Membrane insertion of αA-crystallin is oligomer-size dependent

• We investigate the membrane-binding ability of αA-crystallin.
• αA-crystallin can insert into lipid monolayer.
• Membrane-binding ability of αA-crystallin positively correlates to oligomer size.
• Cataract-causing mutations alter the membrane-binding ability of αA-crystallin.
• Change in membrane association may be one of the mechanisms of cataract genesis.

Vertebrate lens is one of the tissues with the highest soluble protein concentration. The predominant soluble proteins in lens fiber cells are crystallins, and among them, α-crystallins belong to the small heat shock protein family with chaperone-like activity. Although α-crystallins are highly soluble in waters, α-crystallins have been detected in the membrane-bound fraction of lens, which will increase in the aged or cataractous lens. In this research, we found αA-crystallin exhibited a complex thermal transition with remarkable changes in secondary and quaternary structures. Treatment of αA-crystallin at high temperatures induced larger oliogomers with higher hydrophobic exposure. Both heat-treated and untreated αA-crystallin could insert into lipid monolayer directly as revealed by monolayer surface pressure experiments. Heat-treatment facilitated the membrane insertion of αA-crystallin and increased the membrane-bound fraction in the cells. The membrane-binding ability of αA-crystallin could be altered by cataract-causing mutations R116C, R116H and Y118D. Our results suggested that the irreversible changes in oligomer size induced by various stresses might promote the membrane association of αA-crystallin and therefore might play a role in aged cataract. Alternations in the membrane binding ability of α-crystallins might be important to the understanding of both aged and congenital cataracts.

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