Temperature induced structural changes of beta-crystallin and sphingomyelin binding

The study of the binding of alpha-crystallin to membranes is potentially important for understanding the function of alpha-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 alpha-crystallin to sphingomyelin liposomes, and the structural changes of alpha-crystallin and sphingomyelin induced by temperature were studied. The influence of the binding of alpha-crystallin on the mobility of the head group region of liposomes of sphingomyelin was dependent on the thermal history of alpha-crystallin. Binding of alpha-crystallin to sphingomyelin caused a decrease in the anisotropy of the fluorophore NBD-PE at or below 37 degrees C. However, when alpha-crystallin or the mixture of alpha-crystallin/sphingomyelin were preincubated near the secondary structure phase transition temperature of 60 degrees C, an increase of the anisotropy of NBD-PE (decrease of lipid head group mobility) was observed when measured at 22 degrees C or 37 degrees C. An inflection near 47 degrees C in the curve of fluorescence anisotropy of bis-ANS pre-incorporated into the alpha-crystallin corresponded to a 3 degrees or 4 degrees structural change of alpha-crystallin. alpha-Crystallin either increases or decreases the flexibility of the head group of sphingomyelin liposomes depending on its structure.