A Comparison of GFP-Tagged Clathrin Light Chains with Fluorochromated Light Chains In Vivo and In Vitro

De novo endocytic clathrin coats develop curvature at early stages of their formation

Sculpting a flat patch of membrane into an endocytic vesicle requires curvature generation on the cell surface, which is the primary function of the endocytosis machinery. Using super-resolved live cell fluorescence imaging, we demonstrate that curvature generation by individual clathrin-coated pits can be detected in real time within cultured cells and tissues of developing organisms. Our analyses demonstrate that the footprint of clathrin coats increases monotonically during the formation of pits at different levels of plasma membrane tension. These findings are only compatible with models that predict curvature generation at the early stages of endocytic clathrin pit formation. We also found that CALM adaptors associated with clathrin plaques form clusters, whereas AP2 distribution is more homogenous. Considering the curvature sensing and driving roles of CALM, we propose that CALM clusters may increase the strain on clathrin lattices locally, eventually giving rise to rupture and subsequent pit completion at the edges of plaques.

Nanodissected elastically loaded clathrin lattices relax to increased curvature

Clathrin-mediated endocytosis (CME) is the major endocytosis pathway for the specific internalization of large compounds, growth factors, and receptors. Formation of internalized vesicles from the flat plasma membrane is accompanied by maturation of cytoplasmic clathrin coats. How clathrin coats mature and the mechanistic role of clathrin coats are still largely unknown. Maturation models proposed clathrin coats to mature at constant radius or constant area, driven by molecular actions or elastic energy. Here, combining high-speed atomic force microscopy (HS-AFM) imaging, HS-AFM nanodissection, and elasticity theory, we show that clathrin lattices deviating from the intrinsic curvature of clathrin form elastically loaded assemblies. Upon nanodissection of the clathrin network, the stored elastic energy in these lattices drives lattice relaxation to accommodate an ideal area-curvature ratio toward the formation of closed clathrin-coated vesicles. Our work supports that the release of elastic energy stored in curvature-frustrated clathrin lattices could play a major role in CME.

Fluorescence Recovery After Photobleaching to Study the Dynamics of Membrane-Bound Proteins In Vivo Using the Drosophila Embryo

The epithelial-to-mesenchymal transition is a highly dynamic cell process and tools such as fluorescence recovery after photobleaching (FRAP), which allow the study of rapid protein dynamics, enable the following of this process in vivo. This technique uses a short intense pulse of photons to disrupt the fluorescence of a tagged protein in a region of a sample. The fluorescent signal intensity after this bleaching is then recorded and the signal recovery used to provide an indicator of the dynamics of the protein of interest. This technique can be applied to any fluorescently tagged protein, but membrane-bound proteins present an interesting challenge as they are spatially confined and subject to specialized cellular trafficking. Several methods of analysis can be applied which can disentangle these various processes and enable the extraction of information from the recovery curves. Here we describe this technique when applied to the quantification of the plasma membrane-bound E-cadherin protein in vivo using the epidermis of the late embryo of Drosophila melanogaster (Drosophila) as an example of this technique.

Unusual proteins in Giardia duodenalis and their role in survival

The capacity of the parasite Giardia duodenalis to perform complex functions with minimal amounts of proteins and organelles has attracted increasing numbers of scientists worldwide, trying to explain how this parasite adapts to internal and external changes to survive. One explanation could be that G. duodenalis evolved from a structurally complex ancestor by reductive evolution, resulting in adaptation to its parasitic lifestyle. Reductive evolution involves the loss of genes, organelles, and functions that commonly occur in many parasites, by which the host renders some structures and functions redundant. However, there is increasing data that Giardia possesses proteins able to perform more than one function. During recent decades, the concept of moonlighting was described for multitasking proteins, which involves only proteins with an extra independent function(s). In this chapter, we provide an overview of unusual proteins in Giardia that present multifunctional properties depending on the location and/or parasite requirement. We also discuss experimental evidence that may allow some giardial proteins to be classified as moonlighting proteins by examining the properties of moonlighting proteins in general. Up to date, Giardia does not seem to require the numerous redundant proteins present in other organisms to accomplish its normal functions, and thus this parasite may be an appropriate model for understanding different aspects of moonlighting proteins, which may be helpful in the design of drug targets.

