Structural basis of an endocytic checkpoint that primes the AP2 clathrin adaptor for cargo internalization

Biomolecular condensation orchestrates clathrin-mediated endocytosis in plants

Clathrin-mediated endocytosis is an essential cellular internalization pathway involving the dynamic assembly of clathrin and accessory proteins to form membrane-bound vesicles. The evolutionarily ancient TSET-TPLATE complex (TPC) plays an essential, but ill-defined role in endocytosis in plants. Here we show that two highly disordered TPC subunits, AtEH1 and AtEH2, function as scaffolds to drive biomolecular condensation of the complex. These condensates specifically nucleate on the plasma membrane through interactions with anionic phospholipids, and facilitate the dynamic recruitment and assembly of clathrin, as well as early- and late-stage endocytic accessory proteins. Importantly, condensation promotes ordered clathrin assemblies. TPC-driven biomolecular condensation thereby facilitates dynamic protein assemblies throughout clathrin-mediated endocytosis. Furthermore, we show that a disordered region of AtEH1 controls the material properties of endocytic condensates in vivo. Alteration of these material properties disturbs the recruitment of accessory proteins, influences endocytosis dynamics and impairs plant responsiveness. Our findings reveal how collective interactions shape endocytosis.

Leucine Zipper Downregulated in Cancer-1 Interacts with Clathrin Adaptors to Control Epidermal Growth Factor Receptor (EGFR) Internalization and Gefitinib Response in EGFR-Mutated Non-Small Cell Lung Cancer

The epidermal growth factor receptor (EGFR) is a common driver of non-small cell lung cancer (NSCLC). Clathrin-mediated internalization (CMI) sustains EGFR signaling. AXL is associated with resistance to EGFR-tyrosine kinase inhibitors (TKIs) in EGFR-mutated (EGFRM) NSCLC. We investigated the effects of Leucine zipper downregulated in cancer-1 (LDOC1) on EGFR CMI and NSCLC treatment. Coimmunoprecipitation, double immunofluorescence staining, confocal microscopy analysis, cell surface labelling assays, and immunohistochemistry studies were conducted. We revealed that LDOC1 interacts with clathrin adaptors through binding motifs. LDOC1 depletion promotes internalization and plasma membrane recycling of EGFR in EGFRM NSCLC PC9 and HCC827 cells. Membranous and cytoplasmic EGFR decreased and increased, respectively, in LDOC1 (-) NSCLC tumors. LDOC1 depletion enhanced and sustained activation of EGFR, AXL, and HER2 and enhanced activation of HER3 in PC9 and HCC827 cells. Sensitivity to first-generation EGFR-TKIs (gefitinib and erlotinib) was significantly reduced in LDOC1-depleted PC9 and HCC827 cells. Moreover, LDOC1 downregulation was significantly associated (p < 0.001) with poor overall survival in patients with EGFRM NSCLC receiving gefitinib (n = 100). In conclusion, LDOC1 may regulate the efficacy of first-generation EGFR-TKIs by participating in the CMI of EGFR. Accordingly, LDOC1 may function as a prognostic biomarker for EGFRM NSCLC.

How will I recognize you? Insights into endocytic cargo recognition in plants

The plasma membrane (PM) houses a wide variety of proteins, facilitating interactions between the cell and its surroundings. Perception of external stimuli leads to selective internalization of membrane proteins via endocytosis. A multitude of endocytic signals affect protein internalization; however, their coordination and the exact mechanism of their recognition still remain elusive. In this review, we summarized the up-to-date knowledge of different internalization signals in PM cargo proteins and their involvement during protein trafficking.

Comparison of endocytosis pathways of Duck Tembusu virus in BHK-21 and duck embryo fibroblasts

