Sanjay K. Mishra

Disabled-2 exhibits the properties of a cargo-selective endocytic clathrin adaptor

Clathrin‐coated pits at the cell surface select material for transportation into the cell interior. A major mode of cargo selection at the bud site is via the μ2 subunit of the AP‐2 adaptor complex, which recognizes tyrosine‐based internalization signals. Other internalization motifs and signals, including phosphorylation and ubiquitylation, also tag certain proteins for incorporation into a coated vesicle, but the mechanism of selection is unclear. Disabled‐2 (Dab2) recognizes the FXNPXY internalization motif in LDL‐receptor family members via an N‐terminal phosphotyrosine‐binding (PTB) domain. Here, we show that in addition to binding AP‐2, Dab2 also binds directly to phosphoinositides and to clathrin, assembling triskelia into regular polyhedral coats. The FXNPXY motif and phosphoinositides contact different regions of the PTB domain, but can stably anchor Dab2 to the membrane surface, while the distal AP‐2 and clathrin‐binding determinants regulate clathrin lattice assembly. We propose that Dab2 is a typical member of a growing family of cargo‐specific adaptor proteins, including β‐arrestin, AP180, epsin, HIP1 and numb, which regulate clathrin‐coat assembly at the plasma membrane by synchronizing cargo selection and lattice polymerization events.

Epsin 1 is a Polyubiquitin-Selective Clathrin-Associated Sorting Protein

Epsin 1 engages several core components of the endocytic clathrin coat, yet the precise mode of operation of the protein remains controversial. The occurrence of tandem ubiquitin‐interacting motifs (UIMs) suggests that epsin could recognize a ubiquitin internalization tag, but the association of epsin with clathrin‐coat components or monoubiquitin is reported to be mutually exclusive. Here, we show that endogenous epsin 1 is clearly an integral component of clathrin coats forming at the cell surface and is essentially absent from caveolin‐1‐containing structures under normal conditions. The UIM region of epsin 1 associates directly with polyubiquitin chains but has extremely poor affinity for monoubiquitin. Polyubiquitin binding is retained when epsin synchronously associates with phosphoinositides, the AP‐2 adaptor complex and clathrin. The enrichment of epsin within clathrin‐coated vesicles purified from different tissue sources varies and correlates with sorting of multiubiquitinated cargo, and in cultured cells, polyubiquitin, rather than non‐conjugable monoubiquitin, promotes rapid internalization. As epsin interacts with eps15, which also contains a UIM region that binds to polyubiquitin, epsin and eps15 appear to be central components of the vertebrate poly/multiubiquitin‐sorting endocytic clathrin machinery.

The autosomal recessive hypercholesterolemia (ARH) protein interfaces directly with the clathrin-coat machinery

The low density lipoprotein (LDL) receptor plays a pivotal role in cholesterol metabolism. Inherited mutations that disturb the activity of the receptor lead to elevations in plasma cholesterol levels and early-onset coronary atherosclerosis. Defects in either the LDL receptor or apolipoprotein B, the proteinaceous component of LDL particles that binds the LDL receptor, elevate circulating LDL-cholesterol levels in an autosomal-dominant fashion, with heterozygotes displaying values between homozygous and normal individuals. Rarely, similar clinical phenotypes occur with a recessive pattern of inheritance, and several genetic lesions in the autosomal recessive hypercholesterolemia (ARH) gene on chromosome 1 have been mapped in this class of patients. ARH has an N-terminal phosphotyrosine-binding (PTB) domain evolutionarily related to that found in Disabled-2 and numb, two endocytic proteins. PTB domains bind to the consensus sequence FXNPXY, corresponding to the internalization motif of the LDL receptor. We show here that in addition to the FXNPXY sequence, ARH binds directly to soluble clathrin trimers and to clathrin adaptors by a mode involving the independently folded appendage domain of the β subunit. At steady state, ARH colocalizes with endocytic proteins in HeLa cells, and the LDL receptor fluxes through peripheral ARH-positive sites before delivery to early endosomes. Because ARH also binds directly to phosphoinositides, which regulate clathrin bud assembly at the cell surface, our data suggest that in ARH patients, defective sorting adaptor function in hepatocytes leads to faulty LDL receptor traffic and hypercholesterolemia.

