Epsins are a family of highly conserved membrane proteins that are important in creating membrane curvature. Epsins contribute to membrane deformations like endocytosis, and block vesicle formation during mitosis. [1]
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Epsins are a family of highly conserved membrane proteins that are important in creating membrane curvature. Epsins contribute to membrane deformations like endocytosis, and block vesicle formation during mitosis. [1]
Epsin contains various protein domains that aid in function. Starting at the N-terminus is the ENTH domain. ENTH stands for Epsin N-Terminal Homolog. The ENTH domain is approximately 150 amino acids long and is highly conserved across species. [1] It is composed of seven α-helices and an eighth helix that is not aligned with the seven helices that make up a superhelical fold. [1] The role of the ENTH domain is to bind membrane lipids which is currently thought to aid in the invagination of the plasma membrane to form clathrin-coated vesicles. Additionally, located toward the C-terminus of the ENTH domain are two to three ubiquitin interacting motifs which aids in ubiquitin dependent recruitment. [1]
Following the ENTH domain there is not as much conservation in structure across species. However, in higher eukaryotes there are several conserved motifs such as the clathrin-binding motifs which bind clathrin heavy chain, these motifs flank a cluster of up to eight DP repeats which bind to AP2.
In general, most vertebrates contain at least two epsin paralogs. The two paralogs, epsin-1 and epsin-2 are members that contribute to the clathrin coated endocytotic machinery and are localized at the plasma membrane. [1] In mammals, the two main classes of Epsins are expressed throughout tissues but has the highest expression in the brain, whereas the third Epsin has higher expression in the epidermis and the stomach. [2]
Epsins have many different domains to interact with various proteins related to endocytosis. At its N-terminus is an ENTH domain that binds phosphatidylinositol (4,5)-bisphosphate, meaning that it binds a lipid of biological membranes. It has also been postulated that this is a site for cargo-binding. In the middle of the epsin sequence are two UIMs (ubiquitin-interacting motifs). The C-terminus contains multiple binding sites, for example for clathrin and AP2 adaptors. As such, epsins are able to bind to membranes with specific cargo and connect them with the endocytosis machinery, so one may understand epsins as something like Swiss army knives for endocytosis.
Epsins may be the major membrane curvature-driving proteins in many clathrin-coated vesicle budding events. In addition to its primary role as an endocytic adapter, there is evidence the epsins play a role in regulating GTPase activity which provides an alternative mechanism for epsin's role in cell polarity and migration. [2]
In addition, Epsin is thought to play a role in the Notch Signaling Pathway, which is critical for normal embryonic development. Notch Signaling is dependent on the proteolytic cleavage of the Notch receptor intracellular domain. Epsin's role in Notch Signaling is due to Notch's reliance on ligand endocytosis to release the Notch intracellular domain. This occurs through ubiquitination of the D114 notch ligand which provides a docking location of the epsin UIM domain. Current research suggest that this directing of cargo material aids in the recycling in Notch signaling as well. A study on knock out epsins 1 and 2 in mice showed embryonic death at day 10. Further investigation showed vascular defects in the embryo proper, placenta and yolk sac which are characteristic of a loss in notch signaling. [3]
There are four human genes encoding epsin family members: EPN1 , EPN2 , EPN3 , and EPN4 .
The epsin homologue of C. elegans is EPN-1. EPN-1 conserves the UIM, ENTH domain, and clathrin-binding motif.
The epsin homologue of Drosophila melanogaster is liquid facets and was first identified due to its role in eye patterning in flies.
There are three Arabidopsis thaliana genes encoding epsin family members, Epsin1, Epsin2 and Epsin3 that differ in molecular weight and C - terminal domains. [4] Epsin1 has highest expression in cotyledons and flowers while Epsin2 and Epsin3 expression is currently unknown. [5] Little is known about the role plant Epsin plays in clathrin coated vesicle formation.
