ON THE INTERACTION OF ALPHA-CRYSTALLIN WITH UNFOLDED PROTEINS

alpha-Crystallin, a major protein component of the lens, has chaperone -like properties whereby it prevents destabilised proteins from precip itating out of solution. It does so by forming a soluble high-molecula r-weight (HMW) complex. A spectroscopic investigation of the HMW compl ex formed between a variety of unfolded proteins and bovine alpha-crys tallin is presented in this paper. As monitored by fluorescence spectr oscopy, a large amount of the hydrophobic probe, 8-anilino-1-naphthale ne sulfonate (ANS) binds to the HMW complex implying that the complexe d proteins (alcohol dehydrogenase (ADH), gamma-crystallin and rhodanes e) are bound in an unfolded, possibly molten-globule state. The intera ction between the anionic surfactant, sodium dodecyl sulfate (SDS) and ADH at high temperatures gives rise to a similar large increase in AN S fluorescence to that for the complex between alpha-crystallin and AD H. SDS, like alpha-crystallin, therefore complexes to proteins in thei r unfolded state leaving a large hydrophobic surface exposed to solven t. Unlike other chaperones (e.g., GroEL, DnaK and SecB), alpha-crystal lin does not interact with unfolded, hydrophobic but stable proteins ( e.g., reduced and carboxymethylated alpha-lactalbumin and alpha-casein ). It is concluded that alpha-crystallin will only complex with protei ns that are about to precipitate out of solution, i.e., ones that are severely compromised. H-1-NMR spectroscopy of the HMW complex formed b etween alpha-crystallin and gamma-crystallin indicates that the short C-terminal extension of alpha(B)-crystallin, but not that of alpha(A)- crystallin, has lost its flexibility in the complex implying that the former is involved in interactions with the unfolded gamma-crystallin molecule, possibly electrostatically via its two C-terminal lysine res idues.