True Arrestins and Arrestin-Fold Proteins: A Structure-Based Appraisal - Progress in Molecular Biology and Translational Science - page 22

the direction of the central polar core and induces a global conformational
change of arrestin, resulting in release of the C-terminal tail 20 ( Fig. 2.1 ). This
recognition mechanism is conserved for all four human true arrestins. 49 In
the case of b -arrestins, exposure of the C-tail allows the recruitment of the
endocytic machinery (see below). Disruption of the polar core in b -arrestin
1byaR 169 E mutation or a deletion of the C-terminal tail generates a con-
stitutively active b -arrestin, able to bind b 2 -adrenergic receptor in a

phosphorylation-independent manner. 50
Additional receptor-binding multielements exist on the concave sides of
N- and C-arrestin domains, which are organized as a saddle to accommodate
the receptor. The primary binding site of phosphorylated GPCRs is located
on b -strands 5 and 6 (N-domain) for rod arrestin. For b -arrestin 1, it is dis-
tributed on b -strands 5 and 6 and their connecting loop, b -strands 9 and 10
and their connecting loop (N-domain), and b -strands 15 and 16
(C-domain). 5,51 Segment swapping between visual and b -arrestins strongly
modifies the binding specificity toward the recognized receptors. 2,8 Cone
arrestin binds green cone opsin and nonvisual receptors and conversely
b -arrestin 1 is able to bind both visual receptors in addition to nonvisual
receptors. 6

2.3. Binding sites for the endocytic machinery on arrestin ' s

C-terminal tail
The polar core typically consists of three positive residues (R 169 ,K 170 , and
R 393 for bovine b -arrestin 1) balanced by four negative residues (D 26 ,D 29 ,
D 290 , and D 297 ) in the resting arrestin. These charged residues are contrib-
uted by arrestin's N-domain, C-domain, and its C-terminal tail ( Fig. 2.1 ).
Binding of arrestins to activated, phosphorylated receptors leads to the tran-
sition to their active state. Two strategic lysines in the 3-element interaction
cluster (K 10 and K 11 in bovine b -arrestin 1) are able to sense the phosphate
groups of the receptor that are channeled toward the polar core where they
disrupt the charge balance. This induces a conformational change in the
arrestin that liberates its C-terminal tail normally engaged in the polar core.
The binding sites for clathrin (LIELD) and the b -subunit of adaptor protein
AP-2 (comprising R 393 -R 395 ) localized on the C-tail are then free to recruit
the endocytic machinery. This mechanism is wonderfully adapted to bind
arrestin's partners following the activation of the receptor in order to pro-
mote its internalization in clathrin-coated vesicles. 2,3,18,31,49,52-54
Analysis of an alternatively spliced long isoform of b -arrestin 1 reveals a
second clathrin-binding site ([L/I] 2 GxL) in the splice loop. 55 Human visual