Regulation of Protein Synthesis in Chick Oviduct

Abstract

Abstract Administration of estradiol to immature chicks triggers the cytodifferentiation of tubular gland cells which synthesize the major egg white proteins, ovalbumin, conalbumin, ovomucoid, and lysozyme, and modulates the rate at which oviduct proteins are synthesized. Estradiol has three modulatory effects. (a) It produces a 2- to 3-fold increase in the extent of polypeptide initiation, as evidenced by the increase in polysome sizes in the absence of a significant change in the average molecular weight of the proteins being synthesized. Furthermore, the size of ovalbumin-synthesizing polysomes changes significantly after cessation of hormonal treatment. (b) There is about a 40% increase in the rate of polypeptide elongation. The rate of elongation changes more slowly than initiation and is probably rate-limiting for protein synthesis in the estrogen-stimulated chick, whereas initiation is limiting in chicks withdrawn from estrogen treatment. (c) Previous data on polysome accumulation can now be interpreted to indicate that there is a 2- to 3-fold increase in translatable mRNA concentration during hormonal treatment. The product of these three modulatory effects account for the 7-fold increase in the rate of protein synthesis previously reported. The rates of initiation and elongation of ovalbumin and conalbumin were compared after several hormonal treatments known to change their relative rates of synthesis. No differential translational effects were noted indicating that the differences in synthesis of these proteins most likely reflect differences in mRNA concentration. The measured mRNA transit times for egg white proteins at 35° are approximately: 2 min, ovalbumin; 3.6 min, ovomucoid; and 5 min, conalbumin. The ovomucoid transit time is longer than expected considering the size of this polypeptide; however it includes both the time for polypeptide synthesis and assembly of about 25 carbohydrate moieties. Estimates of the number of ovalbumin-synthesizing polysomes and rate of ovalbumin elongation allow a calculation of the amount of ovalbumin synthesized per day. The values obtained are in close agreement with the amount known to be secreted in eggs.