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S.M. Hanson, D.J. Francis, N. Van Eps, C. Hubbell, S.A. Vishnivetskiy, C.S. Klug, W.L. Hubbell, V.V. Gurevich; The Molecular Mechanism and Functional Role of Visual Arrestin Self–Association . Invest. Ophthalmol. Vis. Sci. 2006;47(13):823.
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© ARVO (1962-2015); The Authors (2016-present)
Among arrestin proteins only visual (rod) arrestin forms oligomers at physiological concentrations. Thus, arrestin self–association likely contributes to the specialized functional characteristics of rod photoreceptors. Here we elucidate the structural basis and functional consequences of visual arrestin self–association.
Visible light scattering was used to determine the nature of the visual arrestin oligomer in solution. To elucidate the shape of the tetramer, we introduced spin labels at several locations and analyzed their effects by two independent methods, SDSL EPR spectroscopy and light scattering. Long–range inter–subunit distance measurements were made by double electron electron resonance (DEER) and used to determine the ability of the tetramer to bind rhodopsin and microtubules.
We found that visual arrestin forms tetramers in a cooperative manner (KD tetramer 0.6µM << KD dimer 33µM). The effects of targeted mutations on arrestin self–association were inconsistent with the interfaces that exist in the crystal tetramer. DEER distance measurements unequivocally show that the solution tetramer is different from the tetramer in the crystal. Based on these data a model of the solution tetramer that is consistent with the cooperativity of its formation and other experimental data is proposed. Recently we showed that visual arrestin translocation between compartments of the rod cell is governed by its interactions with light–activated rhodopsin (Rh*) in the outer segment and microtubules (MTs) in the inner segment. Using the DEER signal as a readout we found that the arrestin tetramer can bind MTs whereas only arrestin monomer binds P–Rh* and Rh*.
Visual arrestin cooperatively forms tetramers at physiological concentrations. The shape of the solution tetramer is different from the crystal. The arrestin tetramer likely serves as a "storage" form of arrestin, preventing premature signal termination at low light by buffering the amount of free monomeric arrestin that is available to quench rhodopsin signaling. Tetramer binding to microtubules is likely a part of this regulatory mechanism because it significantly increases the arrestin–binding capacity of microtubules thereby helping to keep arrestin away from the outer segment when maximum signaling efficiency is important.
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