BINARY-LIQUID PHASE-SEPARATION OF LENS PROTEIN SOLUTIONS

We have determined the coexistence curves (plots of phase-separation temperature T versus protein concentration C) for aqueous solutions of purified calf lens proteins. The proteins studied, calf gamma-IIIa-, gamma-IIIb-, and gamma-IVa-crystallin, have very similar amino acid sequences and three-dimensional structures. Both ascending and descending limbs of the coexistence curves were measured. We find that the coexistence curves for each of these proteins and for gamma-II-crystallin can be fit, near the critical point, to the function /(C(c) - C)/C(c)/= A[(T(c) - T)/T(c)]beta, where beta = 0.325, C(c) is the critical protein concentration in mg/ml, T(c) is the critical temperature for phase separation in K, and A is a parameter that characterizes the width of the coexistence curve. We find that A and C(c) are approximately the same for all four coexistence curves (A = 2.6 +/- 0.1, C(c) = 289 +/- 20 mg/ml), but that T(c) is not the same. For gamma-II- and gamma-IIIb-crystallin, T(c) almost-equal-to 5-degrees-C, whereas for gamma-IIIa- and gamma-IVa-crystallin, T(c) almost-equal-to 38-degrees-C. By comparing the published protein sequences for calf, rat, and human gamma-crystallins, we postulate that a few key amino acid residues account for the division of gamma-crystallins into low-T(c) and high-T(c) groups.