Identification of rabaptin-5, rabex-5, and GM130 as putative effectors of rab33b, a regulator of retrograde traffic between the Golgi apparatus and ER.

The role of rab33b, a Golgi-specific rab protein, was investigated. Microinjection of rab33b mutants stabilised in the GTP-specific state resulted in a marked inhibition of anterograde transport within the Golgi and in the recycling of glycosyltransferases from the Golgi to the ER, respectively. A GST-rab33b fusion protein stabilised in its GTP form was found to interact by Western blotting or mass spectroscopy with Golgi protein GM130 and rabaptin-5 and rabex-5, two rab effector molecules thought to function exclusively in the endocytic pathway. A similar binding was seen to rab1 but not to rab6, both Golgi rabs. In contrast, rab5 was as expected, shown to bind rabaptin-5 and rabex-5 as well as the endosomal effector protein EEA1 but not GM130. No binding of EEA1 was seen to any of the Golgi rabs.     

Rabaptin-5alpha/rabaptin-4 serves as a linker between rab4 and gamma(1)-adaptin in membrane recycling from endosomes

Rab4 regulates recycling from early endosomes. We investigated the role of the rab4 effector rabaptin-5alpha and its putative partner gamma(1)-adaptin in membrane recycling. We found that rabaptin-5alpha forms a ternary complex with the gamma(1)-sigma(1) subcomplex of AP-1, via a direct interaction with the gamma(1)-subunit. The binding site for gamma(1)-adaptin is in the hinge region of rabaptin-5alpha, which is distinct from rab4- and rab5-binding domains. Endogenous or ectopically expressed gamma(1)- adaptin localized to both the trans-Golgi network and endosomes. Co-expressed rabaptin-5alpha and gamma(1)-adaptin, however, co-localized in a rab4-dependent manner on recycling endosomes. Transfection of rabaptin-5alpha caused enlarged endosomes and delayed recycling of transferrin. RNAi of rab4 had an opposing effect on transferrin recycling. Collectively, our data show that rab4-GTP acts as a scaffold for a rabaptin-5alpha- gamma(1)-adaptin complex on recycling endosomes and that interactions between rab4, rabaptin-5alpha and gamma(1)-adaptin regulate membrane recycling.     

Interactions between adaptor protein-1 of the clathrin coat and microtubules via type 1a microtubule-associated proteins.

The classical view suggests that adaptor proteins of the clathrin coat mediate the sorting of cargo protein passengers into clathrin-coated pits and the recruitment of clathrin into budding areas in the donor membrane. In the present study, we provide biochemical and morphological evidence that the adaptor protein 1 (AP-1) adaptor of the trans-Golgi network clathrin interacts with microtubules. AP-1 in cytosolic extracts interacted with in vitro assembled microtubules, and these interactions were inhibited by ATP depletion of the extracts or in the presence of 5'-adenylylimidodiphosphate. An overexpressed gamma-subunit of the AP-1 complex associated with microtubules, suggesting that this subunit may mediate the interaction of AP-1 with the cytoskeleton. Purified AP-1 did not interact with purified microtubules, but interaction occurred when an isolated microtubule-associated protein fraction was added to the reaction mix. The gamma-adaptin subunit of AP-1 specifically co-immunoprecipitated with a microtubule-associated protein of type 1a from rat brain cytosol. This suggests that type 1a microtubule-associated protein may mediate the association of AP-1 with microtubules in the cytoplasm. The microtubule binding activity of AP-1 was markedly inhibited in cytosol of mitotic cells. By means of its interaction with microtubule-associated proteins, we propose novel roles for AP-1 adaptors in modulating the dynamics of the cytoskeleton, the stability and shape of coated organelles, and the loading of nascent AP-1-coated vesicles onto appropriate microtubular tracks.     