The AP2 adaptor enhances clathrin coat stiffness

Deformation of the plasma membrane into clathrin-coated vesicles is a critical step in clathrin-mediated endocytosis and requires the orchestrated assembly of clathrin and endocytic adaptors into a membrane-associated protein coat. The individual role of these membrane-bending and curvature-stabilizing factors is subject to current debate. As such, it is unclear whether the clathrin coat itself is stiff enough to impose curvature and if so, whether this could be effectively transferred to the membrane by the linking adaptor proteins. We have recently demonstrated that clathrin alone is sufficient to form membrane buds in vitro. Here, we use atomic force microscopy to assess the contributions of clathrin and its membrane adaptor protein 2 (AP2) to clathrin coat stiffness, which determines the mechanics of vesicle formation. We found that clathrin coats are less than 10-fold stiffer than the membrane they enclose, suggesting a delicate balance between the forces harnessed from clathrin coat formation and those required for membrane bending. We observed that clathrin adaptor protein AP2 increased the stiffness of coats formed from native clathrin, but did not affect less-flexible coats formed from clathrin lacking the light chain subunits. We thus propose that clathrin light chains are important for clathrin coat flexibility and that AP2 facilitates efficient cargo sequestration during coated vesicle formation by modulating clathrin coat stiffness.

Crosstalk between CLCb/Dyn1-Mediated Adaptive Clathrin-Mediated Endocytosis and Epidermal Growth Factor Receptor Signaling Increases Metastasis

Signaling receptors are internalized and regulated by clathrin-mediated endocytosis (CME). Two clathrin light chain isoforms, CLCa and CLCb, are integral components of the endocytic machinery whose differential functions remain unknown. We report that CLCb is specifically upregulated in non-small-cell lung cancer (NSCLC) cells and is associated with poor patient prognosis. Engineered single CLCb-expressing NSCLC cells, as well as "switched" cells that predominantly express CLCb, exhibit increased rates of CME and altered clathrin-coated pit dynamics. This "adaptive CME" resulted from upregulation of dynamin-1 (Dyn1) and its activation through a positive feedback loop involving enhanced epidermal growth factor (EGF)-dependent Akt/GSK3β phosphorylation. CLCb/Dyn1-dependent adaptive CME selectively altered EGF receptor trafficking, enhanced cell migration in vitro, and increased the metastatic efficiency of NSCLC cells in vivo. We define molecular mechanisms for adaptive CME in cancer cells and a role for the reciprocal crosstalk between signaling and CME in cancer progression.

Static Clathrin Assemblies at the Peripheral Vacuole-Plasma Membrane Interface of the Parasitic Protozoan Giardia lamblia

Giardia lamblia is a parasitic protozoan that infects a wide range of vertebrate hosts including humans. Trophozoites are non-invasive but associate tightly with the enterocyte surface of the small intestine. This narrow ecological specialization entailed extensive morphological and functional adaptations during host-parasite co-evolution, including a distinctly polarized array of endocytic organelles termed peripheral vacuoles (PVs), which are confined to the dorsal cortical region exposed to the gut lumen and are in close proximity to the plasma membrane (PM). Here, we investigated the molecular consequences of these adaptations on the Giardia endocytic machinery and membrane coat complexes. Despite the absence of canonical clathrin coated vesicles in electron microscopy, Giardia possesses conserved PV-associated clathrin heavy chain (GlCHC), dynamin-related protein (GlDRP), and assembly polypeptide complex 2 (AP2) subunits, suggesting a novel function for GlCHC and its adaptors. We found that, in contrast to GFP-tagged AP2 subunits and DRP, CHC::GFP reporters have no detectable turnover in living cells, indicating fundamental differences in recruitment to the membrane and disassembly compared to previously characterized clathrin coats. Histochemical localization in electron tomography showed that these long-lived GlCHC assemblies localized at distinctive approximations between the plasma and PV membrane. A detailed protein interactome of GlCHC revealed all of the conserved factors in addition to novel or highly diverged proteins, including a putative clathrin light chain and lipid-binding proteins. Taken together, our data provide strong evidence for giardial CHC as a component of highly stable assemblies at PV-PM junctions that likely have a central role in organizing continuities between the PM and PV membranes for controlled sampling of the fluid environment. This suggests a novel function for CHC in Giardia and the extent of molecular remodeling of endocytosis in this species.

In canonical clathrin mediated endocytosis (CME) models, the concerted action of ca. 50 proteins mediates the uptake of extracellular components. The key player in this process is clathrin which coats transport intermediates called clathrin coated vesicles (CCV). The intestinal parasite Giardia lamblia has undergone extensive remodeling during colonization of the mammalian duodenum. Here, we report on unique features of this parasite’s endocytic system, consisting of fixed peripheral vacuoles (PV) in close proximity to the exposed plasma membrane (PM), with no discernible CCVs. Using state-of-the-art imaging strategies, we show that the surface of Giardia trophozoites is pock-marked with PM invaginations reaching to the underlying PV membrane. Co-immunoprecipitation and analysis of protein dynamics reveal that, in line with the absence of CCVs, giardial clathrin assemblies have no dynamic behavior. CHC still remains associated to AP2 and dynamin, both conserved dynamic CME components, and to a newly identified putative clathrin light chain. The emerging model calls for giardial clathrin organized into static cores surrounded by dynamic interaction partners, and most likely involved in the regulation of fusion between the PM and the PVs in a “kiss-and-flush”-like mechanism. This suggests that Giardia harbors a conceptually novel function for clathrin in endocytosis, which might be a consequence of host-parasite co-evolution.

