Purpose:
To elucidate conformational changes in rod arrestin induced by its binding to light-activated phosphorylated rhodopsin (P-Rh*).
Methods:
Rod arrestin forms tetramers at physiological concentrations. To explore the structure of free (monomer in the solution tetramer) and rhodopsin-bound arrestin, pairs of spin labels were introduced in the molecule. Distances between the pairs were determined by DEER spectroscopy to provide global structural constraints.
Results:
We have previously shown that that the structure of the crystal and solution tetramer is different, that only monomeric arrestin binds to P-Rh*, and that arrestin binds individual rhodopsin molecules. The arrestin crystal structure reveals a tetramer in which the monomers have different conformations in flexible regions of the molecule, including the "finger loop" involved in P-Rh* binding. The set from which pairs were selected are shown in the figure, and include sites in both the N- and C-domains.
Conclusions:
Distance data are consistent with the D chain conformation seen in the crystal structure tetramer in which the finger loop is in a "bent over", rather than extended, conformation. Pairs involving residue 344 show broad distance distributions indicating plasticity of the host loop. Upon binding to P-Rh*, the pattern of observed distance changes suggests that the structure of the cores of both N- and C-domains remains essentially unchanged, but that specific rearrangements of the finger loop and an adjacent loop containing residue 139 take place.
Keywords: protein structure/function • photoreceptors • signal transduction