Protein Secretory Pathways

Abstract

In many cells proteins can be secreted from particular sites of the membrane or in response to specific stimuli. The eukaryotic cell has developed mechanisms to ensure that proteins are targeted to the correct secretory pathway.

Keywords: lipid rafts, secretory pathways; sorting receptors; sorting signals; trans‐Golgi network

Figure 1.

Overview of the secretory pathway. The trans‐Golgi network (TGN) is the cellular compartment where the different proteins start the sorting process that will finally result in secretion. Lysosomal enzymes (squares) bind to mannose‐6‐phosphate receptors (–[) that direct targeting to the lysosomes. Regulated secretory proteins (small circles) begin to aggregate in the TGN and this may, in turn, trigger association of the proteins with membrane lipid rafts (small ellipses). This association directs packaging into immature secretory granules (ISG) which ultimately become the dense‐core mature secretory granules (MSG) of the regulated secretory pathway. In this model, some proteins exit the TGN already sorted and packaged into transport vesicles, destined for the lysosomes or to be constitutively secreted, whereas other proteins enter the ISG only partially sorted. Refinement of the granule components and hence final sorting occurs at the ISG by removal of constitutively secreted proteins and lysosomal proteins during granule maturation. Regulated secretory proteins may be retained in the maturing granule due to their aggregation and association with the membrane. COP: coatomer protein.

Figure 2.

Sorting via lipid rafts. Proteins to be secreted via the regulated secretory pathway (small circles and crosses) are initially sorted in the TGN. Owing to the acidic pH and the high Ca2+ concentration encountered in the TGN, proteins form aggregates that associate with specific microdomains in the membrane termed lipid rafts (small ellipses). This association can occur via association with other proteins that could be acting as a sorting receptor (triangles) and thus anchoring the aggregated proteins to the lipid rafts, or directly with the lipid components of the rafts. The nascent vesicle containing the proteins buds from the TGN to form the ISG, which after maturation and fusion with other granules results in the formation of an MSG whose contents will be secreted upon stimulation of the cell with a secretagogue.

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References

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Further Reading

Barlowe C (1998) COPII and selective export from the endoplasmic reticulum. Biochimica et Biophysica Acta 1404: 67–76.

Brown DA and London E (1998) Functions of lipid rafts in biological membranes. Annual Review of Cell and Developmental Biology 14: 111–136.

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Tooze SA (1998) Biogenesis of secretory granules in the trans‐Golgi network of neuroendocrine and endocrine cells. Biochimica et Biophysica Acta 1404: 231–244.

Zegers MMP and Hoekstra D (1998) Mechanisms and functional features of polarized membrane traffic in epithelial and hepatic cells. Biochemical Journal 336: 257–269.

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How to Cite close
Blázquez, Mercedes, and Shennan, Kathleen IJ(Jan 2006) Protein Secretory Pathways. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0005723]