NMR spectroscopy of alpha-crystallin. Insights into the structure, interactions and chaperone action of small heat-shock proteins.
The subunit molecular mass of alpha-crystallin, like many small heat-shock proteins (sHsps), is around 20 kDa although the protein exists as a large aggregate of average mass around 800 kDa. Despite this large size, a well-resolved 1H NMR spectrum is observed for alpha-crystallin which arises from short, polar, highly-flexible and solvent-exposed C-terminal extensions in each of the subunits, alpha A- and alpha B-crystallin. These extensions are not involved in interactions with other proteins (e.g. beta- and gamma-crystallins) under non-chaperone conditions. As determined by NMR studies on mutants of alpha A-crystallin with alterations in its C-terminal extension, the extensions have an important role in acting as solubilising agents for the relatively-hydrophobic alpha-crystallin molecule and the high-molecular-weight (HMW) complex that forms during the chaperone action. The related sHsp, Hsp25, also exhibits a flexible C-terminal extension. Under chaperone conditions, and in the HMW complex isolated from old lenses, the C-terminal extension of the alpha A-crystallin subunit maintains its flexibility whereas the alpha B-crystallin subunit loses, at least partially, its flexibility, implying that it is involved in interaction with the 'substrate' protein. The conformation of 'substrate' proteins when they interact with alpha-crystallin has been probed by 1H NMR spectroscopy and it is concluded that alpha-crystallin interacts with 'substrate' proteins that are in a disordered molten globule state, but only when this state is on its way to large-scale aggregation and precipitation. By monitoring the 1H and 31P NMR spectra of alpha-crystallin in the presence of increasing concentrations of urea, it is proposed that alpha-crystallin adopts a two-domain structure with the larger C-terminal domain unfolding first in the presence of denaturant. All these data have been combined into a model for the quaternary structure of alpha-crystallin. The model has two layers each of approximately 40 subunits arranged in an annulus or toroid. A large central cavity is present whose entrance is ringed by the flexible C-terminal extensions. A large hydrophobic region in the aggregate is exposed to solution and is available for interaction with 'substrate' proteins during the chaperone action.
Carver
J A
JA
Department of Chemistry, University of Wollongong, NSW, Australia. john_carver@uow.edu.au
Lindner
R A
RA
D000375:Aging / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D003459:Crystallins / Q000737:chemistry* / Q000378:metabolism; D006360:Heat-Shock Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D066298:In Vitro Techniques; D046911:Macromolecular Substances; D009682:Magnetic Resonance Spectroscopy; D051379:Mice; D008958:Models, Molecular; D018832:Molecular Chaperones / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D008970:Molecular Weight; D009154:Mutation; D009363:Neoplasm Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011487:Protein Conformation; D011489:Protein Denaturation; D014508:Urea
D003459:Crystallins; D006360:Heat-Shock Proteins; C486113:Hsbp1 protein, mouse; D046911:Macromolecular Substances; D018832:Molecular Chaperones; D009363:Neoplasm Proteins; D014508:Urea