A. Plangger et al., MECHANISM OF ARRESTIN-2 FUNCTION IN RHABDOMERIC PHOTORECEPTORS, The Journal of biological chemistry, 269(43), 1994, pp. 26969-26975
Arrestins have emerged as one family of proteins that mediate the inac
tivation of G-protein-coupled receptors. We have isolated cDNA clones
encoding two arrestin isoforms of the dipteran visual system, Callipho
ra arrestin 1 (Arrl) and arrestin 2 (Arr2). Microsequencing establishe
d that the arr2 gene encodes the Calliphora 49-kDa protein characteriz
ed previously as a photoreceptor-specific protein that undergoes rever
sible binding to light-activated rhodopsin and thereby activates the p
hosphorylation of metarhodopsin. Ultrastructural localization of Arr2
to the rhabdomeral part of the photoreceptor cell and quantitation of
the amount of Arr2 bound suggest that Arr2 directly interacts with lig
ht-activated rhodopsin. In a reconstituted system containing affinity
purified Arr2 and isolated rhabdomeric membranes, Arr2 binds to non-ph
osphorylated and phosphorylated metarhodopsin with comparable affinity
. Reaction time courses reveal that Arr2 binding precedes phosphorylat
ion of metarhodopsin, contrary to what has been reported so far for ve
rtebrate photoreceptors. The phosphorylation-independent binding of Ar
r2 to metarhodopsin provides a mechanism for the rapid inactivation of
the long-lived activated rhodopsin state which is characteristic for
invertebrate photoreceptors. The dephosphorylation of rhodopsin is cat
alyzed by a Ca2+-dependent protein phosphatase which is shown here for
the first time to exist in a membrane-associated form. Only metarhodo
psin molecules with bound Arr2 are resistant to dephosphorylation. Thu
s, in fly photoreceptors, Arr2 acts as a regulatory protein that contr
ols the phosphorylation as well as the dephosphorylation of the light-
activated visual pigment.