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Protein Secretion: Puzzling Receptors Christoph Thiele, Hans-Hermann Gerdes and Wieland B R496 Dispatch Protein secretion: Puzzling receptors Christoph Thiele, Hans-Hermann Gerdes and Wieland B. Huttner All known sorting receptors for soluble cargo in the sorting at these two levels appears to vary between cell secretory pathway are transmembrane proteins. For types. In the most abundant cell type endowed with regu- sorting to the regulated pathway, however, a lated protein secretion, the neuron, membrane traffic to subpopulation of secretory proteins, associated with the either the somatodendritic or the axonal cell surface membrane but not membrane-spanning, appears to link diverges in the trans-Golgi network [7], which is confined cargo and membrane in storage granule biogenesis. to the neuronal cell body. In neurons, therefore, the trans- Golgi network is likely to be a major site of protein sorting Address: Department of Neurobiology, University of Heidelberg, Im Neuenheimer Feld 364, D-69120 Heidelberg, Germany. during the biogenesis of neuropeptide-containing secre- tory granules, which typically are targeted to the axon, and Current Biology 1997, 7:R496–R500 vesicles targeted to the dendrites. (Some protein sorting may continue as neuronal secretory granules undergo mat- http://biomednet.com/elecref/09609822007R0496 uration during axonal transport.) © Current Biology Ltd ISSN 0960-9822 In exocrine cells, the rate of vesicle traffic to the basolat- Secretory granules are specialized organelles that mediate eral surface for cell maintenance, relative to that of apical regulated protein secretion — the exocytotic release of secretory granules to fulfill the specialized function of the proteins in response to a stimulus. They contain a selected cell, is presumably much less than in a peptidergic neuron set of the soluble and membrane-bound proteins that, fol- with an extensive dendritic tree, such as a cerebellar Purk- lowing synthesis in the rough endoplasmic reticulum, are inje cell. Hence, in exocrine cells, traffic from the trans- transported to, and through, the Golgi complex. This Golgi network into secretory vesicles other than immature selection is the result of protein sorting that occurs during secretory granules may be insignificant. The same appears assembly of the secretory granule matrix and membrane to hold true for certain endocrine cells, such as pancreatic [1,2]. Protein sorting can lead either to enrichment of a islet β-cells. In such endocrine and exocrine cells, most given protein in secretory granules relative to constitutive protein sorting during the course of secretory granule bio- secretory vesicles, as is the case for the regulated secretory genesis is thought to take place in the immature secretory proteins, or to a failure of the protein to appear in secre- granule [6]. tory granules, as is the case for certain membrane proteins destined for the plasma membrane [3] (Figure 1). Neuroendocrine cell lines, which have been frequently used as model systems for studying secretory granule bio- One hypothesis for how secretory proteins are sorted to genesis, resemble neurons in that secretory granule secretory granules postulates the existence of membrane protein sorting occurs at the level of the trans-Golgi receptors that bind regulated secretory proteins at the network [2], but they may in addition exhibit protein level of the trans-Golgi network and direct them to secre- sorting at the level of the immature secretory granule as tory granules [1]. Using pro-opiomelanocortin (POMC) as seen in certain exocrine and endocrine cells [3,6]. Studies their model regulated secretory protein, Loh and col- with pheochromocytoma (PC12) cells have shown [8] that leagues [4] have recently suggested that the membrane- distinct constitutive and regulated secretory markers are associated form of carboxypeptidase E (CPE) — an quantitatively segregated from each other upon exit from enzyme found in secretory granules of neuroendocrine the trans-Golgi network. Thus, markers such as free gly- cells that is involved in the generation of biologically cosaminoglycan chains [2] and α1-antitrypsin (A. Krömer, active peptides from larger precursors [5] — functions as and H.