Secretion-coupled protein degradation: Studies on mammary casein

Abstract

Mammary explants from midpregnant rabbits were cultured for 18 h at 37°C with insulin, prolactin and cortisol. Subsequently, explants were labelled for 2 h with inorganic [ 32P]phosphate, l-[5- 3H]proline or l-[4,5- 3H]leucine, washed and chased for up to 3 h. The radiolabelling profile of [ 32P]casein or [ 3H]casein during the chase period, obtained by isoelectric focussing or immunoprecipitation indicates extensive destruction of neosynthesized casein. The extent of casein destruction in mammary explants in culture (measured after radiolabelling with l-[5- 3H]proline), is inversely related to casein secretion. Least casein degradation is observed in explants after 48 h in culture when casein secretion is maximal (observed histochemically). Subsequently, when the extracellular alveolar lumen is filled with secretion products (72 h), rapid intracellular casein destruction is again observed. When the chase was carried out in the presence of drugs which inhibit degradation and/or secretion, the results indicate that (i) secretion-coupled casein degradation is dependent on an intact functional microfilamentous-microtubular network, (ii) casein is not degraded by an autophagosome requiring process, (iii) degradation is inhibited by leupeptin, (iv) amino-acid analogue containing casein does not undergo secretion-coupled degradation and (v) inhibition of N-glycosylation of intracellular vesicular membrane proteins prevents secretion-coupled degradation. Secretion-coupled protein destruction is discussed in relation to the post-translational regulation of the net production of secretory proteins in eukaryotic cells.