Beclin-phosphatidylinositol 3-kinase complex functions at the trans-Golgi network

Autophagy is an intracellular bulk protein degradation system. Beclin is known to be involved in this process; however, its role is unclear. In this study, we showed that Beclin was co-immunoprecipitated with phosphatidylinositol (PtdIns) 3-kinase, which is also required for autophagy, suggesting that Beclin is a component of the PtdIns 3-kinase complex. Quantitative analyses using a cross-linker showed that all Beclin forms a complex with PtdIns 3-kinase, whereas approximately 50% of PtdIns 3-kinase remains free from Beclin. Indirect immunofluorescence microscopy demonstrated that the majority of Beclin and PtdIns 3-kinase localize to the trans-Golgi network (TGN). Some PtdIns 3-kinase is also distributed in the late endosome. These results suggest that Beclin and PtdIns 3-kinase control autophagy as a complex at the TGN.

Escrt-III: an endosome-associated heterooligomeric protein complex required for mvb sorting

The sorting of transmembrane proteins (e.g., cell surface receptors) into the multivesicular body (MVB) pathway to the lysosomal/vacuolar lumen requires the function of the ESCRT protein complexes. The soluble coiled-coil-containing proteins Vps2, Vps20, Vps24, and Snf7 are recruited from the cytoplasm to endosomal membranes where they oligomerize into a protein complex, ESCRT-III. ESCRT-III contains two functionally distinct subcomplexes. The Vps20-Snf7 subcomplex binds to the endosomal membrane, in part via the myristoyl group of Vps20. The Vps2-Vps24 subcomplex binds to the Vps20-Snf7 complex and thereby serves to recruit additional cofactors to this site of protein sorting. We provide evidence for a role for ESCRT-III in sorting and/or concentration of MVB cargoes.

VPS35 mutations in Parkinson disease

The identification of genetic causes for Mendelian disorders has been based on the collection of multi-incident families, linkage analysis, and sequencing of genes in candidate intervals. This study describes the application of next-generation sequencing technologies to a Swiss kindred presenting with autosomal-dominant, late-onset Parkinson disease (PD). The family has tremor-predominant dopa-responsive parkinsonism with a mean onset of 50.6 ± 7.3 years. Exome analysis suggests that an aspartic-acid-to-asparagine mutation within vacuolar protein sorting 35 (VPS35 c.1858G>A; p.Asp620Asn) is the genetic determinant of disease. VPS35 is a central component of the retromer cargo-recognition complex, is critical for endosome-trans-golgi trafficking and membrane-protein recycling, and is evolutionarily highly conserved. VPS35 c.1858G>A was found in all affected members of the Swiss kindred and in three more families and one patient with sporadic PD, but it was not observed in 3,309 controls. Further sequencing of familial affected probands revealed only one other missense variant, VPS35 c.946C>T; (p.Pro316Ser), in a pedigree with one unaffected and two affected carriers, and thus the pathogenicity of this mutation remains uncertain. Retromer-mediated sorting and transport is best characterized for acid hydrolase receptors. However, the complex has many types of cargo and is involved in a diverse array of biologic pathways from developmental Wnt signaling to lysosome biogenesis. Our study implicates disruption of VPS35 and retromer-mediated trans-membrane protein sorting, rescue, and recycling in the neurodegenerative process leading to PD.

Starvation and ULK1-dependent cycling of mammalian Atg9 between the TGN and endosomes

Autophagy, fundamentally a lysosomal degradation pathway, functions in cells during normal growth and certain pathological conditions, including starvation, to maintain homeostasis. Autophagosomes are formed through a mechanism that is not well understood, despite the identification of many genes required for autophagy. We have studied the mammalian homologue of Atg9p, a multi-spanning transmembrane protein essential in yeast for autophagy, to gain a better understanding of the function of this ubiquitious protein. We show that both the N- and C-termini of mammalian Atg9 (mAtg9) are cytosolic, and predict that mAtg9 spans the membrane six times. We find that mAtg9 is located in the trans-Golgi network and late endosomes and colocalizes with TGN46, the cation-independent mannose-6-phosphate receptor, Rab7 and Rab9. Amino acid starvation or rapamycin treatment, which upregulates autophagy, causes a redistribution of mAtg9 from the TGN to peripheral, endosomal membranes, which are positive for the autophagosomal marker GFP-LC3. siRNA-mediated depletion of the putative mammalian homologue of Atg1p, ULK1, inhibits this starvation-induced redistribution. The redistribution of mAtg9 also requires PI 3-kinase activity, and is reversed after restoration of amino acids. We speculate that starvation-induced autophagy, which requires mAtg9, may rely on an alteration of the steady-state trafficking of mAtg9, in a Atg1-dependent manner.