AP-1 and AP-3 mediate sorting of melanosomal and lysosomal membrane proteins into distinct post-Golgi trafficking pathways

The adaptor complexes AP-1 and AP-3 are localized to endosomes and/or the trans Golgi network (TGN). Because of limitations in analysing intracellular adaptor function directly, their site of function is a matter of ongoing uncertainty. To overcome this problem and to analyse adaptor sorting at the TGN, we reconstituted vesicle formation from Golgi/TGN-enriched membranes in a novel in vitro budding assay. Melanocytes were metabolically labelled followed by a 19°C temperature block to accumulate newly synthesized proteins in Golgi membranes, which were then enriched by subcellular fractionation and used as donor membranes for vesicle formation in vitro . The incorporation of the melanosomal proteins tyrosinase and tyrosinase-related protein 1 (TRP-1) as well as Lamp-1 and 46 kDa mannose-6-phosphate receptor (MPR46) into Golgi/TGN-derived vesicles was temperature, nucleotide, cytosol, ADP ribosylation factor 1 and adaptor dependent. We show that sorting of TRP-1 and MPR46 was AP-1 dependent, while budding of tyrosinase and Lamp-1 required AP-3. Depletion of clathrin inhibited sorting of all four cargo proteins, suggesting that AP-1 and AP-3 are involved in the formation of distinct types of clathrin-coated vesicles, each of which is characterized by the incorporation of specific cargo membrane proteins.     

Association and Colocalization of Eps15 with Adaptor Protein-2 and Clathrin

Eps15 has been identified as a substrate of the EGF receptor tyrosine kinase. In this report, we show that activation of the EGF receptor by either EGF or TGF-α results in phosphorylation of Eps15. Stimulation of cells with PDGF or insulin did not lead to Eps15 phosphorylation, suggesting that phosphorylation of Eps15 is a receptor-specific process. We demonstrate that Eps15 is constitutively associated with both α-adaptin and clathrin. Upon EGF stimulation, Eps15 and α-adaptin are recruited to the EGF receptor. Using a truncated EGF receptor mutant, we demonstrate that the regulatory domain of the cytoplasmic tail of the EGF receptor is essential for the binding of Eps15. Fractionation studies reveal that Eps15 is present in cell fractions enriched for plasma membrane and endosomal membranes. Immunofluorescence studies show that Eps15 colocalizes with adaptor protein-2 (AP-2) and partially with clathrin. No colocalization of Eps15 was observed with the early endosomal markers rab4 and rab5. These observations indicate that Eps15 is present in coated pits and coated vesicles of the clathrin-mediated endocytic pathway, but not in early endosomes. Neither AP-2 nor clathrin are required for the binding of Eps15 to coated pits or coated vesicles, since in membranes lacking AP-2 and clathrin, Eps15 still shows the same staining pattern. These findings suggest that Eps15 may play a critical role in the recruitment of active EGF receptors into coated pit regions before endocytosis of ligand-occupied EGF receptors.     

Clathrin Terminal Domain-Ligand Interactions Regulate Sorting of Mannose 6-Phosphate Receptors Mediated by AP-1 and GGA Adaptors

Clathrin plays important roles in intracellular membrane traffic including endocytosis of plasma membrane proteins and receptors and protein sorting between the trans-Golgi network (TGN) and endosomes. Whether clathrin serves additional roles in receptor recycling, degradative sorting, or constitutive secretion has remained somewhat controversial. Here we have used acute pharmacological perturbation of clathrin terminal domain (TD) function to dissect the role of clathrin in intracellular membrane traffic. We report that internalization of major histocompatibility complex I (MHCI) is inhibited in cells depleted of clathrin or its major clathrin adaptor complex 2 (AP-2), a phenotype mimicked by application of Pitstop® inhibitors of clathrin TD function. Hence, MHCI endocytosis occurs via a clathrin/AP-2-dependent pathway. Acute perturbation of clathrin also impairs the dynamics of intracellular clathrin/adaptor complex 1 (AP-1)- or GGA (Golgi-localized, γ-ear-containing, Arf-binding protein)-coated structures at the TGN/endosomal interface, resulting in the peripheral dispersion of mannose 6-phosphate receptors. By contrast, secretory traffic of vesicular stomatitis virus G protein, recycling of internalized transferrin from endosomes, or degradation of EGF receptor proceeds unperturbed in cells with impaired clathrin TD function. These data indicate that clathrin is required for the function of AP-1- and GGA-coated carriers at the TGN but may be dispensable for outward traffic en route to the plasma membrane.     

