Arf regulates interaction of GGA with mannose-6-phosphate receptor

The host mannose-6-phosphate pathway and viral infection

Viruses, despite their simple structural composition, engage in intricate and complex interactions with their hosts due to their parasitic nature. A notable demonstration of viral behavior lies in their exploitation of lysosomes, specialized organelles responsible for the breakdown of biomolecules and clearance of foreign substances, to bolster their own replication. The man-nose-6-phosphate (M6P) pathway, crucial for facilitating the proper transport of hydrolases into lysosomes and promoting lysosome maturation, is frequently exploited for viral manipulation in support of replication. Recently, the discovery of lysosomal enzyme trafficking factor (LYSET) as a pivotal regulator within the lysosomal M6P pathway has introduced a fresh perspective on the intricate interplay between viral entry and host factors. This groundbreaking revelation illuminates unexplored dimensions of these interactions. In this review, we endeavor to provide a thorough overview of the M6P pathway and its intricate interplay with viral factors during infection. By consolidating the current understanding in this field, our objective is to establish a valuable reference for the development of antiviral drugs that selectively target the M6P pathway.

KDEL Receptor Trafficking to the Plasma Membrane Is Regulated by ACBD3 and Rab4A-GTP

KDEL receptor-1 maintains homeostasis in the early secretory pathway by capturing and retrieving ER chaperones to the ER during heavy secretory activity. Unexpectedly, a fraction of the receptor is also known to reside in the plasma membrane (PM), although it is largely unknown exactly how the KDEL receptor gets exported from the Golgi and travels to the PM. We have previously shown that a Golgi scaffolding protein (ACBD3) facilitates KDEL receptor localization at the Golgi via the regulating cargo wave-induced cAMP/PKA-dependent signaling pathway. Upon endocytosis, surface-expressed KDEL receptor undergoes highly complex itineraries through the Golgi and the endo-lysosomal compartments, where the endocytosed receptor utilizes Rab14A- and Rab11A-positive recycling endosomes and clathrin-decorated tubulovesicular carriers. In this study, we sought to investigate the mechanism through which the KDEL receptor gets exported from the Golgi en route to the PM. We report here that ACBD3 depletion results in greatly increased trafficking of KDEL receptor to the PM via Rab4A-positive tubular carriers emanating from the Golgi. Expression of constitutively activated Rab4A mutant (Q72L) increases the surface expression of KDEL receptor up to 2~3-fold, whereas Rab4A knockdown or the expression of GDP-locked Rab4A mutant (S27N) inhibits KDEL receptor targeting of the PM. Importantly, KDELR trafficking from the Golgi to the PM is independent of PKA- and Src kinase-mediated mechanisms. Taken together, these results reveal that ACBD3 and Rab4A play a key role in regulating KDEL receptor trafficking to the cell surface.

Insulin-Like Growth Factor-II/Cation-Independent Mannose 6-Phosphate Receptor in Neurodegenerative Diseases

The insulin-like growth factor II/mannose 6-phosphate (IGF-II/M6P) receptor is a multifunctional single transmembrane glycoprotein. Recent studies have advanced our understanding of the structure, ligand-binding properties, and trafficking of the IGF-II/M6P receptor. This receptor has been implicated in a variety of important cellular processes including growth and development, clearance of IGF-II, proteolytic activation of enzymes, and growth factor precursors, in addition to its well-known role in the delivery of lysosomal enzymes. The IGF-II/M6P receptor, distributed widely in the central nervous system, has additional roles in mediating neurotransmitter release and memory enhancement/consolidation, possibly through activating IGF-II-related intracellular signaling pathways. Recent studies suggest that overexpression of the IGF-II/M6P receptor may have an important role in regulating the levels of transcripts and proteins involved in the development of Alzheimer's disease (AD)-the prevalent cause of dementia affecting the elderly population in our society. It is reported that IGF-II/M6P receptor overexpression can increase the levels/processing of amyloid precursor protein leading to the generation of β-amyloid peptide, which is associated with degeneration of neurons and subsequent development of AD pathology. Given the significance of the receptor in mediating the transport and functioning of the lysosomal enzymes, it is being considered for therapeutic delivery of enzymes to the lysosomes to treat lysosomal storage disorders. Notwithstanding these results, additional studies are required to validate and fully characterize the function of the IGF-II/M6P receptor in the normal brain and its involvement in various neurodegenerative disorders including AD. It is also critical to understand the interaction between the IGF-II/M6P receptor and lysosomal enzymes in neurodegenerative processes, which may shed some light on developing approaches to detect and prevent neurodegeneration through the dysfunction of the receptor and the endosomal-lysosomal system.