FAPPs control Golgi-to-cell-surface membrane traffic by binding to ARF and PtdIns(4)P

The molecular mechanisms underlying the formation of carriers trafficking from the Golgi complex to the cell surface are still ill-defined; nevertheless, the involvement of a lipid-based machinery is well established. This includes phosphatidylinositol 4-phosphate (PtdIns(4)P), the precursor for phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)). In yeast, PtdIns(4)P exerts a direct role, however, its mechanism of action and its targets in mammalian cells remain uncharacterized. We have identified two effectors of PtdIns(4)P, the four-phosphate-adaptor protein 1 and 2 (FAPP1 and FAPP2). Both proteins localize to the trans-Golgi network (TGN) on nascent carriers, and interact with PtdIns(4)P and the small GTPase ADP-ribosylation factor (ARF) through their plekstrin homology (PH) domain. Displacement or knockdown of FAPPs inhibits cargo transfer to the plasma membrane. Moreover, overexpression of FAPP-PH impairs carrier fission. Therefore, FAPPs are essential components of a PtdIns(4)P- and ARF-regulated machinery that controls generation of constitutive post-Golgi carriers.

Exiting the Golgi complex

The composition and identity of cell organelles are dictated by the flux of lipids and proteins that they receive and lose through cytosolic exchange and membrane trafficking. The trans-Golgi network (TGN) is a major sorting centre for cell lipids and proteins at the crossroads of the endocytic and exocytic pathways; it has a complex dynamic structure composed of a network of tubular membranes that generate pleiomorphic carriers targeted to different destinations. Live-cell imaging combined with three-dimensional tomography has recently provided the temporal and topographical framework that allows the assembly of the numerous molecular machineries so far implicated in sorting and trafficking at the TGN.

Retrograde transport from endosomes to the trans-Golgi network

A subset of intracellular transmembrane proteins such as acid-hydrolase receptors, processing peptidases and SNAREs, as well as extracellular protein toxins such as Shiga toxin and ricin, undergoes 'retrograde' transport from endosomes to the trans-Golgi network. Here, we discuss recent studies that have begun to unravel the molecular machinery that is involved in this process. We also propose a central role for a 'tubular endosomal network' in sorting to recycling pathways that lead not only to the trans-Golgi network but also to different plasma-membrane domains and to specialized storage vesicles.

Role of diacylglycerol in PKD recruitment to the TGN and protein transport to the plasma membrane

Protein kinase D (PKD) is a cytosolic serine-threonine kinase that binds to the trans-Golgi network (TGN) and regulates the fission of transport carriers specifically destined to the cell surface. PKD was found to bind diacylglycerol (DAG), and this binding was necessary for its recruitment to the TGN. Reducing cellular levels of DAG inhibited PKD recruitment and blocked protein transport from the TGN to the cell surface. Thus, a DAG-dependent, PKD-mediated signaling regulates the formation of transport carriers from the TGN in mammalian cells.

Role of the mammalian retromer in sorting of the cation-independent mannose 6-phosphate receptor

The cation-independent mannose 6-phosphate receptor (CI-MPR) mediates sorting of lysosomal hydrolase precursors from the TGN to endosomes. After releasing the hydrolase precursors into the endosomal lumen, the unoccupied receptor returns to the TGN for further rounds of sorting. Here, we show that the mammalian retromer complex participates in this retrieval pathway. The hVps35 subunit of retromer interacts with the cytosolic domain of the CI-MPR. This interaction probably occurs in an endosomal compartment, where most of the retromer is localized. In particular, retromer is associated with tubular–vesicular profiles that emanate from early endosomes or from intermediates in the maturation from early to late endosomes. Depletion of retromer by RNA interference increases the lysosomal turnover of the CI-MPR, decreases cellular levels of lysosomal hydrolases, and causes swelling of lysosomes. These observations indicate that retromer prevents the delivery of the CI-MPR to lysosomes, probably by sequestration into endosome-derived tubules from where the receptor returns to the TGN.

