Excess secretory products fuse with lysosomes

Excess secretory products fuse with lysosomes ells known to spew secretory protein granules had been helpful in deciphering and defining the secretory pathway. But what happened when secretion systems were turned off? Lysosomes were known to degrade foreign proteins taken up by cells and even autodigest intracellular membranous structures—but what was the fate of excess endogenous protein? To ask that question, Smith and Farquhar (1966) needed a secretion system that could be manipulated in the lab (and without the benefit of today's inducible gene expression systems). Lactating rats provided prolactin-secreting pituitary cells that fit the bill as " it was easy to cut off secretion by removing the suckling babies and then ask, how would the cells adapt? " says The cells, says Farquhar, were " devoted to pushing out prolactin, " at least until the babies were removed. At that point the duo brought in the new and powerful technique of enzyme histochemistry to localize lytic activity C The first supper y 1963, lysosomes were well established as an in vitro degradative entity localized to a few fractions (de Duve et al., 1955; de Duve, 1963). But a corresponding in vivo classification was trickier due to the heterogeneity of structures seen in different cell types and within cells. Christian De Duve had grouped lysosome-like entities into a system of four types of compartments: enzyme-storing granules, digestive vacuoles for reabsorbing proteins, autolytic vacuoles, and residual bodies containing the remnants of digestion. These compartments had enzymes such as acid phos-phatase. But did the same compartments have both enzymes and meaningful protein substrates ? The advent of lyso-somal enzyme tests, which gave a lead precipitate reaction product visible by EM (Novikoff and Holt, 1957; Essner and Novikoff, 1961), gave Miller and Palade (1964) a method to test for colocalization. Fritz Miller and George Palade injected rats and mice B Enzymes (black deposits of reaction product) colocalize with substrates (ingested ferritin; small black particles) in lysosomes. PALADE (Miller and Palade, 1964), which could " bridge the gap between [fractionation] biochemistry and EM. " The traditional assumption was that cells would simply store excess secretory granules until they were needed again. So the researchers were surprised to observe that immature granules fused with multi-vesicular bodies (MVBs) and mature granules fused with " dense bodies " or ly-sosomes. Previous work suggested that MVBs were intermediates in lysosomal degradation of endocytosed proteins in renal and nerve …