Immobilization of the C-terminal extension of bovine alphaA-crystallin reduces chaperone-like activity.
alpha-Crystallins occur as multimeric complexes, which are able to suppress precipitation of unfolding proteins. Although the mechanism of this chaperone-like activity is unknown, the affinity of alpha-crystallin for aggregation-prone proteins is probably based on hydrophobic interactions. alpha-Crystallins expose a considerable hydrophobic surface to solution, but nevertheless they are very stable and highly soluble. An explanation for this paradox may be that alpha-crystallin subunits have a polar and unstructured C-terminal extension that functions as a sort of solubilizer. In this paper we have described five alphaA-crystallins in which charged and hydrophobic residues were inserted in the C-terminal extension. Introduction of lysine, arginine, and aspartate does not substantially influence chaperone-like activity. In contrast, introduction of a hydrophobic tryptophan greatly diminishes functional activity. CD experiments indicate that this mutant has a normal secondary structure and fluorescence measurements show that the inserted tryptophan is located in a polar environment. However, NMR spectroscopy clearly demonstrates that the presence of the tryptophan residue dramatically reduces the flexibility of the C-terminal extension. Furthermore, the introduction of this tryptophan results in a considerably decreased thermostability of the protein. We conclude that changing the polarity of the C-terminal extension of alphaA-crystallin by insertion of a highly hydrophobic residue can seriously disturb structural and functional integrity.
Department of Biochemistry, University of Nijmegen, 6500 HB Nijmegen, The Netherlands. w.dejong@bioch.kun.nl
Smulders RHPH
J A
JA
Carver
R A
RA
Lindner
M A
MA
van Boekel
H
H
Bloemendal
W W
WW
de Jong
D000818:Animals; D002417:Cattle; D002850:Chromatography, Gel; D003459:Crystallins / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D018076:DNA, Complementary; D006358:Hot Temperature; D018832:Molecular Chaperones / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D013050:Spectrometry, Fluorescence; D014364:Tryptophan / Q000737:chemistry
D003459:Crystallins; D018076:DNA, Complementary; D018832:Molecular Chaperones; D014364:Tryptophan