Glycosphingolipid synthesis requires FAPP2 transfer of glucosylceramide

The molecular machinery responsible for the generation of transport carriers moving from the Golgi complex to the plasma membrane relies on a tight interplay between proteins and lipids. Among the lipid-binding proteins of this machinery, we previously identified the four-phosphate adaptor protein FAPP2, the pleckstrin homology domain of which binds phosphatidylinositol 4-phosphate and the small GTPase ARF1. FAPP2 also possesses a glycolipid-transfer-protein homology domain. Here we show that human FAPP2 is a glucosylceramide-transfer protein that has a pivotal role in the synthesis of complex glycosphingolipids, key structural and signalling components of the plasma membrane. The requirement for FAPP2 makes the whole glycosphingolipid synthetic pathway sensitive to regulation by phosphatidylinositol 4-phosphate and ARF1. Thus, by coupling the synthesis of glycosphingolipids with their export to the cell surface, FAPP2 emerges as crucial in determining the lipid identity and composition of the plasma membrane.

Retention of prominin in microvilli reveals distinct cholesterol-based lipid micro-domains in the apical plasma membrane

Membrane cholesterol-sphingolipid 'rafts', which are characterized by their insolubility in the non-ionic detergent Triton X-100 in the cold, have been implicated in the sorting of certain membrane proteins, such as placental alkaline phosphatase (PLAP), to the apical plasma membrane domain of epithelial cells. Here we show that prominin, an apically sorted pentaspan membrane protein, becomes associated in the trans-Golgi network with a lipid raft that is soluble in Triton X-100 but insoluble in another non-ionic detergent, Lubrol WX. At the cell surface, prominin remains insoluble in Lubrol WX and is selectively associated with microvilli, being largely segregated from the membrane subdomains containing PLAP. Cholesterol depletion results in the loss of prominin's microvillus-specific localization but does not lead to its complete intermixing with PLAP. We propose the coexistence within a membrane domain, such as the apical plasma membrane, of different cholesterol-based lipid rafts, which underlie the generation and maintenance of membrane subdomains.

Spatial coupling of mTOR and autophagy augments secretory phenotypes

Protein synthesis and autophagic degradation are regulated in an opposite manner by mammalian target of rapamycin (mTOR), whereas under certain conditions it would be beneficial if they occurred in unison to handle rapid protein turnover. We observed a distinct cellular compartment at the trans side of the Golgi apparatus, the TOR-autophagy spatial coupling compartment (TASCC), where (auto)lysosomes and mTOR accumulated during Ras-induced senescence. mTOR recruitment to the TASCC was amino acid- and Rag guanosine triphosphatase-dependent, and disruption of mTOR localization to the TASCC suppressed interleukin-6/8 synthesis. TASCC formation was observed during macrophage differentiation and in glomerular podocytes; both displayed increased protein secretion. The spatial coupling of cells' catabolic and anabolic machinery could augment their respective functions and facilitate the mass synthesis of secretory proteins.

Protein kinase D regulates the fission of cell surface destined transport carriers from the trans-Golgi network

When a kinase inactive form of Protein Kinase D (PKD-K618N) was expressed in HeLa cells, it localized to the trans-Golgi network (TGN) and caused extensive tubulation. Cargo that was destined for the plasma membrane was found in PKD-K618N-containing tubes but the tubes did not detach from the TGN. As a result, the transfer of cargo from TGN to the plasma membrane was inhibited. We have also demonstrated the formation and subsequent detachment of cargo-containing tubes from the TGN in cells stably expressing low levels of PKD-K618N. Our results suggest that PKD regulates the fission from the TGN of transport carriers that are en route to the cell surface.

Early/recycling endosomes-to-TGN transport involves two SNARE complexes and a Rab6 isoform

The molecular mechanisms underlying early/recycling endosomes-to-TGN transport are still not understood. We identified interactions between the TGN-localized putative t-SNAREs syntaxin 6, syntaxin 16, and Vti1a, and two early/recycling endosomal v-SNAREs, VAMP3/cellubrevin, and VAMP4. Using a novel permeabilized cell system, these proteins were functionally implicated in the post-Golgi retrograde transport step. The function of Rab6a' was also required, whereas its closely related isoform, Rab6a, has previously been implicated in Golgi-to-endoplasmic reticulum transport. Thus, our study shows that membrane exchange between the early endocytic and the biosynthetic/secretory pathways involves specific components of the Rab and SNARE machinery, and suggests that retrograde transport between early/recycling endosomes and the endoplasmic reticulum is critically dependent on the sequential action of two members of the Rab6 subfamily.