Epsin is thought to have role in the angiogenesis of tumors; thus, epsin has the potential to be a target for anti-cancer therapies. Several cancers including prostate, breast, lung and skin display an upregulation in epsin. Research indicates that the overexpression could affect the regulation of tumor angiogenesis through defects in the notch pathway. [2] There is also evidence that Epsin could lead to colon cancer through impaired Wnt signaling by reducing the stability of the Wnt effector dishevelled, making epsin a possible target for pharmaceuticals. [6]
Epsin 4, which encodes the protein enthoprotin, now known as clathrin interactor 1 (CLINT1), has been studied for a possible relationship to schizophrenia in four independent studies, though no conclusive evidence has been found in the analysis of SNPs believed to be associated with schizophrenia (rs1186922, rs254664, rs10046055). [7] [8] [9] [10] [11] A genetic abnormality in CLINT1 is assumed to change the way internalisation of neurotransmitter receptors occurs in the brains of people with schizophrenia.
Clathrin is a protein that plays a major role in the formation of coated vesicles. Clathrin was first isolated and named by Barbara Pearse in 1976. It forms a triskelion shape composed of three clathrin heavy chains and three light chains. When the triskelia interact they form a polyhedral lattice that surrounds the vesicle, hence the protein's name, which is derived from the Latin clathrum meaning lattice. Coat-proteins, like clathrin, are used to build small vesicles in order to transport molecules within cells. The endocytosis and exocytosis of vesicles allows cells to communicate, to transfer nutrients, to import signaling receptors, to mediate an immune response after sampling the extracellular world, and to clean up the cell debris left by tissue inflammation. The endocytic pathway can be hijacked by viruses and other pathogens in order to gain entry to the cell during infection.
Receptor-mediated endocytosis (RME), also called clathrin-mediated endocytosis, is a process by which cells absorb metabolites, hormones, proteins – and in some cases viruses – by the inward budding of the plasma membrane (invagination). This process forms vesicles containing the absorbed substances and is strictly mediated by receptors on the surface of the cell. Only the receptor-specific substances can enter the cell through this process.
Vesicular transport adaptor proteins are proteins involved in forming complexes that function in the trafficking of molecules from one subcellular location to another. These complexes concentrate the correct cargo molecules in vesicles that bud or extrude off of one organelle and travel to another location, where the cargo is delivered. While some of the details of how these adaptor proteins achieve their trafficking specificity has been worked out, there is still much to be learned.
AP180 is a protein that plays an important role in clathrin-mediated endocytosis of synaptic vesicles. It is capable of simultaneously binding both membrane lipids and clathrin and is therefore thought to recruit clathrin to the membrane of newly invaginating vesicles. In Drosophila melanogaster, deletion of the AP180 homologue, leads to enlarged but much fewer vesicles and an overall decrease in transmitter release. In D. melanogaster it was also shown that AP180 is also required for either recycling vesicle proteins and/or maintaining the distribution of both vesicle and synaptic proteins in the nerve terminal. A ubiquitous form of the protein in mammals, CALM, is named after its association with myeloid and lymphoid leukemias where some translocations map to this gene. The C-terminus of AP180 is a powerful and specific inhibitor of clathrin-mediated endocytosis.
In molecular biology, BAR domains are highly conserved protein dimerisation domains that occur in many proteins involved in membrane dynamics in a cell. The BAR domain is banana-shaped and binds to membrane via its concave face. It is capable of sensing membrane curvature by binding preferentially to curved membranes. BAR domains are named after three proteins that they are found in: Bin, Amphiphysin and Rvs.
The epsin N-terminal homology (ENTH) domain is a structural domain that is found in proteins involved in endocytosis and cytoskeletal machinery.
Amphiphysin is a protein that in humans is encoded by the AMPH gene.
The AP2 adaptor complex is a multimeric protein that works on the cell membrane to internalize cargo in clathrin-mediated endocytosis. It is a stable complex of four adaptins which give rise to a structure that has a core domain and two appendage domains attached to the core domain by polypeptide linkers. These appendage domains are sometimes called 'ears'. The core domain binds to the membrane and to cargo destined for internalisation. The alpha and beta appendage domains bind to accessory proteins and to clathrin. Their interactions allow the temporal and spatial regulation of the assembly of clathrin-coated vesicles and their endocytosis.
