Differential synthesis and phosphorylation of the α-crystallin A and B chains during bovine lens fiber cell differentiation

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Abstract
[14C]-amino acids and [32P]-orthophosphate incorporation experiments were carried out in bovine lenses in culture to study the synthesis and phosphorylation of α-crystallin A and B polypeptides during differentiation of the lens fiber cells. Following culture, the [14C] or [32P]-labelled α-crystallin was isolated by gel filtration chromatography from four regions of the lens corresponding to: A) quiescent epithelial cells, B) dividing epithelial cells and early stages of elongation, C) young elongating fibers, and D) mature fibers from the superficial cortex. The incorporation of label into the α-crystallin primary gene products αA2 and αB2 and their respective phosphorylated forms αA1 and αB1 was determined by isoelectric focusing and radioautography. Different synthesis and phosphorylation patterns were observed in αA and αB polypeptides. Synthesis and phosphorylation of the αB chain occurs most actively in the epithelial cells, both processes decrease during differentiation and there is no net accumulation of the phosphorylated form αB1 in the mature fiber cell. In contrast to the B chain, the A chain synthesis, minimal in the epithelial cell, increases with differentiation. Most striking, the A chain phosphorylation, not detectable in the epithelial cells, increases with differentiation. In the mature fiber cell, the phosphorylated form αA1 accounts for one third of the A chain. These observations indicate that the two chains may have different functions. The synthesis and phosphorylation patterns of αA suggest a lens-specific function of this polypeptide in the fiber cell and in the terminal differentiation process. The synthesis and phosphorylation patterns of αB suggest a non-lens-specific function in the epithelial cell.