Sorting Nexin-1 Mediates Tubular Endosome-to-TGN Transport through Coincidence Sensing of High- Curvature Membranes and 3-Phosphoinositides

BACKGROUND

Sorting nexins (SNXs) are phox homology (PX) domain-containing proteins thought to regulate endosomal sorting of internalized receptors. The prototypical SNX is sorting nexin-1 (SNX1), a protein that through its PX domain binds phosphatidylinositol 3-monophosphate [PtdIns(3)P] and phosphatidylinositol 3,5-bisphosphate [PtdIns(3,5)P(2)]. SNX1 is associated with early endosomes, from where it has been proposed to regulate the degradation of internalized epidermal growth factor (EGF) receptors through modulating endosomal-to-lysosomal sorting.

RESULTS

We show here that SNX1 contains a BAR (Bin/Amphiphysin/Rvs) domain, a membrane binding domain that endows SNX1 with the ability to form dimers and to sense membrane curvature. We present evidence that through coincidence detection, the BAR and PX domains efficiently target SNX1 to a microdomain of the early endosome defined by high curvature and the presence of 3-phosphoinositides. In addition, we show that the BAR domain endows SNX1 with an ability to tubulate membranes in-vitro and drive the tubulation of the endosomal compartment in-vivo. Using RNA interference (RNAi), we establish that SNX1 does not play a role in EGF or transferrin receptor sorting; rather it specifically perturbs endosome-to-trans Golgi network (TGN) transport of the cation-independent mannose-6-phosphate receptor (CI-MPR). Our data support an evolutionarily conserved function for SNX1 from yeast to mammals and provide functional insight into the molecular mechanisms underlying lipid-mediated protein targeting and tubular-based protein sorting.

CONCLUSIONS

We conclude that through coincidence detection SNX1 associates with a microdomain of the early endosome-characterized by high membrane curvature and the presence of 3-phosphoinositides-from where it regulates tubular-based endosome-to-TGN retrieval of the CI-MPR.

Systematic Analysis of SNARE Molecules in Arabidopsis: Dissection of the post-Golgi Network in Plant Cells

In all eucaryotic cells, specific vesicle fusion during vesicular transport is mediated by membrane-associated proteins called SNAREs (soluble N-ethyl-maleimide sensitive factor attachment protein receptors). Sequence analysis identified a total of 54 SNARE genes (18 Qa-SNAREs/Syntaxins, 11 Qb-SNAREs, 8 Qc-SNAREs, 14 R-SNAREs/VAMPs and 3 SNAP-25) in the Arabidopsis genome. Almost all of them were ubiquitously expressed through out all tissues examined. A series of transient expression assays using green fluorescent protein (GFP) fused proteins revealed that most of the SNARE proteins were located on specific intracellular compartments: 6 in the endoplasmic reticulum, 9 in the Golgi apparatus, 4 in the trans-Golgi network (TGN), 2 in endosomes, 17 on the plasma membrane, 7 in both the prevacuolar compartment (PVC) and vacuoles, 2 in TGN/PVC/vacuoles, and 1 in TGN/PVC/plasma membrane. Some SNARE proteins showed multiple localization patterns in two or more different organelles, suggesting that these SNAREs shuttle between the organelles. Furthermore, the SYP41/SYP61-residing compartment, which was defined as the TGN, was not always located along with the Golgi apparatus, suggesting that this compartment is an independent organelle distinct from the Golgi apparatus. We propose possible combinations of SNARE proteins on all subcellular compartments, and suggest the complexity of the post-Golgi membrane traffic in higher plant cells.

Adaptor-related proteins

Two new adaptor-related protein complexes, AP-3 and AP-4, have recently been identified, and both have been implicated in protein sorting at the trans-Golgi network (TGN) and/or endosomes. In addition, two families of monomeric proteins with adaptor-related domains, the GGAs and the stoned B family, have also been identified and shown to act at the TGN and plasma membrane, respectively. Together with the two conventional adaptors, AP-1 and AP-2, these proteins may act to direct different types of cargo proteins to different post-Golgi membrane compartments.

