The stability of human acidic beta-crystallin oligomers and hetero-oligomers.

Crystallins are bulk structural proteins of the eye lens that have to last a life time. They gradually become modified with age, denature and form light scattering centres. High thermodynamic and kinetic stability of the crystallins enables them to resist unfolding and delay cataract. Here we have made recombinant human betaA1-, betaA3-, and betaA4-crystallins. The betaA3-crystallin formed higher oligomers that lead to precipitation at ambient temperature. Heat-induced precipitation of betaA3-crystallin was compared with human and calf betaB2-crystallins, showing that the human proteins start to precipitate above 50 degrees C while the calf betaB2-crystallin stays in solution even when unfolded. The stabilities of these human acidic beta-crystallin homo-oligomers have been estimated by measuring their unfolding in urea at neutral pH. BetaA3/1/betaB1 and betaA4/betaB1-crystallin hetero-oligomers have been prepared from homo-oligomers by subunit exchange. The resolution of the methodology used was insufficient to detect a stabilization of the betaA4-crystallin subunit in the hetero-oligomer, the betaA1-crystallin subunit was clearly stabilized by its interaction with betaB1-crystallin. Circular dichroism and fluorescence spectroscopies show that homo-dimer surface tryptophans become buried in the betaA3/1/betaB1-crystallin hetero-dimer concomitant with changes in polypeptide chain conformation.

Characterization of two novel human small heat shock proteins: protein kinase-related HspB8 and testis-specific HspB9.

Using search profiles based on the conserved alpha-crystallin domain that is characteristic for small heat shock proteins (sHsps), we traced two new human sHsps. One of these, being the eighth known human sHsp and thus named HspB8, was recently described as a serine-threonine protein kinase (H11), but not identified as an sHsp (C.C. Smith, Y.X. Yu, M. Kulka, L. Aurelian, J. Biol. Chem. 275 (2000)). Northern blotting showed that HspB8/H11 is predominantly transcribed in skeletal muscle and heart, like most other sHsps. The other, named HspB9, is specifically expressed in testis, notably in the spermatogenic cells from late pachytene spermatocyte stage till elongate spermatid stage. While mammalian sHsps are generally highly conserved, mouse HspB9 shows 38% sequence difference with human HspB9, which may confirm its sex-related role.