-H. G., unpublished observations) are largely pack- such a sorting receptor. Before discussing the evidence for aged into constitutive secretory vesicles, and markers such and against this proposition, it is useful to review certain as chromogranin B and secretogranin II are largely pack- aspects of protein sorting during secretory granule biogen- aged into immature secretory granules. esis that have a bearing on the case. In neuroendocrine cells where there is significant mem- Two levels of sorting to the regulated pathway of protein brane traffic into both constitutive secretory vesicles and secretion can be distinguished: sorting in the trans-Golgi immature secretory granules, passive flow — traffic by network during the formation of immature secretory gran- ‘default’ — of secretory cargo from the trans-Golgi network ules (vesicular intermediates in secretory granule biogene- appears to occur largely into constitutive secretory vesicles sis), and sorting in the immature secretory granules during rather than immature secretory granules. Thus, most sul- their maturation [1–3,6] (Figure 1). The relative extent of fophilin — an artificial secretory protein consisting of Dispatch R497 Figure 1 (a) Neurons and neuroendocrine cell lines (b) Endocrine and exocrine cells TGN TGN 1 2 3 132 ISG ISG 4 5 45 CSV IDV MSG CSV IDV MSG Constitutive Constitutive-like Regulated Constitutive Constitutive-like Regulated secretion secretion secretion secretion secretion secretion 1 Constitutive secretory proteins (red circles) exit from the trans-Golgi network (TGN) by passive flow in constitutive secretory vesicles (CSVs) 2 Aggregates of regulated secretory proteins (light green symbols) exit from the TGN in immature secretory granules (ISGs), via interaction with their membrane-associated counterparts (dark green symbols); constitutive secretory proteins are excluded from these aggregates 3 Lysosomal enzymes (blue circles) exit from the TGN in ISGs bound to mannose-6-phosphate receptors (black forks), which interact with a patchy clathrin coat 4 Receptor-bound lysosomal enzymes are removed from the ISG in clathrin-coated, immature secretory granule-derived vesicles (IDVs); IDVs also remove some soluble processing products of the regulated secretory proteins (green triangles), which are excluded from the condensing regulated secretory proteins and enter the IDVs by passive flow 5 During ISG maturation, regulated secretory proteins condense; they remain with the mature secretory granule (MSG) © 1997 Current Biology Secretory proteins can be sorted at either of two levels during arrow), sorting occurs predominantly at the ISGs. In these cells, some secretory granule biogenesis — at the trans-Golgi network (TGN) or of the constitutive secretory proteins, though excluded from the the immature secretory granule (ISG). In neurons and neuroendocrine aggregates of regulated secretory proteins, exit from the TGN in ISGs. cell lines (left), in which there is signficant membrane traffic into both These constitutive secretory proteins are removed from the ISGs by constitutive secretory vesicles (CSVs) and ISGs — indicated by the immature secretory granule-derived vesicles (IDVs). Note that equal thickness of the arrows — sorting occurs primarily at the TGN. In lysosomal enzymes may also exit from the TGN in distinct clathrin- certain endocrine and exocrine cells (right), in which there is minimal coated vesicles, which are not shown for clarity. membrane traffic into CSVs (thin arrow) relative to the ISGs (thick twelve tandem repeats of the heptapeptide tyrosine sulfa- B [10]. This implies that the immunoglobulin-bearing chro- tion site of the regulated secretory protein chromogranin B mogranin B molecules interacted with other molecules of [9] and so unlikely to contain a sorting signal for packaging the nascent immature secretory granule, and hence the exit into constitutive secretory vesicles — nonetheless exits of chromogranin B from the trans-Golgi network in imma- from the trans-Golgi network in constitutive secretory vesi- ture secretory granules is not by default (Figure 1). cles (A. Krömer and H.-H.G., unpublished observations). Furthermore, in PC12 cells, an immunoglobulin — nor- Three observations provide insight into the intermolecular mally a constitutive secretory protein — was found to be interactions of regulated secretory proteins during sorting diverted to secretory granules when bound to chromogranin to immature secretory granules. One is that regulated R498 Current Biology, Vol 7 No 8 secretory proteins are present at high concentrations in the that of chromogranin B [16], is known to mediate its dimer- lumen of the secretory pathway and exhibit a tendency to ization (C.T. and W.B.H., unpublished observations). self-associate in the lumenal milieu [2]. This leads to their Therefore, the loop-mediated interaction of soluble chro- aggregation in the trans-Golgi network, which underlies mogranin molecules with each other and, presumably, with the electron-dense matrix seen by electron microscopy in their membrane-associated counterparts, is likely to be the trans-Golgi network of cells forming secretory granules required for their sorting to immature secretory granules. [2]. Like the condensation of regulated secretory proteins that occurs in the immature secretory granule — such as POMC also contains an amino-terminal disulfide-bonded that of the
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