On the interaction of α-crystallin with unfolded proteins

Abstract

α-Crystallin, a major protein component of the lens, has chaperone-like properties whereby it prevents destabilised proteins from precipitating out of solution. It does so by forming a soluble high-molecular-weight (HMW) complex. A spectroscopic investigation of the HMW complex formed between a variety of unfolded proteins and bovine α-crystallin is presented in this paper. As monitored by fluorescence spectroscopy, a large amount of the hydrophobic probe, 8-anilino-1-naphthalene sulfonate (ANS) binds to the HMW complex implying that the complexed proteins (alcohol dehydrogenase (ADH), γ-crystallin and rhodanese) are bound in an unfolded, possibly molten-globule state. The interaction between the anionic surfactant, sodium dodecyl sulfate (SDS) and ADH at high temperatures gives rise to a similar large increase in ANS fluorescence to that for the complex between α-crystallin and ADH. SDS, like α-crystallin, therefore complexes to proteins in their unfolded state leaving a large hydrophobic surface exposed to solvent. Unlike other chaperones (e.g., GroEL, DnaK and SecB), α-crystallin does not interact with unfolded, hydrophobic but stable proteins (e.g., reduced and carboxymethylated α-lactalbumin and α-casein). It is concluded that α-crystallin will only complex with proteins that are about to precipitate out of solution, i.e., ones that are severely compromised. 1H-NMR spectroscopy of the HMW complex formed between α-crystallin and γ-crystallin indicates that the short C-terminal extension of α B-crystallin, but not that of α A-crystallin, has lost its flexibility in the complex implying that the former is involved in interactions with the unfolded γ-crystallin molecule, possibly electrostatically via its two C-terminal lysine residues.