Structural Determinants of Arrestin Functions - Progress in Molecular Biology and Translational Science - page 68

nonvisual arrestin with high receptor selectivity. Both the 70% success rate
in changing receptor preference and additive effects of individual mutations
show that it opened a promising new direction. However, a lot of additional
experimentation is necessary to construct the arrestins that fit every GPCR
whose signaling needs to be corrected to cure a particular human disorder.
The same approach can be used to rein in excessive signaling by normal
GPCRs that develops for various reasons in numerous human disorders.
4. INTERACTIONS WITH OTHER SIGNALING PROTEINS
Clathrin was the first nonreceptor-binding partner of arrestin proteins
identified. 145 Since then arrestins were shown to interact with an amazing
variety of trafficking and signaling proteins (see Chapter 1 ). The molecular
mechanisms of most of these interactions remain to be elucidated.
By comparison, it appears that we know a lot about the mechanics of recep-
tor binding, although even in this area there are more questions than
answers.
4.1. Where do the other partners bind?
Only two proteins have been shown to interact with the receptor-binding
surface of arrestins: microtubules 35 and Ca 2 þ -liganded calmodulin. 146 In
both cases, receptors bind arrestin with much higher affinity, easily winning
the competition with these partners. 32,146 Most other proteins interact with
receptor-bound arrestins, 145,147-150 which suggests that they engage
nonreceptor-binding elements localized either on the convex sides of the
two domains, or on the arrestin C-tail that is released upon receptor binding.
Considering the potential biological importance of these interactions,
surprisingly few binding sites of nonreceptor partners have been identified,
and most of those very imprecisely. The best characterized among these are
clathrin and AP2 interaction sites, both of which are localized in the C-tail of
arrestin-2 and -3. The main clathrin site is a short LIELD or LIEFE motif in
the part of the C-tail upstream of its contact with the N-domain, 151 which is
not visible in any of the crystal structures of free arrestins, likely because it is
inherently disordered. 19-21 This interaction was resolved in the cocrystal of
arrestin-2 with the clathrin b -propeller domain, where this arrestin element
binds between blades 1 and 2. 152 Interestingly, in the prevalent long splice
variant of arrestin-2, 8 another C-tail element also interacts with clathrin,