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 /(Cc-C)/Cc/ = A [(Tc - T)/Tc]beta, where beta = 0.325, Cc is the critical protein concentration in mg/ml, Tc 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 Cc are approximately the same for all four coexistence curves (A = 2.6 +/- 0.1, Cc = 289 +/- 20 mg/ml), but that Tc is not the same. For gamma II- and gamma IIIb-crystallin, Tc approximately 5 degrees C, whereas for gamma IIIa- and gamma IVa-crystallin, Tc approximately 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-Tc and high-Tc groups.
Broide
M L
ML
Department of Physics, Massachusetts Institute of Technology, Cambridge 02139.
Berland
C R
CR
Pande
J
J
Ogun
O O
OO
Benedek
G B
GB
D000818:Animals; D002417:Cattle; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003459:Crystallins / Q000737:chemistry* / Q000302:isolation & purification; D003461:Crystallography; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D012996:Solutions; D013696:Temperature
D003459:Crystallins; D012996:Solutions