α2-adrenergic agonist enrichment of spinophilin at the cell surface involves βγ subunits of G_i proteins and is preferentially induced by the α_2A-subtype

Agonist activation regulates reciprocal interactions of spinophilin and arrestin with the α_2A- and α_2B-adrenergic receptor (AR) subtypes via their 3i loop. Because arrestin association with G protein-coupled receptor is preceded by redistribution of arrestin to the cell surface, the present studies explored whether agonist activation of the α_2A- and α_2B-AR subtypes also led to spinophilin enrichment at the cell surface. Live cell imaging studies using a green fluorescent protein-tagged spinophilin examined spinophilin localization and its regulation by α₂-AR agonist. Agonist activation of α_2A-AR preferentially, compared with the α_2B- AR, led to spinophilin enrichment at the cell surface in human embryonic kidney 293 cells and in mouse embryo fibroblasts derived from spinophilin null mice. Activation of the ΔLEESSSSα_2A-AR, which has enriched association with spinophilin compared with the wild-type (WT) α_2A-AR, does not show an enhanced redistribution of spinophilin to the surface compared with WT α_2A-AR, demonstrating that the ability or affinity of the receptor in binding spinophilin may be independent of the ability of the receptor to effect spinophilin redistribution to the surface. Agonist-evoked enrichment of spinophilin at the cell surface seems to involve downstream signaling events, manifested both by the pertussis toxin sensitivity of the process and by the marked attenuation of spinophilin redistribution in cells expressing the β-adrenergic receptor kinase-C tail, which sequesters βγ subunits of G proteins. Together, the data suggest that agonist-evoked spinophilin enrichment at the cell surface is caused by receptor-evoked signaling pathways and is independent of the affinity of the receptor for the spinophilin molecule.