Non-stoichiometric relationship between clathrin heavy and light chains revealed by quantitative comparative proteomics of clathrin-coated vesicles from brain and liver

We used tandem mass spectrometry with peptide counts to identify and to determine the relative levels of expression of abundant protein components of highly enriched clathrin-coated vesicles (CCVs) from rat liver. The stoichiometry of stable protein complexes including clathrin heavy chain and clathrin light chain dimers and adaptor protein (AP) heterotetramers was assessed. We detected a deficit of clathrin light chain compared with clathrin heavy chain in non-brain tissues, suggesting a level of regulation of clathrin cage formation specific to brain. The high ratio of AP-1 to AP-2 in liver CCVs is reversed compared with brain where there is more AP-2 than AP-1. Despite this, general endocytic cargo proteins were readily detected in liver but not in brain CCVs, consistent with the previous demonstration that a major function for brain CCVs is recycling synaptic vesicles. Finally we identified 21 CCV-associated proteins in liver not yet characterized in mammals. Our results further validate the peptide accounting approach, reveal new information on the properties of CCVs, and allow for the use of quantitative proteomics to compare abundant components of organelles under different experimental and pathological conditions.     

Oligomerization and dissociation of AP-1 adaptors are regulated by cargo signals and by ArfGAP1-induced GTP hydrolysis

The mechanism of AP-1/clathrin coat formation was analyzed using purified adaptor proteins and synthetic liposomes presenting tyrosine sorting signals. AP-1 adaptors recruited in the presence of Arf1.GTP and sorting signals were found to oligomerize to high-molecular-weight complexes even in the absence of clathrin. The appendage domains of the AP-1 adaptins were not required for oligomerization. On GTP hydrolysis induced by the GTPase-activating protein ArfGAP1, the complexes were disassembled and AP-1 dissociated from the membrane. AP-1 stimulated ArfGAP1 activity, suggesting a role of AP-1 in the regulation of the Arf1 "GTPase timer." In the presence of cytosol, AP-1 could be recruited to liposomes without sorting signals, consistent with the existence of docking factors in the cytosol. Under these conditions, however, AP-1 remained monomeric, and recruitment in the presence of GTP was short-lived. Sorting signals allowed stable recruitment and oligomerization also in the presence of cytosol. These results suggest a mechanism whereby initial assembly of AP-1 with Arf1.GTP and ArfGAP1 on the membrane stimulates Arf1 GTPase activity, whereas interaction with cargo induces oligomerization and reduces the rate of GTP hydrolysis, thus contributing to efficient cargo sorting.     

ARF1.GTP,tyrosine-based signals,and phosphatidylinositol 4,5-bisphosphate constitute a minimal machinery to recruit the AP-1 clathrin adaptor to membranes

At the trans-Golgi network, clathrin coats containing AP-1 adaptor complexes are formed in an ARF1-dependent manner, generating vesicles transporting cargo proteins to endosomes. The mechanism of site-specific targeting of AP-1 and the role of cargo are poorly understood. We have developed an in vitro assay to study the recruitment of purified AP-1 adaptors to chemically defined liposomes presenting peptides corresponding to tyrosine-based sorting motifs. AP-1 recruitment was found to be dependent on myristoylated ARF1, GTP or nonhydrolyzable GTP-analogs, tyrosine signals, and small amounts of phosphoinositides, most prominently phosphatidylinositol 4,5-bisphosphate, in the absence of any additional cytosolic or membrane bound proteins. AP-1 from cytosol could be recruited to a tyrosine signal independently of the lipid composition, but the rate of recruitment was increased by phosphatidylinositol 4,5-bisphosphate. The results thus indicate that cargo proteins are involved in coat recruitment and that the local lipid composition contributes to specifying the site of vesicle formation.     

Biochemical dissection of AP-1 recruitment onto Golgi membranes

Recruitment of the Golgi-specific AP-1 adaptor complex onto Golgi membranes is thought to be a prerequisite for clathrin coat assembly on the TGN. We have used an in vitro assay to examine the translocation of cytosolic AP-1 onto purified Golgi membranes. Association of AP-1 with the membranes required GTP or GTP analogues and was inhibited by the fungal metabolite, brefeldin A. In the presence of GTP gamma S, binding of AP-1 to Golgi membranes was strictly dependent on the concentration of cytosol added to the assay. AP-1 recruitment was also found to be temperature dependent, and relatively rapid at 37 degrees C, following a lag period of 3 to 4 min. Using only an adaptor-enriched fraction from cytosol, purified myristoylated ARF1, and Golgi membranes, the GTP gamma S-dependent recruitment of AP-1 could be reconstituted. Our results show that the association of the AP-1 complex with Golgi membranes, like the coatomer complex, requires ARF, which accounts for the sensitivity of both to brefeldin A. In addition, they provide the basis for a model for the early biochemical events that lead to clathrin-coated vesicle formation on the TGN.     