Durable protein lattices of clathrin that can be functionalized with nanoparticles and active biomolecules

Biological molecules that self-assemble and interact with other molecules are attractive building blocks for engineering biological devices. DNA has been widely used for the creation of nanomaterials, but the use of proteins remains largely unexplored. Here, we show that clathrin can form homogeneous and extended two-dimensional lattices on a variety of substrates, including glass, metal, carbon and plastic. Clathrin is a three-legged protein complex with unique self-assembling properties and is relevant in the formation of membrane transport vesicles in eukaryotic cells. We used a fragment of the adaptor protein epsin to immobilize clathrin lattices on the substrates. The lattices span multiple square millimetres with a regular periodicity of 30 nm and can be functionalized via modified subunits of clathrin with either inorganic nanoparticles or active enzymes. The lattices can be stored for months after crosslinking and stabilization with uranyl acetate. They could be dehydrated and rehydrated without loss of function, offering potential applications in sensing and as biosynthetic reactors.

Flat clathrin lattices: stable features of the plasma membrane

Clathrin-mediated endocytosis (CME) is a fundamental property of eukaryotic cells. Classical CME proceeds via the formation of clathrin-coated pits (CCPs) at the plasma membrane, which invaginate to form clathrin-coated vesicles, a process that is well understood. However, clathrin also assembles into flat clathrin lattices (FCLs); these structures remain poorly described, and their contribution to cell biology is unclear. We used quantitative imaging to provide the first comprehensive description of FCLs and explore their influence on plasma membrane organization. Ultrastructural analysis by electron and superresolution microscopy revealed two discrete populations of clathrin structures. CCPs were typified by their sphericity, small size, and homogeneity. FCLs were planar, large, and heterogeneous and present on both the dorsal and ventral surfaces of cells. Live microscopy demonstrated that CCPs are short lived and culminate in a peak of dynamin recruitment, consistent with classical CME. In contrast, FCLs were long lived, with sustained association with dynamin. We investigated the biological relevance of FCLs using the chemokine receptor CCR5 as a model system. Agonist activation leads to sustained recruitment of CCR5 to FCLs. Quantitative molecular imaging indicated that FCLs partitioned receptors at the cell surface. Our observations suggest that FCLs provide stable platforms for the recruitment of endocytic cargo.

Hsc70-induced changes in clathrin-auxilin cage structure suggest a role for clathrin light chains in cage disassembly

The molecular chaperone, Hsc70, together with its co-factor, auxilin, facilitates the ATP-dependent removal of clathrin during clathrin-mediated endocytosis in cells. We have used cryo-electron microscopy to determine the 3D structure of a complex of clathrin, auxilin^401-910 and Hsc70 at pH 6 in the presence of ATP, frozen within 20 seconds of adding Hsc70 in order to visualize events that follow the binding of Hsc70 to clathrin and auxilin before clathrin disassembly. In this map, we observe density beneath the vertex of the cage that we attribute to bound Hsc70. This density emerges asymmetrically from the clathrin vertex, suggesting preferential binding by Hsc70 for one of the three possible sites at the vertex. Statistical comparison with a map of whole auxilin and clathrin previously published by us reveals the location of statistically significant differences which implicate involvement of clathrin light chains in structural rearrangements which occur after Hsc70 is recruited. Clathrin disassembly assays using light scattering suggest that loss of clathrin light chains reduces the efficiency with which auxilin facilitates this reaction. These data support a regulatory role for clathrin light chains in clathrin disassembly in addition to their established role in regulating clathrin assembly.

The role of clathrin in mitotic spindle organisation

Clathrin, a protein best known for its role in membrane trafficking, has been recognised for many years as localising to the spindle apparatus during mitosis, but its function at the spindle remained unclear. Recent work has better defined the role of clathrin in the function of the mitotic spindle and proposed that clathrin crosslinks the microtubules (MTs) comprising the kinetochore fibres (K-fibres) in the mitotic spindle. This mitotic function is unrelated to the role of clathrin in membrane trafficking and occurs in partnership with two other spindle proteins: transforming acidic coiled-coil protein 3 (TACC3) and colonic hepatic tumour overexpressed gene (ch-TOG; also known as cytoskeleton-associated protein 5, CKAP5). This review summarises the role of clathrin in mitotic spindle organisation with an emphasis on the recent discovery of the TACC3-ch-TOG-clathrin complex.