Syp1 is a conserved endocytic adaptor that contains domains involved in cargo selection and membrane tubulation.

Internalization of diverse transmembrane cargos from the plasma membrane requires a similarly diverse array of specialized adaptors, yet only a few adaptors have been characterized. We report the identification of the muniscin family of endocytic adaptors that is conserved from yeast to human beings. Solving the structures of yeast muniscin domains confirmed the unique combination of an N‐terminal domain homologous to the crescent‐shaped membrane‐tubulating EFC/F‐BAR domains and a C‐terminal domain homologous to cargo‐binding μ homology domains (μHDs). In vitro and in vivo assays confirmed membrane‐tubulation activity for muniscin EFC/F‐BAR domains. The μHD domain has conserved interactions with the endocytic adaptor/scaffold Ede1/eps15, which influences muniscin localization. The transmembrane protein Mid2, earlier implicated in polarized Rho1 signalling, was identified as a cargo of the yeast adaptor protein. These and other data suggest a model in which the muniscins provide a combined adaptor/membrane‐tubulation activity that is important for regulating endocytosis.

A Novel AP-2 Adaptor Interaction Motif Initially Identified in the Long-splice Isoform of Synaptojanin 1, SJ170

Phosphoinositides play a fundamental role in clathrin-coat assembly at the cell surface. Several endocytic components and accessory factors contain independently folded phosphoinositide-binding modules that facilitate, in part, membrane placement at the bud site. As the clathrin-coat assembly process progresses toward deeply invaginated buds, focally synthesized phosphoinositides are dephosphorylated, principally through the action of the phosphoinositide polyphosphatase synaptojanin 1. Failure to catabolize polyphosphoinositides retards the fission process and endocytic activity. The long-splice isoform of synaptojanin 1, termed SJ170, contains a carboxyl-terminal extension that harbors interaction motifs for engaging several components of the endocytic machinery. Here, we demonstrate that in addition to DPF and FXDXF sequences, the SJ170 carboxyl terminus contains a novel AP-2 binding sequence, the WXXF motif. The WXXF sequence engages the independently folded α-subunit appendage that projects off the heterotetrameric AP-2 adaptor core. The endocytic protein kinases AAK1 and GAK also contain functional WXX(FW) motifs in addition to two DPF repeats, whereas stonin 2 harbors three tandem WXXF repeats. Each of the discrete SJ170 adaptor-interaction motifs bind to appendages relatively weakly but, as tandemly arrayed within the SJ170 extension, can cooperate to bind bivalent AP-2 with good apparent affinity. These interactions likely contribute to the appropriate targeting of certain endocytic components to clathrin bud sites assembling at the cell surface.

Functional Dissection of an AP-2 β2 Appendage-binding Sequence within the Autosomal Recessive Hypercholesterolemia Protein

The autosomal recessive hypercholesterolemia (ARH) protein plays a critical role in regulating plasma low density lipoprotein (LDL) levels. Inherited defects in ARH lead to a hypercholesterolemia that closely phenocopies that caused by a defective LDL receptor. The elevated serum LDL-cholesterol levels typical of ARH patients and the pronounced accumulation of the LDL receptor at the cell surface of hepatocytes in ARH-null mice argue that ARH operates by promoting the internalization of the LDL receptor within clathrin-coated vesicles. ARH contains an amino-terminal phosphotyrosine-binding domain that associates physically with the LDL receptor internalization sequence and with phosphoinositides. The carboxyl-terminal half of ARH contains a clathrin-binding sequence and a separate AP-2 adaptor binding region providing a plausible mechanism for how ARH can act as an endocytic adaptor or CLASP (clathrin-associated sorting protein) to couple LDL receptors with the clathrin machinery. Because the interaction with AP-2 is highly selective for the independently folded appendage domain of the β2 subunit, we have characterized the ARH β2 appendage-binding sequence in detail. Unlike the known α appendage-binding motifs, ARH requires an extensive sequence tract to bind the β appendage with comparably high affinity. A minimal 16-residue sequence functions autonomously and depends upon ARH residues Asp253, Phe259, Leu262, and Arg266. We suggested that biased β subunit engagement by ARH and the only other β2 appendage selective adaptor, β-arrestin, promotes efficient incorporation of this mechanistically distinct subset of CLASPs into clathrin-coated buds.