Retromer

The retromer is a heteropentameric complex that associates with the cytosolic face of endosomes and mediates retrograde transport of transmembrane cargo from endosomes to the trans-Golgi network. The mammalian retromer complex comprises a sorting nexin dimer composed of a still undefined combination of SNX1, SNX2, SNX5 and SNX6, and a cargo-recognition trimer composed of Vps26, Vps29 and Vps35. The SNX subunits contain PX and BAR domains that allow binding to PI(3)P enriched, highly curved membranes of endosomal vesicles and tubules, while Vps26, Vps29 and Vps35 have arrestin, phosphoesterase and alpha-solenoid folds, respectively. Recent studies have implicated retromer in a broad range of physiological, developmental and pathological processes, underscoring the critical nature of retrograde transport mediated by this complex.

Asymmetric CLASP-dependent nucleation of noncentrosomal microtubules at the trans-Golgi network

Proper organization of microtubule arrays is essential for intracellular trafficking and cell motility. It is generally assumed that most if not all microtubules in vertebrate somatic cells are formed by the centrosome. Here we demonstrate that a large number of microtubules in untreated human cells originate from the Golgi apparatus in a centrosome-independent manner. Both centrosomal and Golgi-emanating microtubules need gamma-tubulin for nucleation. Additionally, formation of microtubules at the Golgi requires CLASPs, microtubule-binding proteins that selectively coat noncentrosomal microtubule seeds. We show that CLASPs are recruited to the trans-Golgi network (TGN) at the Golgi periphery by the TGN protein GCC185. In sharp contrast to radial centrosomal arrays, microtubules nucleated at the peripheral Golgi compartment are preferentially oriented toward the leading edge in motile cells. We propose that Golgi-emanating microtubules contribute to the asymmetric microtubule networks in polarized cells and support diverse processes including post-Golgi transport to the cell front.

Endocytic and secretory traffic in Arabidopsis merge in the trans-Golgi network/early endosome, an independent and highly dynamic organelle

Plants constantly adjust their repertoire of plasma membrane proteins that mediates transduction of environmental and developmental signals as well as transport of ions, nutrients, and hormones. The importance of regulated secretory and endocytic trafficking is becoming increasingly clear; however, our knowledge of the compartments and molecular machinery involved is still fragmentary. We used immunogold electron microscopy and confocal laser scanning microscopy to trace the route of cargo molecules, including the BRASSINOSTEROID INSENSITIVE1 receptor and the REQUIRES HIGH BORON1 boron exporter, throughout the plant endomembrane system. Our results provide evidence that both endocytic and secretory cargo pass through the trans-Golgi network/early endosome (TGN/EE) and demonstrate that cargo in late endosomes/multivesicular bodies is destined for vacuolar degradation. Moreover, using spinning disc microscopy, we show that TGN/EEs move independently and are only transiently associated with an individual Golgi stack.

The exosome pathway in K562 cells is regulated by Rab11

During maturation, reticulocytes lose some membrane proteins that are not required on the mature red cell surface. The proteins are released into the extracellular medium associated with vesicles that are formed by budding of the endosomal membrane into the lumen of the compartment; this process results in the formation of multivesicular bodies (MVBs). Fusion of MVBs with the plasma membrane results in secretion of the small internal vesicles, termed exosomes. K562 cells release exosomes with similar characteristics to reticulocyte exosomes, in particular the transferrin receptor (TfR) is found associated with the vesicles. Interestingly, this cell line has been shown to possess high amounts of Rab11 compared with other Rab proteins. To assess the regulation of transferrin receptor release via exosome secretion by Rab11 in this cell type, K562 cells were stably transfected with GFP-Rab11wt or the GTP- and GDP-locked mutants. The distribution of the proteins was assessed by fluorescence microscopy. Transferrin recycling and the number of TfRs present on the surface of the transfected cells were reduced by overexpression of either Rab11wt or the mutants. The amount of released exosomes was analyzed by measuring different molecular markers present on these vesicles either biochemically or by western blot. Overexpression of the dominant-negative mutant Rab11S25N inhibited exosome release, whereas the secretion of exosomes was slightly stimulated in cells transfected with Rab11wt. Taken together, the results demonstrate that in K562 cells Rab11 modulates the exosome pathway although the exact step involved is still not known.

