α-Crystallin polypeptides as markers of lens cell differentiation

Abstract

THE acquisition of specific biochemical characteristics during cell differentiation is thought to be due to differential gene activation, and the vertebrate eye lens provides a useful tool for investigation of the process^1,2. The lens arises from epithelial cells arranged in a monolayer at the anterior side of the organ and on differentiation they pass through an elongation zone, becoming fibre-like concomitant with a large increase in volume and rapid synthesis of crystallins (Fig. 1). One of the prominent lens proteins, α-crystallin (composed of subunits αA₁, αA₂, αB₁ and αB₂³ has been suggested as a marker for various biological processes, particularly terminal differentiation, because changes in the subunit composition occur during the transition from epithelial to fibre cells. Delcour and Papaconstantinou reported a change in the stoichiometry of the αB₂ and αA₂ chains of α-crystallin during differentiation from 1 : 2 in the epithelial cell to 1 : 3 in the lens fibres⁴. However, changes in subunit composition also occur with ageing of the fibre cell due to the alteration of pre-existing subunits in that αA₁ and αB₁ probably arise from deamidation of αA₂ and αB₂^5,6. Furthermore, other chains are formed by post-trans-lational degradation starting from the C-terminus of preexisting polypeptides^{7, 8}. We have now studied protein synthesis in two different parts of the epithelial monolayer and found further evidence for differential gene activation such that in the central region more αB₂ is synthesised than αA₂.