3.2.1. “Coincidence Detector” Mechanism
Properly folded functional arrestins can be expressed in cell-free translation, where proteins with high specific activity can be generated by replacing a cold amino acid with a hot one. The use of radioactive arrestins in direct binding assay with purified rhodopsin [
55,
81,
83,
85,
97] or other GPCRs [
45,
46,
98,
99] ensures femtomolar sensitivity of the assay. In the case of arrestin-1 binding to rhodopsin this assay revealed that although light-activated phosphorylated rhodopsin (P-Rh*) is the preferred partner, there is detectable binding to inactive (dark) phosphorhodopsin (P-Rh) and light-activated unphosphorylated rhodopsin (Rh*), whereas in the case of inactive unphosphorylated rhodopsin negligible binding was detected [
83,
85](
Figure 2). This suggested that arrestin-1 must have independent sites recognizing the activation state of rhodopsin and receptor-attached phosphates, which mediate its binding to Rh* and inactive P-Rh, respectively. However, arrestin-1 binding to P-Rh* exceeds that to P-Rh or Rh* 10-20-fold (
Figure 2). This differential is too large to be explained by two-site cooperativity. Hence, the model of sequential multi-site binding was proposed: phosphate and active receptor binding sites were suggested to serve as sensors. Low-affinity binding to P-Rh or Rh*, both of which can engage only one sensor, results in rapid dissociation. P-Rh* engages both sensors simultaneously, which triggers arrestin-1 transition into high-affinity receptor-binding conformation, bringing additional elements into contact with rhodopsin, thereby increasing the energy of the interaction, yielding high-affinity binding [
83]. This model (reviewed in [
95]) satisfactorily explained arrestin preference and therefore was widely accepted in the field.
Based on the structure of a GPCR that simultaneously interacts with arrestin and G protein [
100], where the arrestin engages exclusively phosphorylated receptor C-terminus, it was hypothesized that arrestins first bind the elements containing receptor-attached phosphates (“tail only” configuration in [
100]) and then bind the transmembrane cavity of the receptor. Several lines of evidence contradict this idea. One, arrestin-1 binds Rh* and arrestin-2 and -3 bind active unphosphorylated non-visual GPCRs (none of which has any attached phosphates) a lot better than inactive forms of the same receptors (
Figure 2) [
45,
46,
81,
83,
85,
98,
99,
101,
102]. Two, a variety of phosphorylation-independent mutants of arrestin-1, -2, and -3 bind cognate unphosphorylated receptors with high affinity [
45,
81,
83,
85,
97,
98,
99,
102,
103,
104,
105,
106,
107,
108,
109,
110]. Three, several vertebrate wild type GPCRs bind wild type non-visual arrestins without receptor phosphorylation [
111,
112,
113,
114], as do invertebrate rhodopsins [
52,
115]. In all of these cases arrestins must bind unphosphorylated receptor elements first and last, as there are no receptor-attached phosphates to assist in the process. The original hypothesis that arrestins can bind either phosphates on the receptor or the active GPCR first, engaging the other feature second [
83], does not contradict available data.
3.2.2. Identification of Sensors in Arrestins
Phosphate and active receptor sensors needed to be identified. The finger loop of arrestin-1 (
Figure 1) that binds in the cavity between transmembrane helices [
78], which opens upon the activation of rhodopsin [
79] and other GPCRs [
116], was shown to serve as the sensor of receptor activation [
105]. All three classes of proteins that preferentially bind activated GPCRs engage this cavity: G proteins [
77], GRKs [
117], and arrestins [
78], as the appearance of this cavity is a hallmark of GPCR activation. In solved structures of arrestin-1 and -2 complexes with GPCRs the finger loop can remain unstructured (with M2 muscarinic [
88], β
1-adrenergic [
90], V2 vasopressin [
91], and CB1 cannabinoid [
94] receptors) or form a short α-helix (with rhodopsin [
78], neurotensin [
87,
89], and serotonin 5HT
2B receptors). Non-visual arrestin-2 and -3 bind many different GPCRs (reviewed in [
118,
119]). It appears that this loop can engage this cavity in different receptors in distinct manner, as mutations in it differentially affect the interaction: e.g., in arrestin-3 Asp70Pro kills the binding to M2 muscarinic, but only reduces the binding to dopamine D2 receptors, whereas Leu69Arg totally eliminates the binding to M2, but does not appreciably affect the binding to D2 [
63]. Finger loops of both arrestin-2 and -3 have residues with diverse side chains (large hydrophobic, hydrophilic uncharged, as well as positively and negatively charged [
105]). This, along with the conformational flexibility of the finger loop, likely allows it to fit into the cavity in different receptors lined with the side chains of different chemical nature.