Different Pathways to the Lysosome: Sorting out Alternatives

Considerable research supports a model in which hydrolytic enzymes of mammalian lysosomes are sorted to their destinations in a receptor-dependent mechanism. The ligand for the mammalian sorting receptors is mannose 6-phosphate (M6P). Two M6P receptors have been defined in mammals. Here, we review the foundational evidence supporting this mechanism and highlight the remaining gaps in our understanding of the mammalian mechanism, including evidence for M6P-independent sorting, and its relevance to lysosomal enzyme sorting in metazoa.

A PH domain in the Arf GTPase-activating protein (GAP) ARAP1 binds phosphatidylinositol 3,4,5-trisphosphate and regulates Arf GAP activity independently of recruitment to the plasma membranes

ARAP1 is a phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3))-dependent Arf GTPase-activating protein (GAP) with five PH domains that regulates endocytic trafficking of the epidermal growth factor receptor (EGFR). Two tandem PH domains are immediately N-terminal of the Arf GAP domain, and one of these fits the consensus sequence for PtdIns(3,4,5)P(3) binding. Here, we tested the hypothesis that PtdIns(3,4,5)P(3)-dependent recruitment mediated by the first PH domain of ARAP1 regulates the in vivo and in vitro function of ARAP1. We found that PH1 of ARAP1 specifically bound to PtdIns(3,4,5)P(3), but with relatively low affinity (approximately 1.6 microm), and the PH domains did not mediate PtdIns(3,4,5)P(3)-dependent recruitment to membranes in cells. However, PtdIns(3,4,5)P(3) binding to the PH domain stimulated GAP activity and was required for in vivo function of ARAP1 as a regulator of endocytic trafficking of the EGFR. Based on these results, we propose a variation on the model for the function of phosphoinositide-binding PH domains. In our model, ARAP1 is recruited to membranes independently of PtdIns(3,4,5)P(3), the subsequent production of which triggers enzymatic activity.

Arf GAP2 is positively regulated by coatomer and cargo

Arf GAP2 is one of four Arf GAPs that function in the Golgi apparatus. We characterized the kinetics of Arf GAP2 and its regulation. Purified Arf GAP2 had little activity compared to purified Arf GAP1. Of the potential regulators we examined, coatomer had the greatest effect, stimulating activity one to two orders of magnitude. The effect was biphasic, with half-maximal activation observed at 50 nM coatomer and activation peaking at approximately 150 nM coatomer. Activation by coatomer was greater for Arf GAP2 than has been reported for Arf GAP1. The effects of phosphoinositides and changes in vesicle curvature on GAP activity were small compared to coatomer; however, both increased coatomer-dependent activity. Peptides from p24 cargo proteins increased Arf GAP2 activity by an additional 2- to 4-fold. The effect of cargo peptide was dependent on coatomer. Overexpressing the cargo protein p25 decreased cellular Arf1*GTP levels. The differential sensitivity of Arf GAP1 and Arf GAP2 to coatomer could coordinate their activities. Based on the common regulatory features of Arf GAP1 and 2, we propose a mechanism for cargo selection in which GTP hydrolysis triggered by cargo binding to the coat protein is coupled to coat polymerization.

