Temperature Induced Structural Changes ofβ-Crystallin and Sphingomyelin Binding

Abstract

The study of the binding of α-crystallin to membranes is potentially important for understanding the function of α-crystallin in the ocular lens and the formation of cataracts. Using fluorescence probes,N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine, triethylammonium salt (NBD-PE) and (1,1′-bi(4-anilino)naphthalene-5,5′-disulfonic acid, dipotassium salt (bis-ANS), the temperature dependence of the binding of α-crystallin to sphingomyelin liposomes, and the structural changes of α-crystallin and sphingomyelin induced by temperature were studied. The influence of the binding of α-crystallin on the mobility of the head group region of liposomes of sphingomyelin was dependent on the thermal history of α-crystallin. Binding of α-crystallin to sphingomyelin caused adecreasein the anisotropy of the fluorophore NBD-PE at or below 37°C. However, when α-crystallin or the mixture of α-crystallin/sphingomyelin were preincubated near the secondary structure phase transition temperature of 60°C, anincreaseof the anisotropy of NBD-PE (decrease of lipid head group mobility) was observed when measured at 22°C or 37°C. An inflection near 47°C in the curve of fluorescence anisotropy of bis-ANS pre-incorporated into the α-crystallin corresponded to a 3° or 4° structural change of α-crystallin. α-Crystallin either increases or decreases the flexibility of the head group of sphingomyelin liposomes depending on its structure.