INTERMOLECULAR INTERACTION OF LENS CRYSTALLINS - FROM ROTATIONALLY MOBILE TO IMMOBILE STATES AT HIGH-PROTEIN CONCENTRATIONS

The conformation of lens crystallins in vivo or in a highly concentrat ed solution is not well established. Most studies were carried out in dilute solutions in which protein-protein interaction is minimal. In o rder to see whether there is conformational change (tertiary and secon dary structures) when crystallin solutions are brought to high concent rations, we have performed the following molecular spectroscopic measu rements: circular dichroism (CD) and Fourier transform infrared (FTIR) . Near-UV CD measurements showed a more than two-fold increase in CD i ntensity (molar ellipticity) for the total water-soluble (WS) protein from young calf lens nucleus in a highly concentrated solution (>300 m g/ml in a 0.01-mm cell), when compared with a dilute solution (1000-fo ld dilution in a 10-mm cell), The individual crystallins in concentrat ed solutions also showed an increase in CD intensity, but of different magnitude: alpha-crystallin > beta-crystallin > gamma-crystallin. The increased CD indicates that lens crystallins are in a more compact st ructure in highly concentrated solutions; they likely undergo a transi tion from a mobile to an immobile state. Change in near-UV CD usually is caused by restricted mobility of aromatic side groups, particularly Trp. The transition involves not only a change in protein tertiary an d/or quaternary structure, but also in protein backbone structure. The change of protein backbone structure was drawn from FTIR measurements . FTIR spectra, sensitive to the secondary structure in the amide I re gion, could be measured for a highly concentrated solution for which f ar-UV CD measurement is not feasible. The secondary structure that sho wed prominent change for alpha-crystallin in a highly concentrated sol ution was beta-conformation: increase in beta-turn with a concomitant decrease of alpha-helix structure. (C) 1998 Academic Press.