Nonvisual arrestin oligomerization and cellular localization are regulated by inositol hexakisphosphate binding.

Arrestins are known to interact with phosphoinositides and regulate membrane signaling and trafficking processes. Their role in clathrin-mediated endocytosis and light adaptation in Drosophila has been established. However, the regulation of arrestin functions in the nucleus and cytosol remains unclear. No prior work had resolved the ligand dependence of these intracellular roles. The involvement of inositol hexakisphosphate (IP6) in these processes has not been fully explored. Researchers have proposed that arrestins may have additional roles beyond membrane signaling. The structural and functional details of how IP6 interacts with arrestins are not well defined. This gap motivated the current investigation into the molecular and cellular effects of IP6 on arrestin-2. Understanding these interactions could clarify how arrestins regulate intracellular signaling.

This study aimed to investigate the structural and functional interactions between arrestin-2 and inositol hexakisphosphate (IP6). The researchers sought to determine how IP6 binding affects arrestin-2 oligomerization and cellular localization. They focused on identifying the binding sites and their affinity for IP6. The study also examined whether IP6 influences the ability of arrestin-2 to form homo- and hetero-oligomers. The authors wanted to assess the impact of IP6 binding on interactions with known arrestin partners like clathrin and ERK2. They also aimed to explore the subcellular distribution of arrestin-2 in response to IP6. The study was designed to test the hypothesis that IP6 binding regulates arrestin oligomerization and localization. This approach could reveal how arrestins modulate signaling in different cellular compartments.

The researchers used crystallography to determine the structure of the arrestin-2.IP6 complex at 2.9 Å resolution. They identified two potential IP6-binding sites on the protein monomer. Mutagenesis was combined with isothermal titration calorimetry to assess binding affinities. Tritiated IP6 binding assays were used to confirm the presence of two distinct binding sites. Native gel electrophoresis was employed to observe changes in oligomerization. Gel filtration and analytical ultracentrifugation were used to quantify the extent of IP6-induced oligomerization. The study also included mammalian cell experiments to evaluate homo- and hetero-oligomerization. Subcellular localization was analyzed using fluorescence microscopy and fractionation techniques.

The crystal structure revealed two IP6-binding sites on arrestin-2, one in the N-domain and one in the C-domain. Isothermal titration calorimetry showed a low-affinity site in the N-domain and a high-affinity site in the C-domain. IP6 binding was confirmed to promote arrestin-2 oligomerization through both sites. Native gel electrophoresis and analytical ultracentrifugation supported this oligomerization effect. Arrestin-2 was found to form homo-oligomers in the presence of IP6. The protein also hetero-oligomerized with arrestin-3 in an IP6-dependent manner. Mutation of either IP6-binding site disrupted oligomerization. However, interactions with clathrin, AP-2, and ERK2 remained unaffected by these mutations. Subcellular localization studies showed that oligomeric arrestin-2 was cytoplasmic. Monomeric arrestin-2, in contrast, exhibited increased nuclear localization.

The study suggests that IP6 binding regulates the oligomerization and localization of arrestin-2. The two IP6-binding sites appear to mediate these effects independently. Oligomerization was shown to depend on both the N- and C-domain sites. The ability of arrestin-2 to form hetero-oligomers with arrestin-3 was also IP6-dependent. Mutation of either binding site disrupted this process. However, interactions with other proteins like clathrin and ERK2 were not affected. The cytoplasmic localization of oligomeric arrestin-2 implies a shift in function. The authors propose that IP6 binding may negatively regulate arrestin interactions with membrane and nuclear proteins. These findings support a model where IP6 modulates arrestin signaling in different cellular compartments.