The tertiary and quaternary structure of α-crystallin is still a matter of controversy. We have examined α-crystallins isolated at different temperature (4°C, 20°C and 33°C), using equilibrium sedimentation and light scattering. Both techniques give the same structural and interaction parameters (molar mass, second virial coefficient) and complementary information (hydrodynamic radius, hydrodynamic volume).
The quaternary structure changes as a function of the temperature of isolation and processing. On cooling the cytoplasma below 30 °C, the quaternary structure of α-crystallin slowly changes to a larger particle which is unstable at 20 °C. On cooling further to lower temperatures (4 °C), the α-crystallin apparently recovers its stability, so it can be stored for longer times. The structural transition between 33 °C and 4 °C is reversible as we can conclude from our data of α-crystallin isolated and measured at 33 °C and α-crystallin isolated at 4 °C, stored at 33 °C for 24 h and measured at 33 °C.
The high hydrodynamic volume of α-crystallin suggests a very loose structure for this particle: a string of beads or a random coil. This loose structure suggests a rather limited interaction between the peptides and dramatically reduces the light scattering.
This structure can also explain the chaperone activity of the α-crystallin. The loose interaction between the crystallin peptides allows the interaction of the latter with the hydrophobic surfaces of the stressed proteins.
So both functions of α-crystallin, its chaperone activity and its low scattering capacity even at high concentration, are enhanced by its expanded quaternary structure.