Rates of protein synthesis in explanted embryonic chick lens epithelia: differential stimulation of delta-crystallin synthesis.

Cells in the central region of 6-day-old embryonic chick lens epithelia display morphological and biochemical changes, when cultured in medium supplemented with fetal calf serum, comparable to those of lens fiber cells differentiating in vivo. In the present study the rates of synthesis of total protein and of δ-crystallin were quantitated during the first day of culture by measuring (1) ³H-valine incorporation into bulk proteins and into δ-crystallin (isolated by quantitative immunoprecipitation), (2) the specific radioactivity of picomolar amounts of intracellular valine (determined by analysis of the ¹⁴C-dansyl-derivative of ³H-valine), (3) the amount of protein degradation occurring during the labeling period (estimated by “pulse-chase” experiments with cycloheximide), and (4) the number of cells in the explants (counted following dispersal with trypsin-EDTA). The results showed that total protein synthesis increased 1.7-fold per cell during the first 24 hrs in vitro. In contrast, δ-crystallin synthesis increased 2.8-fold per cell during this time. These experiments establish that δ-crystallin synthesis is differentially stimulated in epithelia cultured in serum-supplemented medium, and provide the basis for quantitative analysis of the mechanism controlling differential protein synthesis during lens fiber differentiation in vitro.