Sorting of lysosomal proteins

Lysosomes are composed of soluble and transmembrane proteins that are targeted to lysosomes in a signal-dependent manner. The majority of soluble acid hydrolases are modified with mannose 6-phosphate (M6P) residues, allowing their recognition by M6P receptors in the Golgi complex and ensuing transport to the endosomal/lysosomal system. Other soluble enzymes and non-enzymatic proteins are transported to lysosomes in an M6P-independent manner mediated by alternative receptors such as the lysosomal integral membrane protein LIMP-2 or sortilin. Sorting of cargo receptors and lysosomal transmembrane proteins requires sorting signals present in their cytosolic domains. These signals include dileucine-based motifs, DXXLL or [DE]XXXL[LI], and tyrosine-based motifs, YXXØ, which interact with components of clathrin coats such as GGAs or adaptor protein complexes. In addition, phosphorylation and lipid modifications regulate signal recognition and trafficking of lysosomal membrane proteins. The complex interaction of both luminal and cytosolic signals with recognition proteins guarantees the specific and directed transport of proteins to lysosomes.

PI4P promotes the recruitment of the GGA adaptor proteins to the trans-Golgi network and regulates their recognition of the ubiquitin sorting signal

Phosphatidylinositol 4 phosphate (PI4P) is highly enriched in the trans-Golgi network (TGN). Here we establish that PI4P is a key regulator of the recruitment of the GGA clathrin adaptor proteins to the TGN and that PI4P has a novel role in promoting their recognition of the ubiquitin (Ub) sorting signal. Knockdown of PI4KIIalpha by RNA interference (RNAi), which depletes the TGN's PI4P, impaired the recruitment of the GGAs to the TGN. GGAs bind PI4P primarily through their GAT domain, in a region called C-GAT, which also binds Ub but not Arf1. We identified two basic residues in the GAT domain that are essential for PI4P binding in vitro and for the recruitment of GGAs to the TGN in vivo. Unlike wild-type GGA, GGA with mutated GATs failed to rescue the abnormal TGN phenotype of the GGA RNAi-depleted cells. These residues partially overlap with those that bind Ub, and PI4P increased the affinity of the GAT domain for Ub. Because the recruitment of clathrin adaptors and their cargoes to the TGN is mediated through a web of low-affinity interactions, our results show that the dual roles of PI4P can promote specific GGA targeting and cargo recognition at the TGN.

Mutational analysis of the Arf1*GTP/Arf GAP interface reveals an Arf1 mutant that selectively affects the Arf GAP ASAP1

Arf1 is a GTP binding protein that functions at a number of cellular sites to control membrane traffic and actin remodeling. Arf1 is regulated by site-specific GTPase-activating proteins (GAPs). The combined results of crystallographic and biochemical studies have led to the proposal that Arf1 GAPs differ in the specific interface formed with Arf1. To test this hypothesis, we have used mutagenesis to examine the interaction of three Arf GAPs (ASAP1, AGAP1, and ArfGAP1) with switch 1, switch 2, and alpha helix3 of Arf1. The GAPs were similar in being affected by mutations in switch 1 and 2. However, effects of a mutation within alpha helix3 and specific mutations within switch 1 and 2 differed among the GAPs. The largest differences were observed with a change of isoleucine 46 to aspartate ([I46D]Arf1), which reduced ASAP1-induced catalysis by approximately 10,000-fold but had a 3-fold effect on AGAP1. The reduction was due to an isolated effect on the catalytic rate, k(cat). In vivo [I46D]Arf1 had no detectable effect on the Golgi apparatus but, instead, functioned as a constitutively active mutant in the cell periphery, affecting the localization of ASAP1 and paxillin. Based on our results, we conclude that the contribution of specific residues within switch 1 of Arf to binding and achieving a transition state toward GTP hydrolysis differs among Arf GAPs.

Arf GAPs and membrane traffic

The selective transfer of material between membrane-delimited organelles is mediated by protein-coated vesicles. In many instances, formation of membrane trafficking intermediates is regulated by the GTP-binding protein Arf. Binding and hydrolysis of GTP by Arf was originally linked to the assembly and disassembly of vesicle coats. Arf GTPase-activating proteins (GAPs), a family of proteins that induce hydrolysis of GTP bound to Arf, were therefore proposed to regulate the disassembly and dissociation of vesicle coats. Following the molecular identification of Arf GAPs, the roles for GAPs and GTP hydrolysis have been directly examined. GAPs have been found to bind cargo and known coat proteins as well as directly contribute to vesicle formation, which is consistent with the idea that GAPs function as subunits of coat proteins rather than simply Arf inactivators. In addition, GTP hydrolysis induced by GAPs occurs largely before vesicle formation and is required for sorting. These results are the primary basis for modifications to the classical model for the function of Arf in transport vesicle formation, including a recent proposal that Arf has a proofreading, rather than a structural, role.