Cortactin is a monomeric protein located in the cytoplasm of cells that can be activated by external stimuli to promote polymerization and rearrangement of the actin cytoskeleton, especially the actin cortex around the cellular periphery. It is present in all cell types. When activated, it will recruit Arp2/3 complex proteins to existing actin microfilaments, facilitating and stabilizing nucleation sites for actin branching. Cortactin is important in promoting lamellipodia formation, invadopodia formation, cell migration, and endocytosis.
AP-2 complex subunit mu is a protein that in humans is encoded by the AP2M1 gene.
AP-1 complex subunit mu-1 is a protein that in humans is encoded by the AP1M1 gene.
Dynamin-1 is a protein that in humans is encoded by the DNM1 gene.
AP-1 complex subunit beta-1 is a protein that in humans is encoded by the AP1B1 gene.
Clathrin interactor 1 (CLINT1), also known as EPSIN4, is a protein which in humans is encoded by the CLINT1 gene.
Epsin-1 is a protein that in humans is encoded by the EPN1 gene.
Epsin-2 is a protein that in humans is encoded by the EPN2 gene.
Membrane curvature is the geometrical measure or characterization of the curvature of membranes. The membranes can be naturally occurring or man-made (synthetic). An example of naturally occurring membrane is the lipid bilayer of cells, also known as cellular membranes. Synthetic membranes can be obtained by preparing aqueous solutions of certain lipids. The lipids will then "aggregate" and form various phases and structures. According to the conditions and the chemical structures of the lipid, different phases will be observed. For instance, the lipid POPC tends to form lamellar vesicles in solution, whereas smaller lipids, such as detergents, will form micelles if the CMC was reached. There are five commonly proposed mechanisms by which membrane curvature is created, maintained, or controlled: lipid composition, shaped transmembrane proteins, protein motif insertion/BAR domains, protein scaffolding, and cytoskeleton scaffolding.
Clathrin adaptor proteins, also known as adaptins, are vesicular transport adaptor proteins associated with clathrin. These proteins are synthesized in the ribosomes, processed in the endoplasmic reticulum and transported from the Golgi apparatus to the trans-Golgi network, and from there via small carrier vesicles to their final destination compartment. The association between adaptins and clathrin are important for vesicular cargo selection and transporting. Clathrin coats contain both clathrin and adaptor complexes that link clathrin to receptors in coated vesicles. Clathrin-associated protein complexes are believed to interact with the cytoplasmic tails of membrane proteins, leading to their selection and concentration. Therefore, adaptor proteins are responsible for the recruitment of cargo molecules into a growing clathrin-coated pits. The two major types of clathrin adaptor complexes are the heterotetrameric vesicular transport adaptor proteins (AP1-5), and the monomeric GGA adaptors. Adaptins are distantly related to the other main type of vesicular transport proteins, the coatomer subunits, sharing between 16% and 26% of their amino acid sequence.
The C-terminal domain ofBeta2-adaptin is a protein domain is involved in cell trafficking by aiding import and export of substances in and out of the cell.
In molecular biology, the Ubiquitin-Interacting Motif (UIM), or 'LALAL-motif', is a sequence motif of about 20 amino acid residues, which was first described in the 26S proteasome subunit PSD4/RPN-10 that is known to recognise ubiquitin. In addition, the UIM is found, often in tandem or triplet arrays, in a variety of proteins either involved in ubiquitination and ubiquitin metabolism, or known to interact with ubiquitin-like modifiers. Among the UIM proteins are two different subgroups of the UBP family of deubiquitinating enzymes, one F-box protein, one family of HECT-containing ubiquitin-ligases (E3s) from plants, and several proteins containing ubiquitin-associated UBA and/or UBX domains. In most of these proteins, the UIM occurs in multiple copies and in association with other domains such as UBA, UBX, ENTH domain, EH, VHS, SH3 domain, HECT, VWFA, EF-hand calcium-binding, WD-40, F-box, LIM, protein kinase, ankyrin, PX, phosphatidylinositol 3- and 4-kinase, C2 domain, OTU, DnaJ domain, RING-finger or FYVE-finger. UIMs have been shown to bind ubiquitin and to serve as a specific targeting signal important for monoubiquitination. Thus, UIMs may have several functions in ubiquitin metabolism each of which may require different numbers of UIMs.