Interaction of α‐crystallin with lens plasma membranes

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Abstract
The binding of the major water-soluble lens protein α-crystallin to the lens plasma membrane has been investigated by reassociating purified α-crystallin with α-crystallin-depleted membranes and with phospholipid vesicles in which the lens membrane protein MP26 had been reconstituted. α-Crystallin reassociates at high affinity (Kd= 13 × 10−8 M) with alkali-washed lens plasma membranes but not with lens plasma membranes treated with guanidine/HCl, nor with phospholipid vesicles or erythrocyte membranes. Binding to lens plasma membranes is dependent on salt, temperature and pH and occurs in a saturable manner. Reconstitution of MP26 into phospholipid vesicles and subsequent analysis of α-crystallin binding suggests the involvement of this transmembrane protein. Binding ist not influenced by pretreatment of membranes with proteases, suggesting that the 4-kDa cytoplasmic fragment of MP26 is not necessary for α-crystallin binding. Labeling experiments using (trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine as a probe for intrinsic membrane proteins further showed that α-crystallin contains hydrophobic regions on its surface which might enable this protein to make contact with the lipid bilayer. Newly synthesized α-crystallin, in lens culture, is not associated with the plasma membrane, suggesting that the assembly of α-crystallin aggregates does not take place in a membrane-bound mode.