Factors influencing α-crystallin association with phospholipid vesicles

α-Crystallin is the major protein of vertebrate lenses. It is found in the water soluble and insoluble fractions of lens fiber cells and can represent up to 50% of the total soluble protein [1,2]. α-Crystallin is a large heteromeric complex containing 30 to 40 copies of two closely related subunits, αA(αA) and αB-crystallin (αB), in roughly a 3:1 ratio in humans [2]. The αA protein is found exclusively in the lens whereas αB is also found in skeletal muscle, skin, brain, and several other tissues [3,4]. Both subunits are members of the small heat shock protein family and can suppress stress-induced protein aggregation in vitro while conferring heat shock tolerance to mammalian tissue culture cells [5-7]. The biological function of α-crystallin in the lens is still not completely evident. Its ability to inhibit stress-induced protein aggregation likely represents a prominent role in the maintenance of lens transparency through blocking the formation of protein inclusion bodies, which could scatter light and cause opacification [5]. However, this chaperone-like activity (CLA) is not the only functional capability observed for α-crystallin. It has been shown that α-crystallin can associate with both lens plasma membranes as well as certain types of intermediate filaments, particularly the beaded filament, yet the functional significance of these interactions is not well understood [8]. As the lens ages, changes are seen in the distribution of α-crystallin within fiber cells. Soluble α-crystallin is slowly lost to the water insoluble fraction of the lens with age and at the onset of cataract; however, the nature of the water insoluble fraction is not well characterized [1,9-12]. Water insoluble αcrystallin could be disorganized aggregates of protein and/or filamentor membrane-bound protein. Interestingly, other in vivo studies have shown that the amount of membrane associated crystallin protein, particularly α-crystallin, increases with age and with the onset of cataract, supporting the hypothesis that water insoluble α-crystallin may actually be membrane bound [13,14]. In addition, our recent data on the characterization of the αA R116C congenital cataract mutant supports a model where increased membrane binding may be a critical event in the mechanism of some forms of cataract [15]. The membranes in the lens fiber cells are quite unusual with respect to other cell membranes. They are extremely rich in aquaporin 0, a lens-specific water channel which makes up nearly 50% of the total intrinsic membrane protein in the lens [16]. Other major protein components include gap junctions and cell adhesion molecules [17]. Interestingly, vertebrate lenses have a peculiar lipid composition (Table 1). An estimated 43% of all phospholipids in the lens is 4,5dihydrosphingomyelin (DH-SPH) [18-20]. Prior to 1994, this © 2002 Molecular Vision