The identification of the arrestin phosphate sensor was convoluted. Early finding that charge neutralization of Arg175 in arrestin-1, which is located within a cluster of phosphate-binding positively charged residues, yields a mutant that does not require phosphates (binds Rh* fairly well) [
81] focused the attention on this residue. Its replacement with the other 19 residues showed that with the positive charge in this position arrestin-1 requires rhodopsin phosphorylation for tight binding, with the other residues it shows varying levels of phosphorylation independence, and with the negative charge in this position the binding becomes independent of the presence of rhodopsin-attached phosphates [
97]. This suggested that it likely interacts with a negatively charged residue in the basal state of arrestin, and that the phosphate breaks this interaction by neutralizing its charge [
120], like the mutations, thereby allowing arrestin to bind with high affinity. Charge reversal of homologous arginines in arrestin-2 [
99,
110,
121] and -3 [
98,
122] also made these subtypes phosphorylation-independent, suggesting that similar mechanism operates in all arrestins. The finding that this arginine is located in the polar core of arrestin-1 [
19,
20], and its homologs are in the polar cores of the other subtypes [
48,
49,
50,
51] (
Figure 1) supported the idea of its functional significance. The structures identified potential negatively charged counterparts of this arginine. Follow-up mutagenesis showed that Asp296 is the key in the bovine arrestin-1: its charge reversal also yielded phosphorylation-independent mutants, whereas simultaneous reversal of both charges, which restored the salt bridge in an opposite configuration, yielded arrestin-1 that required rhodopsin phosphorylation, essentially like wild type [
107]. Arginine substitution of Asp296, as well as its homologs in mouse arrestin-1 and non-visual subtypes, greatly increased the binding to phosphorylated and unphosphorylated receptors [
109,
122]. All these data were consistent with the idea that the polar core salt bridge between Arg175 and Asp296 (and their homologs in the other arrestins) serves as the phosphate sensor: receptor-attached phosphates break this bridge, enabling arrestin transition into high-affinity binding state (this idea was first suggested in [
107]; reviewed in detail in [
95]). This model looked attractive because it made perfect sense biologically: the sequences of the cytoplasmic elements of GPCRs vary greatly [
31], while attached phosphates are the only common theme, so that this mechanism explains how the non-visual subtypes can bind hundreds of different GPCRs.
While in all structures of the arrestin complexes with receptors [
78,
86,
87,
88,
89,
90,
91,
92,
94,
100], phosphorylated receptor peptides [
120,
123], and with IP
6 [
63] the polar core is broken, in none of them the polar core arginine interacts with a phosphate. These data buried that model. However, a residue in the lariat loop (which supplies two negative charges to the polar core) Lys294 in arrestin-2 (
Figure 1) and homologous Lys301 in mouse arrestin-1 were found in contact with the phosphate in the bound receptor (rhodopsin [
86], neurotensin receptor [
89], or engineered multi-phosphorylated V2 receptor peptide attached to the M2 muscarinic [
88] and β
2-adrenergic receptor [
100]) or one of the phosphates in IP
6 [
63] or bound phosphopeptide [
123]. This suggested the idea that the phosphate of the activator can destabilize the polar core in a different way: by pulling out the lariat loop, thereby removing two negative charges from their basal positions. However, the charge of this lysine (Lys295 and Lys300 in bovine arrestin-3 and -1, respectively) was eliminated (Lys->Ala) and reversed (Lys->Glu) with no detectable negative effect on the receptor binding [
109,
122]. These experimental results buried yet another beautiful idea.