The Duck Tembusu virus (DTMUV) is a zoonotic flavivirus characterized by nonsuppurative encephalitis and decreasing egg production that has adversely affected the poultry industry. While the way of invasion of DTMUV into different host cells, especially primary cells, remains elusive. In the present study, the ultrastructural pathological characteristics showed that DTMUV underwent a typical maturation and replication process: progeny virus particles gathered in rough endoplasmic reticulum (RER) cisternae, reached the cell membrane via Golgi body's endocrine channel, then were released in the infected baby hamster kidney-21 (BHK-21) and duck embryo fibroblast (DEF). Endoplasmic reticulum vesicles in BHK-21 were short rods and densely arranged like honeycombs, whereas vesicles in DEF were round and dispersed. Further study showed that the virus replication peak in mammalian BHK-21 cells was at 48 hpi, whereas in avian DEF cells was at 24 hpi. DTMUV entry into BHK-21 and DEF cells was blocked by clathrin inhibitor, chlorpromazine (CPZ), indicating that the flavivirus DTMUV enters BHK-21 and DEF both via a clathrin-mediated endocytosis (CME) pathway rather than caveola-mediated endocytosis or micropinocytosis. The endocytic difference in DTMUV entry into BHK-21 and DEF cells might provide insight into understanding the underlying virulence difference between passaged cells and cultured cells.

Lipid nanodiscs as a template for high-resolution cryo-EM structures of peripheral membrane proteins

Peripheral membrane proteins are ubiquitous throughout cell biology and are required for a variety of cellular processes such as signal transduction, membrane trafficking, and autophagy. Transient binding to the membrane has a profound impact on protein function, serving to induce conformational changes and alter biochemical and biophysical parameters by increasing the local concentration of factors and restricting diffusion to two dimensions. Despite the centrality of the membrane in serving as a template for cell biology, there are few reported high-resolution structures of peripheral membrane proteins bound to the membrane. We analyzed the utility of lipid nanodiscs to serve as a template for cryo-EM analysis of peripheral membrane proteins. We tested a variety of nanodiscs and we report a 3.3 Å structure of the AP2 clathrin adaptor complex bound to a 17-nm nanodisc, with sufficient resolution to visualize a bound lipid head group. Our data demonstrate that lipid nanodiscs are amenable to high-resolution structure determination of peripheral membrane proteins and provide a framework for extending this analysis to other systems.

SGIP1 in axons prevents internalization of desensitized CB1R and modifies its function

In the central nervous system (CNS), cannabinoid receptor 1 (CB1R) is preferentially expressed in axons where it has a unique property, namely resistance to agonist-driven endocytosis. This review aims to summarize what we know about molecular mechanisms of CB1R cell surface stability in axonal compartments, how these impact CB1R signaling, and to consider their physiological consequences. This review then focuses on a potential candidate for maintaining axonal CB1R at the cell surface, Src homology 3-domain growth factor receptor-bound 2-like endophilin interacting protein 1 (SGIP1). SGIP1 may contribute to the polarized distribution of CB1R and modify its signaling in axons. In addition, deletion of SGIP1 results in discrete behavioral changes in modalities controlled by the endocannabinoid system in vivo. Several drugs acting directly via CB1R have important therapeutic potential, however their adverse effects limit their clinical use. Future studies might reveal chemical approaches to target the SGIP1-CB1R interaction, with the aim to exploit the endocannabinoid system pharmaceutically in a discrete way, with minimized undesired consequences.

O-GlcNAc transferase modulates the cellular endocytosis machinery by controlling the formation of clathrin-coated pits

Clathrin-mediated endocytosis (CME) controls the internalization and function of a wide range of cell surface proteins. CME occurs by the assembly of clathrin and many other proteins on the inner leaflet of the plasma membrane into clathrin-coated pits (CCPs). These structures recruit specific cargo destined for internalization, generate membrane curvature, and in many cases undergo scission from the plasma membrane to yield intracellular vesicles. The diversity of functions of cell surface proteins controlled via internalization by CME may suggest that regulation of CCP formation could be effective to allow cellular adaptation under different contexts. Of interest is how cues derived from cellular metabolism may regulate CME, given the reciprocal role of CME in controlling cellular metabolism. The modification of proteins with O-linked β-GlcNAc (O-GlcNAc) is sensitive to nutrient availability and may allow cellular adaptation to different metabolic conditions. Here, we examined how the modification of proteins with O-GlcNAc may control CCP formation and thus CME. We used perturbation of key enzymes responsible for protein O-GlcNAc modification, as well as specific mutants of the endocytic regulator AAK1 predicted to be impaired for O-GlcNAc modification. We identify that CCP initiation and the assembly of clathrin and other proteins within CCPs are controlled by O-GlcNAc protein modification. This reveals a new dimension of regulation of CME and highlights the important reciprocal regulation of cellular metabolism and endocytosis.