Model-guided microarray implicates the retromer complex in Alzheimer's disease

Although, in principle, gene expression profiling is well suited to isolate pathogenic molecules associated with Alzheimer's disease (AD), techniques such as microarray present unique analytic challenges when applied to disorders of the brain. Here, we addressed these challenges by first constructing a spatiotemporal model, predicting a priori how a molecule underlying AD should behave anatomically and over time. Then, guided by the model, we generated gene expression profiles of the entorhinal cortex and the dentate gyrus, harvested from the brains of AD cases and controls covering a broad age span. Among many expression differences, the retromer trafficking molecule VPS35 best conformed to the spatiotemporal model of AD. Western blotting confirmed the abnormality, establishing that VPS35 levels are reduced in brain regions selectively vulnerable to AD. VPS35 is the core molecule of the retromer trafficking complex and further analysis revealed that VPS26, another member of the complex, is also downregulated in AD. Cell culture studies, using small interfering RNAs or expression vectors, showed that VPS35 regulates Abeta peptide levels, establishing the relevance of the retromer complex to AD. Reviewing our findings in the context of recent studies suggests how downregulation of the retromer complex in AD can regulate local levels of Abeta peptide.

Activation of Endogenous Cdc42 Visualized in Living Cells

Signaling proteins are tightly regulated spatially and temporally to perform multiple functions. For Cdc42 and other guanosine triphosphatases, the subcellular location of activation is a critical determinant of cell behavior. However, current approaches are limited in their ability to examine the dynamics of Cdc42 activity in living cells. We report the development of a biosensor capable of visualizing the changing activation of endogenous, unlabeled Cdc42 in living cells. With the use of a dye that reports protein interactions, the biosensor revealed localized activation in the trans-Golgi apparatus, microtubule-dependent Cdc42 activation at the cell periphery, and activation kinetics precisely coordinated with cell extension and retraction.

Rab11 regulates the compartmentalization of early endosomes required for efficient transport from early endosomes to the trans-golgi network

Several GTPases of the Rab family, known to be regulators of membrane traffic between organelles, have been described and localized to various intracellular compartments. Rab11 has previously been reported to be associated with the pericentriolar recycling compartment, post-Golgi vesicles, and the trans-Golgi network (TGN). We compared the effect of overexpression of wild-type and mutant forms of Rab11 on the different intracellular transport steps in the endocytic/degradative and the biosynthetic/exocytic pathways in HeLa cells. We also studied transport from endosomes to the Golgi apparatus using the Shiga toxin B subunit (STxB) and TGN38 as reporter molecules. Overexpression of both Rab11 wild-type (Rab11wt) and mutants altered the localization of the transferrrin receptor (TfR), internalized Tf, the STxB, and TGN38. In cells overexpressing Rab11wt and in a GTPase-deficient Rab11 mutant (Rab11Q70L), these proteins were found in vesicles showing characteristics of sorting endosomes lacking cellubrevin (Cb). In contrast, they were redistributed into an extended tubular network, together with Cb, in cells overexpressing a dominant negative mutant of Rab11 (Rab11S25N). This tubularized compartment was not accessible to Tf internalized at temperatures <20 degrees C, suggesting that it is of recycling endosomal origin. Overexpression of Rab11wt, Rab11Q70L, and Rab11S25N also inhibited STxB and TGN38 transport from endosomes to the TGN. These results suggest that Rab11 influences endosome to TGN trafficking primarily by regulating membrane distribution inside the early endosomal pathway.

Sorting of mannose 6-phosphate receptors mediated by the GGAs

The delivery of soluble hydrolases to lysosomes is mediated by the cation-independent and cation-dependent mannose 6-phosphate receptors. The cytosolic tails of both receptors contain acidic-cluster-dileucine signals that direct sorting from the trans-Golgi network to the endosomal-lysosomal system. We found that these signals bind to the VHS domain of the Golgi-localized, gamma-ear-containing, ARF-binding proteins (GGAs). The receptors and the GGAs left the trans-Golgi network on the same tubulo-vesicular carriers. A dominant-negative GGA mutant blocked exit of the receptors from the trans-Golgi network. Thus, the GGAs appear to mediate sorting of the mannose 6-phosphate receptors at the trans-Golgi network.