GGA function is required for maturation of neuroendocrine secretory granules

Secretory granule (SG) maturation has been proposed to involve formation of clathrin-coated vesicles (CCVs) from immature SGs (ISGs). We tested the effect of inhibiting CCV budding by using the clathrin adaptor GGA (Golgi-associated, gamma-ear-containing, ADP-ribosylation factor-binding protein) on SG maturation in neuroendocrine cells. Overexpression of a truncated, GFP-tagged GGA, VHS (Vps27, Hrs, Stam)-GAT (GGA and target of myb (TOM))-GFP led to retention of MPR, VAMP4, and syntaxin 6 in mature SGs (MSGs), suggesting that CCV budding from ISGs is inhibited by the SG-localizing VHS-GAT-GFP. Furthermore, VHS-GAT-GFP-overexpression disrupts prohormone convertase 2 (PC2) autocatalytic cleavage, processing of secretogranin II to its product p18, and the correlation between PC2 and p18 levels. All these effects were not observed if full-length GGA1-GFP was overexpressed. Neither GGA1-GFP nor VHS-GAT-GFP perturbed SG protein budding from the TGN, or homotypic fusion of ISGs. Reducing GGA3 levels by using short interfering (si)RNA also led to VAMP4 retention in SGs, and inhibition of PC2 activity. Our results suggest that inhibition of CCV budding from ISGs downregulates the sorting from the ISGs and perturbs the intragranular activity of PC2.

In vitro assays of Arf1 interaction with GGA proteins

ADP-ribosylation factor 1 (Arf1) is a GTP-binding protein that regulates membrane traffic. This function of Arf1 is, at least in part, mediated by Arf1 x GTP binding to coat proteins such as coatomer, clathrin adaptor protein (AP) complexes 1 and 3, and gamma-adaptin homology-Golgi associated Arf-binding (GGA) proteins. Binding to Arf1 x GTP recruits these coat proteins to membranes, leading to the formation of transport vesicles. Whereas coatomer and the AP complexes are hetero-oligomers, GGAs are single polypeptide chains. Therefore, working with recombinant GGAs is straightforward compared to the other Arf1 effectors. Consequently, the GGAs have been used as a model for studying Arf1 interactions with effectors and as reagents to determine Arf1 x GTP levels in cells. In this chapter, we describe in vitro assays for analysis of GGA interaction with Arf1 x GTP and for determining intracellular Arf1 x GTP levels.

The Arf GAPs AGAP1 and AGAP2 distinguish between the adaptor protein complexes AP-1 and AP-3

ADP ribosylation factors (Arf) regulate membrane trafficking at multiple intracellular sites by recruiting coat proteins to membranes. The site-specific regulation of Arf is thought to be mediated by regulatory proteins including the guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Here, we test this hypothesis by comparing the site of action of the Arf GAP AGAP2 to the closely related AGAP1. AGAP1 has previously been found to associate with the adaptor protein complex AP-3 and regulate the function of AP-3 endosomes. We found that AGAP2 directly interacted with AP-1. AGAP2 colocalized with AP-1, transferrin receptor and Rab4 on endosomes. Overexpression of AGAP2 changed the intracellular distribution of AP-1 and promoted Rab4-dependent fast recycling of transferrin. Based on these results, we concluded that the closely related Arf GAPs, AGAP1 and AGAP2, distinguish between these related heterotetrameric adaptor protein complexes to specifically regulate AP-3 endosomes and AP-1 recycling endosomes.