Redistribution of clathrin-coated vesicle adaptor complexes during adipocytic differentiation of 3T3-L1 cells

Mechanisms for intracellular retention of proteins are induced during adipocytic differentiation of 3T3-L1 cells. To investigate the potential role of clathrin lattices in these retention processes, we performed a morphological and biochemical analysis of coated vesicle components in 3T3-L1 cells. Optical sectioning and image restoration revealed a marked increase in the staining of clathrin and beta adaptins in the perinuclear region of cells with differentiation. In addition, predominance of beta (subunit of the AP-2, plasma membrane adaptor) over beta' (subunit of the AP-1, Golgi adaptor) adaptin was observed in immunoblots of clathrin-coated vesicles purified from nondifferentiated fibroblasts, and this ratio was reversed in coated vesicles purified from differentiated adipocytes. These results indicate that the relative abundance of TGN-derived clathrin lattices increases markedly during adipocytic differentiation. Subcellular fractionation indicated that cytosolic AP-1 and AP-2 adaptors comprised approximately 70% of the total cellular adaptor pool. Interestingly, neither the concentration nor the relative ratio of cytosolic AP-1 to AP-2 adaptors increased significantly during differentiation. These data suggest that the increase in TGN-derived lattices results from differentiation-induced mechanisms for enhanced assembly or stabilization of adaptors on Golgi membranes. Interestingly, double- immunofluorescence microscopy also revealed that whereas extensive colocalization between clathrin and beta adaptins occurred both in fibroblasts and adipocytes, structures stained only with anti-adaptin antibody could be detected. Taken together these results suggest that membranes coated with adaptors, but not clathrin, can exist in these cells.     

Tandem arrangement of the clathrin and AP-2 binding domains in amphiphysin 1 and disruption of clathrin coat function by amphiphysin fragments comprising these sites

Amphiphysin 1 and 2 are proteins implicated in the recycling of synaptic vesicles in nerve terminals. They interact with dynamin and synaptojanin via their COOH-terminal SH3 domain, whereas their central regions contain binding sites for clathrin and for the clathrin adaptor AP-2. We have defined here amino acids of amphiphysin 1 crucial for binding to AP-2 and clathrin. Overexpression in Chinese hamster ovary cells of an amphiphysin 1 fragment that binds both AP-2 and clathrin resulted in a segregation of clathrin, which acquired a diffuse distribution, from AP-2, which accumulated at patches also positive for Eps15. These effects correlated with a block in clathrin-mediated endocytosis. A fragment selectively interacting with clathrin produced a similar effect. These results can be explained by the binding of amphiphysin to the NH(2)-terminal domain of clathrin and by a competition with the binding of this domain to the beta-subunit of AP-2 and AP180. The interaction of amphiphysin 1 with either clathrin or AP-2 did not prevent its interaction with dynamin, supporting the existence of tertiary complexes between these proteins. Together with previous evidence indicating a direct interaction between amphiphysin and membrane lipids, these findings support a model in which amphiphysin acts as a multifunctional adaptor linking the membrane to coat proteins and coat proteins to dynamin and synaptojanin.     

Yeast epsin-related proteins required for Golgi-endosome traffic define a gamma-adaptin ear-binding motif

Clathrin-coated vesicles (CCVs) are a central component of endocytosis and traffic between the trans-Golgi network (TGN) and endosomes. Although endocytic CCV formation is well characterized, much less is known about CCV formation at internal membranes. Here we describe two epsin amino-terminal homology (ENTH) domain-containing proteins, Ent3p and Ent5p, that are intimately involved in clathrin function at the Golgi. Both proteins associate with the clathrin adaptor Gga2p in vivo; Ent5p also interacts with the clathrin adaptor complex AP-1 and clathrin. A novel, conserved motif that mediates the interaction of Ent3p and Ent5p with gamma-ear domains of Gga2p and AP-1 is defined. Ent3p and Ent5p colocalize with clathrin, and cells lacking both Ent proteins exhibit defects in clathrin localization and traffic between the Golgi and endosomes. The findings suggest that Ent3p and Ent5p constitute a functionally related pair that co-operate with Gga proteins and AP-1 to recruit clathrin and promote formation of clathrin coats at the Golgi/endosomes. On the basis of our results and the established roles of epsin and epsin-related proteins in endocytosis, we propose that ENTH-domain-containing proteins are a universal component of CCV formation.     