Clathrin Regulates the Association of PIPKIγ661 with the AP-2 Adaptor β2 Appendage

The AP-2 clathrin adaptor differs fundamentally from the related AP-1, AP-3, and AP-4 sorting complexes because membrane deposition does not depend directly on an Arf family GTPase. Instead phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) appears to act as the principal compartmental cue for AP-2 placement at the plasma membrane as well as for the docking of numerous other important clathrin coat components at the nascent bud site. This PtdIns(4,5)P2 dependence makes type I phosphatidylinositol 4-phosphate 5-kinases (PIPKIs) lynchpin enzymes in the assembly of clathrin-coated structures at the cell surface. PIPKIγ is the chief 5-kinase at nerve terminals, and here we show that the 26-amino acid, alternatively spliced C terminus of PIPKIγ661 is an intrinsically unstructured polypeptide that binds directly to the sandwich subdomain of the AP-2 β2 subunit appendage. An aromatic side chain-based, extended interaction motif that also includes the two bulky C-terminal residues of the short PIPKIγ635 variant is necessary for β2 appendage engagement. The clathrin heavy chain accesses the same contact surface on the AP-2 β2 appendage, but because of additional clathrin binding sites located within the unstructured hinge segment of the β2 subunit, clathrin binds the β2 chain with a higher apparent affinity than PIPKIγ661. A clathrin-regulated interaction with AP-2 could allow PIPKIγ661 to be strategically positioned for regional PtdIns(4,5)P2 generation during clathrin-coated vesicle assembly at the synapse.

Impaired pancreatic development in Hif2-alpha deficient mice.

Accumulating data suggest the existence of a link between hypoxia and maintenance of the undifferentiated cell state, but little is known about the cellular signaling mechanisms underlying this process. Recent reports reveal a direct link between components of the hypoxia signaling pathway and Notch pathway in maintaining precursor cells in an undifferentiated state. Here, we report that in the developing mouse pancreas, Hif2-alpha is expressed in pancreatic progenitor cells, but its expression is lost in committed endocrine progenitors as well as in differentiated endocrine and exocrine cells. In an attempt to analyze the function of HIF2-alpha in the developing pancreas, we studied Hif2-alpha(-/-) pancreas. Our analyses revealed that in addition to the decreased size and branching, the Hif2-alpha deficient pancreas also displayed impaired notch signaling and cell differentiation. Finally, we found that HIF2-alpha binds directly to Notch-IC and that the responsible site for this interaction is within the RAM domain of Notch protein. These results suggest that HIF2-alpha is required for normal mouse pancreatic development.

The CFTR trafficking mutation F508del inhibits the constitutive activity of SLC26A9

Several members of the SLC26A family of anion transporters associate with CFTR, forming complexes in which CFTR and SLC26A functions are reciprocally regulated. These associations are thought to be facilitated by PDZ scaffolding interactions. CFTR has been shown to be positively regulated by NHERF-1, and negatively regulated by CAL in airway epithelia. However, it is unclear which PDZ-domain protein(s) interact with SLC26A9, a SLC26A family member found in airway epithelia. We have previously shown that primary, human bronchial epithelia (HBE) from non-CF donors exhibit constitutive anion secretion attributable to SLC26A9. However, constitutive anion secretion is absent in HBE from CF donors. We examined whether changes in SLC26A9 constitutive activity could be attributed to a loss of CFTR trafficking, and what role PDZ interactions played. HEK293 coexpressing SLC26A9 with the trafficking mutant F508del CFTR exhibited a significant reduction in constitutive current compared with cells coexpressing SLC26A9 and wt CFTR. We found that SLC26A9 exhibits complex glycosylation when coexpressed with F508del CFTR, but its expression at the plasma membrane is decreased. SLC26A9 interacted with both NHERF-1 and CAL, and its interaction with both significantly increased with coexpression of wt CFTR. However, coexpression with F508del CFTR only increased SLC26A9s interaction with CAL. Mutation of SLC26A9s PDZ motif decreased this association with CAL, and restored its constitutive activity. Correcting aberrant F508del CFTR trafficking in CF HBE with corrector VX-809 also restored SLC26A9 activity. We conclude that when SLC26A9 is coexpressed with F508del CFTR, its trafficking defect leads to a PDZ motif-sensitive intracellular retention of SLC26A9.