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Y.V. Sergeev, J.F. Hejtmancik, P.T. Wingfield; Energetics Of Beta–crystallins Association . Invest. Ophthalmol. Vis. Sci. 2004;45(13):3974.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Beta–crystallins are major protein constituents of the mammalian lens, where their stability and association into higher order complexes are critical for lens clarity and refraction. They undergo modification as the lens ages, including cleavage of their terminal extensions.Methods: The energetics of betaA3– and betaB2–crystallin association was studied using site–directed mutagenesis and analytical ultracentrifugation. Recombinant murine wild type betaA3– and betaB2–crystallins were modified by removal of either the N–terminal extension of betaA3 or betaB2, or both the N– and C–terminal extensions of betaB2. The proteins were expressed in Sf9 insect cells or E.coli and purified by gel–filtration and ion–exchange chromatography. Results: All beta–crystallins studied demonstrated fast reversible monomer–dimer equilibria over the temperature range studied (5 oC to 35 oC) with a tendency to form tighter dimers at higher temperatures. The N–terminal deletion of betaA3 significantly increases the enthalpy (+10.9 kcal/mol) and entropy (+40.7 cal/deg mol) of binding relative to unmodified protein. Removal of both N– and C– terminal extensions of betaB2 also increases these parameters but to a lesser degree. Deletion of the betaB2–crystallin N–terminal extension alone gave almost no change relative to betaB2. Conclusions: The resultant net negative changes in the binding energy suggest that betaA3–and betaB2–crystallin association is entropically driven. The thermodynamic consequences of the loss of betaA3–crystallin terminal extensions by in vivo proteolytic processing could increase their tendency to associate and so promote the formation of higher order associates in the aging and cataractous lens.
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