All arrestins have two lysines in the β-strand I, right next to the hydrophobic residues mediating its interactions with the α-helix and the C-terminus (three-element interaction in
Figure 1) [
1]. Alanine substitutions of these lysines were shown to prevent the binding of wild type arrestin-1 to P-Rh*, while having minimal effect on the binding of several structurally distinct phosphorylation-independent mutants [
102]. At least one of these lysines was found to contact a receptor-attached phosphate in all structures where the phosphorylated receptor elements were resolved. Lysines in homologous positions are conserved in all arrestins, from
C. elegans to mammals [
124]. Collectively, this evidence makes β-strand I lysines the most likely candidates for the role of the phosphate sensor in arrestins [
109]. Plausible mechanism of their action was proposed in 2000 [
102]: even a small shift of β-strand I due to the pull on these lysines (or either one of them) by a receptor-attached phosphate would move the hydrophobic side chains holding the α-helix and the C-terminus out of position favorable for these interactions, destabilizing the three-element interaction (
Figure 1), thereby facilitating the release of the C-terminus, which removes another conserved arginine (Arg382 in bovine arrestin-1 and its homologues in other arrestins) from the polar core, thereby destabilizing it, as well. Mutagenesis [
98,
104,
106,
109,
122] shows that the release of the arrestin C-terminus is sufficient to facilitate arrestin binding to phosphorylated and unphosphorylated GPCRs.
Modeling suggests that both the polar core and three-element interaction need to be destabilized by the receptor to allow arrestin transition into receptor-bound conformation [
125]. It appears that both of these are destabilized by the receptor-attached phosphates. However, recent mutagenesis studies suggest that there are additional mechanisms enhancing arrestin-1 selectivity for P-Rh*. Several arrestin-1 residues that are not part of either sensor specifically suppress its binding to Rh* [
106,
108]. Apparently, arrestin-1 selectivity for P-Rh* is so important biologically, that some native residues do this even at the expense of somewhat reducing the binding to the preferred target P-Rh* (i.e., their replacement increases P-Rh* binding [
106,
108]). As both arrestin-2 and -3 are a lot less selective for the active phosphorylated form of their cognate GPCRs, as compared to inactive phosphorylated (
Figure 2), this additional mechanism might not exist in non-visual subtypes.
3.2.3. The Role of Receptor-Attached Phosphates
In most cases, arrestins bind active phosphorylated GPCRs (reviewed in [
95,
118]). However, there are exceptions: arrestin binding to M1 muscarinic [
113], substance P [
111], or leukotriene B4 [
112] receptors were reported not to require phosphorylation for arrestin binding. While alanine substitution of the phosphorylatable serines and threonines in β2AR reduces (but does not eliminate) arrestin-2 and -3 binding, elimination of phosphorylation sites in M2 muscarinic receptor does not appreciably change the binding of both non-visual arrestins [
114]. The action of receptor-attached phosphates on the phosphate sensor of arrestins and their role in the interaction should be distinguished: the elimination of the two β-strand I lysines greatly reduces the binding of arrestin-1, -2, and -3 to P-Rh*, but does not appreciably affect the binding of the two non-visual subtypes to β2AR, M2 muscarinic, or D2 dopamine receptors [
114]. Thus, in the case of β2AR attached phosphates participate in the interaction not only via triggering the phosphate sensor, but likely via interaction with other phosphate-binding arrestin elements. Negatively charged at physiological pH phosphates must interact with positively charged arrestin residues (Lys, Arg, possibly His). Lys, Arg, and His can interact with negatively charged glutamic and aspartic acids, which are fairly abundant in the cytoplasmic elements of most GPCRs. Thus, Asp and Glu in the receptor can serve as substitutes for the GRK-attached phosphates. Indeed, negatively charged side chains were shown to participate in the arrestin binding of leukotriene B4 [
112] and luteinizing hormone [
126] receptors. Several Glu and Asp side chains of rhodopsin were found in contact with bound arrestin-1 in the complex [
86].
In Drosophila photoreceptors arrestins (two different subtypes are expressed) are as critical for signal termination as arrestin-1 is in vertebrates [
127]. Yet the phosphorylation of fly rhodopsin is not required for arrestin binding, whereas its activation by light is [
115]. This appears to be the general rule in invertebrates. The two lysines in the β-strand I that most likely serve as the phosphate sensor are conserved in invertebrate arrestins [
124]. Charged residues of the polar core are less conserved, and the polar core in the only solved structure of an invertebrate arrestin is composed of an unconventional set of side chains [
52]. Yet the three-element interaction and the polar core are present in the structure of squid arrestin [
52], both of which likely need to be destabilized to allow high-affinity receptor binding. It is tempting to speculate that negatively charged residues in the invertebrate rhodopsins act on arrestins in lieu of receptor-attached phosphates. However, as fly arrestins selectively bind light-activated forms of rhodopsins, this hypothesis requires an additional assumption that arrestin-binding negatively charged side chains in rhodopsin are inaccessible when it’s inactive and become accessible upon light activation. To test whether this is the case the structures of a fly rhodopsin in inactive and activated state are needed.
