Pinocytosis and protein degradation in the rat visceral yolk sac

Abstract

The rat visceral yolk sac (17.5 days gestation) was incubated in vitro to try to elucidate some of the factors important in controlling pinocytosis and protein degradation. An investigation of four closely- related ribonuclease molecules revealed that a minor structural change in a protein (possibly of a type that occurs in vivo) greatly enhanced its rate of uptake by the yolk-sac tissue, supporting the suggestion that proteolytic "nicking" may be a signal for removal of a protein from the circulation. This study also showed there to be no mannose receptor on the surface of the yolk sac, and that the presence of a small oligosaccharide on the surface of the ribonuclease molecule neither hindered its entry into the tissue, nor affected its subsequent rate of digestion.
The maximum pinocytic rate was exhibited when yolk sacs were incubated in serum-free medium. Addition of serum (up to 50%, v/v) led to a progressive fall in both the pinocytic rate and the rate of degradation of internalized radiolabelled-protein. Addition of amino acids and formaldehyde-denatured BSA to the incubation medium also led to a fall in the rate of degradation of endogenous proteins (data provided by F.J.B. and S.E.K.). These findings support the suggestion that the control of pinocytosis and autophagy may be related by virtue of them both being membrane-related processes.
The effects of a variety of agents (insulin, cycloheximide, puromycin and bacitracin) on pinocytosis and on proteolysis of exogenous and endogenous proteins (the latter data compiled by F.J.B. and S.E.K.) were examined, and compared with the effects in other tissues, as reported in recent literature. Neither of two hypotheses (one based on membrane movement and one on response to intracellular amino acid levels), postulated to relate the rate of pinocytosis to the rate of degradation of endogencms proteins, is supported by the data.