the pig yolk sac II. Analysis of synthesized proteins.

Since previous morphological studies have revealed abundant rough endoplasmic reticulum in the yolk sac endoderm, pig yolk sac explants from 30 day old embryos were incubated for 3-12 h with [3H]-L-leucine in order to study their protein biosynthesis. They were fractionated into a 12,000 X g-pellet, 105,000 X g-pellet, and supernatant. Newly synthesized proteins in these tissue fractions, and proteins discharged into the culture medium, were analysed with the aid of scintillation technique and identified by column chromatography, SDS-polyacrylamide gel electrophoresis with urea, isoelectrofocusing, and 2D-electrophoresis. Most of the radioactivity incorporated into the tissue fractions was regained from the coarse pellet and was located in the molecular weight region between 70,000 and 45,000 daltons, indicating that most of the newly synthesized proteins are membrane bound and include albumin. Albumin, an acid protein of a MW around 80,000 daltons, and many neutral and basic peptides were present in the culture medium. The yolk sac endodermal cells of the pig synthesize less proteins than those of the cat, although the pig cells display much larger amounts of endoplasmic reticulum.