Both vertebrate non-visual arrestins preferentially bind phosphorylated forms of their cognate receptors (
Figure 2). Receptor phosphorylation alone, without activation, greatly increases the binding of arrestin-2 and -3, in sharp contrast to exceptional selectivity of arrestin-1 for P-Rh* (
Figure 2). This raises a question how do receptor-attached phosphates facilitate the binding of arrestin-2 and -3, considering that phosphate-binding lysines in the β-strand I [
114] and the lariat loop [
122] do not appear to be required. This is particularly intriguing considering that at least arrestin-2 can hold onto the receptor while interacting solely with its phosphorylated element (admittedly, with the help of bound antibodies that are not present in cells) [
100]. This question can be addressed only by methods revealing the dynamics of the process, not by structures of the complexes that show the final result, but not the sequence of events whereby it is achieved. Importantly, GPCR activation without phosphorylation induces lower arrestin-2 and -3 binding than phosphorylation without activation (
Figure 2), suggesting that activation-dependent exposure of negative charges in the receptor (as proposed above for invertebrate rhodopsins) is unlikely to play a significant role in the recruitment of vertebrate non-visual subtypes to GPCRs.
3.2.4. Conformational Rearrangements Associated with Receptor Binding
The idea that arrestin-1 binding to rhodopsin involves a global conformational change was proposed more then 30 years ago on the basis of unusually high Arrhenius activation energy [
128]. Soon thereafter it was demonstrated that the binding to phosphorhodopsin involves the release of the arrestin-1 C-terminus [
129,
130]. The finding that C-terminal truncation greatly increases rhodopsin binding of arrestin-1 [
83,
85,
104] suggested that the release of its C-terminus is necessary for the binding. Enhanced rhodopsin binding of the C-terminally truncated splice variant of arrestin-1, p44 [
131,
132], supported this idea. Enhanced binding of arrestin-2 and -3 with deleted C-termini to their cognate GPCRs [
98,
99,
122] indicated that this part of the mechanism is conserved in the arrestin family. Double electron-electron resonance (DEER) (pulse EPR technique) studies of arrestin-1 [
133,
134], -2, and -3 [
135] revealed receptor binding-induced release of the C-termini of all three subtypes. Interestingly, while in the case of arrestin-1 the released C-terminus likely “flops around” (wide distributions of distances between the C-terminus and points in the rest of the molecule), in arrestin-2 and -3 the released C-terminus appears to have a preferred position. Recent single-molecule study of arrestin-2 suggests that there might be two distinct preferred positions of its released C-terminus [
136].
The comparison of the structures of free arrestins [
19,
20,
48,
49,
50,
51,
52,
53] and GPCR-bound arrestin-1 [
78,
86] and -2 [
87,
88,
89,
90,
91,
92] reveals that the release of the arrestin C-terminus is not the only conformational change associated with GPCR binding. In receptor-bound state the two domains twist relative to each other by ~20
o, as compared to their position in free arrestins. Similar domain twist was observed in one of the structures of short arrestin-1 splice variant p44 [
137] and in the IP
6-induced trimer of arrestin-3 [
63], suggesting that arrestin can achieve this conformation independently of GPCRs. Several additional conformational rearrangements in receptor-bound arrestins were revealed by EPR [
138] and the structures of the arrestin-receptor complexes [
78,
86,
87,
88,
89,
90,
91,
92,
94], the most prominent of which is a large movement of the middle loop (originally termed 139-loop in arrestin-1 [
138,
139]) (
Figure 1) from its basal position overhanging the cavity of the C-domain in the direction of the N-domain.
When a protein engages the elements in both arrestin domains (as have been shown for many arrestin binding partners [
140,
141,
142]), it is easy to explain its conformational preference for receptor-bound or basal arrestins: only in one of these conformations the position of interacting elements would be favorable for the binding. Based on this idea, one can predict that partners interacting with only one domain should not have a significant preference for either of these arrestin conformations. This needs to be tested experimentally. The difficulty is that no arrestin partner so far has been shown to interact with only one domain.