Clathrin adaptors mediate two sequential pathways of intra-Golgi recycling

The pathways of membrane traffic within the Golgi apparatus are not fully known. This question was addressed using the yeast Saccharomyces cerevisiae, in which the maturation of individual Golgi cisternae can be visualized. We recently proposed that the AP-1 clathrin adaptor mediates intra-Golgi recycling late in the process of cisternal maturation. Here, we demonstrate that AP-1 cooperates with the Ent5 clathrin adaptor to recycle a set of Golgi transmembrane proteins, including some that were previously thought to pass through endosomes. This recycling can be detected by removing AP-1 and Ent5, thereby diverting the AP-1/Ent5–dependent Golgi proteins into an alternative recycling loop that involves traffic to the plasma membrane followed by endocytosis. Unexpectedly, various AP-1/Ent5–dependent Golgi proteins show either intermediate or late kinetics of residence in maturing cisternae. We infer that the AP-1/Ent5 pair mediates two sequential intra-Golgi recycling pathways that define two classes of Golgi proteins. This insight can explain the polarized distribution of transmembrane proteins in the Golgi.     

Adaptor complex-independent clathrin function in yeast

Clathrin-associated adaptor protein (AP) complexes are major structural components of clathrin-coated vesicles, functioning in clathrin coat assembly and cargo selection. We have carried out a systematic biochemical and genetic characterization of AP complexes in Saccharomyces cerevisiae. Using coimmunoprecipitation, the subunit composition of two complexes, AP-1 and AP-2R, has been defined. These results allow assignment of the 13 potential AP subunits encoded in the yeast genome to three AP complexes. As assessed by in vitro binding assays and coimmunoprecipitation, only AP-1 interacts with clathrin. Individual or combined disruption of AP-1 subunit genes in cells expressing a temperature-sensitive clathrin heavy chain results in accentuated growth and alpha-factor pheromone maturation defects, providing further evidence that AP-1 is a clathrin adaptor complex. However, in cells expressing wild-type clathrin, the same AP subunit deletions have no effect on growth or alpha-factor maturation. Furthermore, gel filtration chromatography revealed normal elution patterns of clathrin-coated vesicles in cells lacking AP-1. Similarly, combined deletion of genes encoding the beta subunits of the three AP complexes did not produce defects in clathrin-dependent sorting in the endocytic and vacuolar pathways or alterations in gel filtration profiles of clathrin-coated vesicles. We conclude that AP complexes are dispensable for clathrin function in S. cerevisiae under normal conditions. Our results suggest that alternative factors assume key roles in stimulating clathrin coat assembly and cargo selection during clathrin-mediated vesicle formation in yeast.     

AP-1 membrane-cytoplasm recycling regulated by mu1A-adaptin.

The adaptor protein complex AP-1 mediates vesicular protein sorting between the trans Golgi network and endosomes. AP-1 recycles between membranes and the cytoplasm together with clathrin during transport vesicle formation and vesicle uncoating. AP-1 recycles independent of clathrin, indicating binding to unproductive membrane domains and premature termination of vesicle budding. Membrane recruitment requires ADP ribosylation factor-1-GTP, a transmembrane protein containing an AP-1-binding motif and phosphatidyl-inositol phosphate (PI-4-P). Little is known about the regulation of AP-1 membrane-cytoplasm recycling. We identified the N-terminal domain of micro1A-adaptin as being involved in the regulation of AP-1 membrane-cytoplasm recycling by constructing chimeras of micro1A and its homologue micro2. The AP-1* complex containing this mu2-micro1A chimera had slowed down recycling kinetics, resulting in missorting of mannose 6-phosphate receptors. The N-terminal domain is only accessible from the cytoplasmic AP-1 surface. None of the proteins known to influence AP-1 membrane recycling bound to this micro1A domain, indicating the regulation of AP-1 membrane-cytoplasm recycling by an yet unidentified cytoplasmic protein.     