Epidermal growth factor-dependent phosphorylation of the GGA3 adaptor protein regulates its recruitment to membranes

The Golgi-localized, Gamma-ear-containing, Arf-binding (GGA) proteins are monomeric clathrin adaptors that mediate the sorting of transmembrane cargo at the trans-Golgi network and endosomes. Here we report that one of these proteins, GGA3, becomes transiently phosphorylated upon activation of the epidermal growth factor (EGF) receptor. This phosphorylation takes place on a previously unrecognized site in the "hinge" segment of the protein, S368, and is strictly dependent on the constitutive phosphorylation of another site, S372. The EGF-induced phosphorylation of S368 does not require internalization of the EGF receptor or association of GGA3 with membranes. This phosphorylation can be blocked by inhibitors of both the mitogen-activated protein kinase and phosphatidylinositol 3-kinase pathways that function downstream of the activated EGF receptor. Phosphorylation of GGA3 on S368 causes an increase in the hydrodynamic radius of the protein, indicating a transition to a more asymmetric shape. Mutation of S368 and S372 to a phosphomimic aspartate residue decreases the association of GGA3 with membranes. These observations indicate that EGF signaling elicits phosphorylation events that regulate the association of GGA3 with organellar membranes.

Common principles in clathrin-mediated sorting at the Golgi and the plasma membrane

Clathrin-mediated vesicular trafficking events underpin the vectorial transfer of macromolecules between several eukaryotic membrane-bound compartments. Classical models for coat operation, focused principally on interactions between clathrin, the heterotetrameric adaptor complexes, and cargo molecules, fail to account for the full complexity of the coat assembly and sorting process. New data reveal that targeting of clathrin adaptor complexes is generally supported by phosphoinositides, that cargo recognition by heterotetrameric adaptors depends on phosphorylation-driven conformational alterations, and that dedicated clathrin-associated sorting proteins (CLASPs) exist to promote the selective trafficking of specific categories of cargo. A host of accessory factors also participate in coat polymerization events, and the independently folded appendage domains that project off the heterotetrameric adaptor core function as recruitment platforms that appear to oversee assembly operations. It is also now clear that focal polymerization of branched actin microfilaments contributes to clathrin-coated vesicle assembly and movement at both plasma membrane and Golgi sites. This improved appreciation of the complex mechanisms governing clathrin-dependent sorting events reveals several common principles of clathrin operation at the Golgi and the plasma membrane.

Multiple phosphorylation events regulate the subcellular localization of GGA1

GGAs comprise a family of Arf-dependent coat proteins or adaptors that regulate vesicle traffic from the trans-Golgi network (TGN). GGAs bind activated Arf, cargo, and additional components necessary for vesicle budding through interactions with their four functional domains: VHS, GAT, hinge, and GAE. We identified three sites of phosphorylation in GGA1 by tandem mass spectrometry: S268 and T270 in the GAT domain and S480 in the hinge. Expression of HA-GGA1 in mammalian cells and comparison to endogenous GGA1 confirmed their localization to late Golgi compartments. In contrast, mutations that mimic the phosphoprotein (HA-GGA1[S268D] or HA-GGA1[T270D]) at either of the sites in the GAT domain caused a decrease in the colocalization with markers of the Golgi and TGN and an increase in puncta in cytoplasm. Quantitative comparisons of the extent of colocalization of GGA1 proteins with the known components of GGA1 vesicles revealed that the composition of those markers tested in HA-GGA1[S268D] and HA-GGA1[T270D] vesicles were indistinguishable from those of HA-GGA1 vesicles. We conclude that phosphorylation of the GAT domain can stabilize the coat proteins bound and thus regulate the rate of coat protein dissociation.

An Ear-Core Interaction Regulates the Recruitment of the AP-3 Complex to Membranes

AP-3 is a heterotetrameric adaptor involved in the biogenesis of lysosome-related organelles. The function of AP-3 as an adaptor relies on its ability to bind to membranes in an Arf-dependent fashion and to recognize sorting signals in the cytosolic tails of the transmembrane cargo. Here, we report an interdomain interaction involving the ear domain of the delta subunit and the sigma3 subunit of AP-3. This interaction interferes with the binding of AP-3 to Arf but not to dileucine-based sorting signals. As a consequence, the delta-ear inhibits the recruitment of AP-3 to membranes both in vitro and in vivo and impairs the sorting of lysosomal membrane proteins. These observations suggest a new regulatory mechanism for the recruitment of AP-3 to membranes involving delta-ear-sigma3 interactions.