Plasma membrane reshaping during endocytosis is revealed by time-resolved electron tomography

Endocytosis, like many dynamic cellular processes, requires precise temporal and spatial orchestration of complex protein machinery to mediate membrane budding. To understand how this machinery works, we directly correlated fluorescence microscopy of key protein pairs with electron tomography. We systematically located 211 endocytic intermediates, assigned each to a specific time window in endocytosis, and reconstructed their ultrastructure in 3D. The resulting virtual ultrastructural movie defines the protein-mediated membrane shape changes during endocytosis in budding yeast. It reveals that clathrin is recruited to flat membranes and does not initiate curvature. Instead, membrane invagination begins upon actin network assembly followed by amphiphysin binding to parallel membrane segments, which promotes elongation of the invagination into a tubule. Scission occurs on average 9 s after initial bending when invaginations are ∼100 nm deep, releasing nonspherical vesicles with 6,400 nm2 mean surface area. Direct correlation of protein dynamics with ultrastructure provides a quantitative 4D resource.

Diversity of clathrin function: new tricks for an old protein

Clathrin is considered the prototype vesicle coat protein whose self-assembly mediates sorting of membrane cargo and recruitment of lipid modifiers. Detailed knowledge of clathrin biochemistry, structure, and interacting proteins has accumulated since the first observation, almost 50 years ago, of its role in receptor-mediated endocytosis of yolk protein. This review summarizes that knowledge, and focuses on properties of the clathrin heavy and light chain subunits and interaction of the latter with Hip proteins, to address the diversity of clathrin function beyond conventional receptor-mediated endocytosis. The distinct functions of the two human clathrin isoforms (CHC17 and CHC22) are discussed, highlighting CHC22's specialized involvement in traffic of the GLUT4 glucose transporter and consequent role in human glucose metabolism. Analysis of clathrin light chain function and interaction with the actin-binding Hip proteins during bacterial infection defines a novel actin-organizing function for CHC17 clathrin. By considering these diverse clathrin functions, along with intracellular sorting roles and influences on mitosis, further relevance of clathrin function to human health and disease is established.

The generation of curved clathrin coats from flat plaques

Flat clathrin lattices or 'plaques' are commonly believed to be the precursors to clathrin-coated buds and vesicles. The sequence of steps carrying the flat hexagonal lattice into a highly curved polyhedral cage with exactly 12 pentagons remains elusive, however, and the large numbers of disrupted interclathrin connections in previously proposed conversion pathways make these scenarios rather unlikely. The recent notion that clathrin can make controlled small conformational transitions opens new avenues. Simulations with a self-assembling clathrin model suggest that localized conformational changes in a plaque can create sufficiently strong stresses for a dome-like fragment to break apart. The released fragment, which is strongly curved but still hexagonal, may subsequently grow into a cage by recruiting free triskelia from the cytoplasm, thus building all 12 pentagonal faces without recourse to complex topological changes. The critical assembly concentration in a slightly acidic in vitro solution is used to estimate the binding energy of a cage at 25-40 k(B) T/clathrin.

Mitotic moonlighting functions for membrane trafficking proteins

In recent years, cell biologists have uncovered a number of new functions for proteins that were previously thought to operate solely in membrane trafficking. These alternative roles, termed moonlighting functions, can occur at distinct intracellular sites or at different stages of the cell cycle. Here, I evaluate the evidence for mitotic moonlighting functions of proteins that have membrane trafficking roles during interphase. The aim is to identify key issues facing the field and to outline important questions for future work.

A TACC3/ch-TOG/clathrin complex stabilises kinetochore fibres by inter-microtubule bridging

Kinetochore fibres (K-fibres) of the spindle apparatus move chromosomes during mitosis. These fibres are discrete bundles of parallel microtubules (MTs) that are crosslinked by inter-MT 'bridges' that are thought to improve fibre stability during chromosomal movement. The identity of these bridges is unknown. Clathrin is a multimeric protein that has been shown to stabilise K-fibres during early mitosis by a mechanism independent of its role in membrane trafficking. In this study, we show that clathrin at the mitotic spindle is in a transforming acidic colied-coil protein 3 (TACC3)/colonic, hepatic tumour overexpressed gene (ch-TOG)/clathrin complex. The complex is anchored to the spindle by TACC3 and ch-TOG. Ultrastructural analysis of clathrin-depleted K-fibres revealed a selective loss of a population of short inter-MT bridges and a general loss of MTs. A similar loss of short inter-MT bridges was observed in TACC3-depleted K-fibres. Finally, immunogold labelling confirmed that inter-MT bridges in K-fibres contain clathrin. Our results suggest that the TACC3/ch-TOG/clathrin complex is an inter-MT bridge that stabilises K-fibres by physical crosslinking and by reducing rates of MT catastrophe.