each lysine can be interlinked forming
specific polyubiquitin chains, each with distinct roles ( Fig. 7.2 ). Poly-
ubiquitination consisting of successive ubiquitin moieties linked at lysine
48 is called “kiss of death” because it often targets the ubiquitinated proteins
to multi-subunit, megadalton, barrel-shaped protein complexes called 26S
proteasomes, which are the primary nonlysosomal enzymatic degradation
machines in cells. 44 Ubiquitin itself is not degraded; rather, it is cleaved from