Structural Determinants of Arrestin Functions - Progress in Molecular Biology and Translational Science - page 76

question, as only monomeric arrestin binds receptor, 120 and no other signal-
ing protein in photoreceptors has an oligomeric storage form. Interestingly,
cone-specific arrestin-4 is a constitutive monomer, 169 but it represents only

2% of total arrestin complement in cones, 170 the rest being arrestin-1 that
readily self-associates. This finding, along with apparent conservation of self-
association among mammalian arrestin-1 homologues, 121 strongly suggests
that oligomerization of this subtype has an important biological function that
needs to be elucidated experimentally.
Both nonvisual arrestins were also found to self-associate 171-173 and even
form heterodimers. 171,172 The mechanism of self-association of arrestin-1
and nonvisual subtypes appears to be different, as IP6 inhibits oligomeriza-
tion of arrestin-1, while promoting self-association of nonvisual
arrestins. 169,172 Two IP6 binding sites in arrestin-2 were identified by
crystallography and confirmed by structure-based mutagenesis. 172
Self-association was shown to regulate the subcellular distribution of
arrestin-2; oligomers were predominantly cytoplasmic, whereas monomers
showed increased nuclear localization. 172 Oligomerization of arrestin-3 was
reported to be required for its interaction with the E3 ubiquitin ligase
Mdm2, and elimination of IP6 binding residues inhibited Mdm2 binding
and p53-dependent antiproliferative effects of arrestin-3. 173 It should be
noted that while self-association of arrestin-1 certainly stops at tetramers that
have the shape of closed diamonds, where all potential interaction surfaces
are fully engaged by sister subunits, 122 it is not clear whether oligomerization
of nonvisual arrestins has a natural limit. Crystallography suggests that they
might form chains of any length 172 ; biochemical data obtained with pure
protein indicate that arrestin-2 can form at least tetramers but do not provide
evidence that the process stops at that stage. 169 Further experimentation
with arrestin-2, arrestin-3, and a mixed population of the two nonvisual
arrestins in the presence and absence of IP6 is necessary to resolve these issues
and attempt construction of constitutively monomeric forms of these pro-
teins with all other functions intact. These tools are necessary to determine
the biological role of the oligomerization of nonvisual arrestins.

5.3. Manipulating MAPK signaling
MAP kinases regulate many vital cellular functions, from proliferation to
apoptotic death. 174 MAPK cascades consist of three kinases that sequentially
activate each other by phosphorylation: upstream MAPKKKs (e.g., ASK1
and cRaf1), MAPKKs (e.g., MEK1/2, MKK4/7), and MAPKs (e.g.,