Eps15 mediates vesicle trafficking from the trans-Golgi network via an interaction with the clathrin adaptor AP-1

Eps15 (EGFR pathway substrate clone 15) is well known for its role in clathrin-coated vesicle formation at the plasma membrane through interactions with other clathrin adaptor proteins such as AP-2. Interestingly, we observed that in addition to its plasma membrane localization, Eps15 is also present at the trans-Golgi network (TGN). Therefore, we predicted that Eps15 might associate with clathrin adaptor proteins at the TGN and thereby mediate the formation of Golgi-derived vesicles. Indeed, we have found that Eps15 and the TGN clathrin adaptor AP-1 coimmunoprecipitate from rat liver Golgi fractions. Furthermore, we have identified a 14-amino acid motif near the AP-2-binding domain of Eps15 that is required for binding to AP-1, but not AP-2. Disruption of the Eps15-AP-1 interaction via siRNA knockdown of AP-1 or expression of mutant Eps15 protein, which lacks a 14-amino acid motif representing the AP-1 binding site of Eps15, significantly reduced the exit of secretory proteins from the TGN. Together, these findings indicate that Eps15 plays an important role in clathrin-coated vesicle formation not only at the plasma membrane but also at the TGN during the secretory process.     

Human stoned B interacts with AP-2 and synaptotagmin and facilitates clathrin-coated vesicle uncoating

Synaptic vesicle biogenesis involves the recycling of synaptic vesicle components by clathrin-mediated endocytosis from the presynaptic membrane. stoned B, a protein encoded by the stoned locus in Drosophila melanogaster has been shown to regulate vesicle recycling by interacting with synaptotagmin. We report here the identification and characterization of a human homolog of stoned B (hStnB). Human stoned B is a brain-specific protein which co-enriches with other endocytic proteins such as AP-2 in a crude synaptic vesicle fraction and at nerve terminals. A domain with homology to the medium chain of adaptor complexes binds directly to both AP-2 and synaptotagmin and competes with AP-2 for the same binding site within synaptotagmin. Finally we show that the µ2 homology domain of hStnB stimulates the uncoating of both clathrin and AP-2 adaptors from clathrin-coated vesicles. We hypothesize that hStnB regulates synaptic vesicle recycling by facilitating vesicle uncoating.     

Dual interaction of synaptotagmin with mu2- and alpha-adaptin facilitates clathrin-coated pit nucleation

The synaptic vesicle protein synaptotagmin was proposed to act as a major docking site for the recruitment of clathrin coats implicated in endocytosis, including the recycling of synaptic vesicles. We show here that the C2B domain of synaptotagmin binds mu2- and alpha-adaptin, two of the four subunits of the endocytic adaptor complex AP-2. mu2 represents the major interacting subunit of AP-2 within this complex. Its binding to synaptotagmin is mediated by a site in subdomain B that is distinct from the binding site for tyrosine-based sorting motifs located in subdomain A. The presence of the C2B domain of synaptotagmin at the surface of liposomes enhances the recruitment of AP-2 and clathrin. Conversely, perturbation of the interaction between synaptotagmin and AP-2 by synprint, the cytoplasmic synaptotagmin-binding domain of N-type calcium channels, inhibits transferrin internalization in living cells. We conclude that a dual interaction of synaptotagmin with the clathrin adaptor AP-2 plays a key physiological role in the nucleation of endocytic clathrin-coated pits.     

The clathrin adaptor AP-1A mediates basolateral polarity

Clathrin and the epithelial-specific clathrin adaptor AP-1B mediate basolateral trafficking in epithelia. However, several epithelia lack AP-1B, and mice knocked out for AP-1B are viable, suggesting the existence of additional mechanisms that control basolateral polarity. Here, we demonstrate a distinct role of the ubiquitous clathrin adaptor AP-1A in basolateral protein sorting. Knockdown of AP-1A causes missorting of basolateral proteins in MDCK cells, but only after knockdown of AP-1B, suggesting that AP-1B can compensate for lack of AP-1A. AP-1A localizes predominantly to the TGN, and its knockdown promotes spillover of basolateral proteins into common recycling endosomes, the site of function of AP-1B, suggesting complementary roles of both adaptors in basolateral sorting. Yeast two-hybrid assays detect interactions between the basolateral signal of transferrin receptor and the medium subunits of both AP-1A and AP-1B. The basolateral sorting function of AP-1A reported here establishes AP-1 as a major regulator of epithelial polarity.