The insulin-like growth factor-II/mannose-6-phosphate receptor: structure, distribution and function in the central nervous system

The insulin-like growth factor-II/mannose-6-phosphate (IGF-II/M6P) receptor is a multifunctional single transmembrane glycoprotein which, along with the cation-dependent M6P (CD-M6P) receptor, mediates the trafficking of M6P-containing lysosomal enzymes from the trans-Golgi network (TGN) to lysosomes. Cell surface IGF-II/M6P receptors also function in the degradation of the non-glycosylated IGF-II polypeptide hormone, as well as in the capture and activation/degradation of extracellular M6P-bearing ligands. In recent years, the multifaceted role of the receptor has become apparent, as several lines of evidence have indicated that in addition to its role in lysosomal enzyme trafficking, clearance and/or activation of a variety of growth factors and endocytosis-mediated degradation of IGF-II, the IGF-II/M6P receptor may also mediate transmembrane signal transduction in response to IGF-II binding under certain conditions. However, very little is known about the physiological significance of the receptor in the function of the central nervous system (CNS). This review aims to delineate what is currently known about IGF-II/M6P receptor structure, its ligand binding properties and role in lysosomal enzyme transport. It also summarizes the recent data regarding the role of the receptor in the CNS, including its distribution, possible importance for normal and activity-dependent functioning as well as its implications in neurodegenerative disorders such as Alzheimer's disease (AD).

Arf GAPs: multifunctional proteins that regulate membrane traffic and actin remodelling

The ADP-ribosylation factor (Arf) Arf GTPase-activating proteins (GAPs) are a family of proteins that induce hydrolysis of GTP bound to Arf. A conserved domain containing a zinc finger motif mediates catalysis. The substrate, Arf.GTP, affects membrane trafficking and actin remodelling. Consistent with activity as an Arf regulator, the Arf GAPs affect both of these pathways. However, the Arf GAPs are likely to have Arf-independent activities that contribute to their cellular functions. Structures of the Arf GAPs are diverse containing catalytic, protein-protein interaction and lipid interaction domains in addition to the Arf GAP domain. Some Arf GAPs have been identified and characterized on the basis of activities other than Arf GAP. Here, we describe the Arf GAP family, enzymology of some members of the Arf GAP family and known functions of the proteins. The results discussed illustrate roles for both Arf-dependent and -independent activities in the regulation of cellular architecture.

The GGA proteins: adaptors on the move

The GGA proteins are a family of ubiquitously expressed, Arf-dependent clathrin adaptors that mediate the sorting of mannose-6-phosphate receptors between the trans-Golgi network and endosomes. Recent studies have elucidated the biochemical and structural bases for the interaction of the GGA proteins with many binding partners, and have shed light on the molecular and cellular mechanisms by which the GGA proteins participate in protein sorting.

Arf and its many interactors

Arf GTP-binding proteins regulate membrane traffic and actin remodeling. Similar to other GTP-binding proteins, a complex of Arf-GTP with an effector protein mediates Arf function. Arf interacts with at least three qualitatively different types of effectors. First, it interacts with structural proteins, the vesicle coat proteins. The second type of effector is lipid-metabolizing enzymes, and the third comprises those proteins that bind to Arf-GTP but whose biochemical or biological functions are not yet clearly defined. Arf interacts with two other families of proteins, the exchange factors and the GTPase-activating proteins. Recent work examining the functional relationships among the diverse Arf interactors has led to reconsideration of the prevailing paradigms for Arf action.

The Structure and Function of GGAs, the Traffic Controllers at the TGN Sorting Crossroads

GGAs (Golgi-localizing, gamma-adaptin ear homology domain, ARF-binding proteins) are a family of monomeric clathrin adaptor proteins that are conserved from yeasts to humans. Data published during the past four years have provided detailed pictures of the localization, domain organization and structure-function relationships of GGAs. GGAs possess four conserved functional domains, each of which interacts with cargo proteins including mannose 6-phosphate receptors, the small GTPase ARF, clathrin, or accessory proteins including Rabaptin-5 and gamma-synergin. Together with or independent of the adaptor protein complex AP-1, GGAs regulate selective transport of cargo proteins, such as mannose 6-phosphate receptors, from the trans-Golgi network to endosomes mediated by clathrin-coated vesicles.