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Arabinogalactan Protein–Rare Earth Element Complexes Activate Plant Endocytosis

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Arabinogalactan Protein–Rare Earth Element Complexes Activate Plant Endocytosis Arabinogalactan protein–rare earth element complexes activate plant endocytosis Lihong Wanga,b,1, Mengzhu Chenga,1, Qing Yanga,1, Jigang Lic, Xiang Wanga, Qing Zhoub, Shingo Nagawad,e, Binxin Xiab, Tongda Xud,e, Rongfeng Huangd,e, Jingfang Hea, Changjiang Lic, Ying Fuc, Ying Liua, Jianchun Baoa, Haiyan Weia, Hui Lie,f, Li Tane, Zhenhong Gue, Ao Xiaa, Xiaohua Huanga,2, Zhenbiao Yangg,h, and Xing Wang Dengi,2 aNational and Local Joint Engineering Research Center of Biomedical Functional Materials, Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing Normal University, 210023 Nanjing, China; bState Key Laboratory of Food Science and Technology, Jiangnan University, 214122 Wuxi, China; cChina State Key Laboratory of Plant Physiology and Biochemistry, College of Biological Sciences, China Agricultural University, 100193 Beijing, China; dFujian Agriculture and Forestry University–University of California, Riverside Joint Center for Horticultural Biology and Metabolomics, Haixia Institute of Science and Technology, Fujian Agriculture and Forestry University, 350002 Fuzhou, China; eShanghai Center for Plant Stress Biology, Shanghai Institute of Biological Sciences, Chinese Academy of Sciences, 201602 Shanghai, China; fSchool of Life Sciences, East China Normal University, 200241 Shanghai, China; gCenter for Plant Cell Biology, Institute of Integrative Genome Biology, University of California, Riverside, CA 92521; hDepartment of Botany and Plant Sciences, University of California, Riverside, CA 92521; and iState Key Laboratory of Protein and Plant Gene Research, The Peking-Tsinghua Center for Life Sciences, School of Advanced Agricultural Sciences and School of Life Sciences, Peking University, 100871 Beijing, China Contributed by Xing Wang Deng, May 29, 2019 (sent for review February 12, 2019; reviewed by Liwen Jiang and Enrique Rojo) Endocytosis is essential to all eukaryotes, but how cargoes are challenges, such as in the identification and visualization of cargo selected for internalization remains poorly characterized. Extra- receptor proteins and their interaction with cargoes in vivo (1–4). cellular cargoes are thought to be selected by transmembrane By visualizing the dynamic of cargo or its receptor protein using receptors that bind intracellular adaptors proteins to initiate stimulated emission depletion (STED) microscopy, and in- endocytosis. Here, we report a mechanism for clathrin-mediated vestigating the mechanism for the endocytosis of rare earth el- endocytosis (CME) of extracellular lanthanum [La(III)] cargoes, ements (REEs), e.g., lanthanum [La(III)], we have identified a which requires extracellular arabinogalactan proteins (AGPs) that mechanism for the activation of CME and cargo recognition. are anchored on the outer face of the plasma membrane. AGPs The REEs in the periodic table of elements comprise 15 lan- PLANT BIOLOGY were colocalized with La(III) on the cell surface and in La(III)- thanide elements plus scandium and yttrium, which have similar Arabidopsis induced endocytic vesicles in leaf cells. Superresolu- properties in atomic radius and charge (5) and are considered to tion imaging showed that La(III) triggered AGP movement across be nonessential elements of living organisms. REEs are exten- the plasma membrane. AGPs were then colocalized and physically sively used in agriculture, industry, national defense, environ- associated with the μ subunit of the intracellular adaptor protein 2 mental protection, medicine, etc. (6, 7). For decades, REEs have (AP2) complexes. The AGP-AP2 interaction was independent of been ingredients of fertilizers for the improvement of plant growth CME, whereas AGP’s internalization required CME and AP2. More- and crop yields mainly via foliage spraying, but the mechanisms for over, we show that AGP-dependent endocytosis in the presence of La(III) also occurred in human cells. These findings indicate that extracellular AGPs act as conserved CME cargo receptors, thus chal- Significance lenging the current paradigm about endocytosis of extracellular cargoes. Due to the vital role for eukaryotes, clathrin-mediated endo- cytosis (CME) has attracted increasing attention, but great arabinogalactan proteins | endocytosis | extracellular cargo | lanthanum | challenges still stand. Here, by overcoming the biggest chal- superresolution imaging lenge in CME research, we visualized the dynamic of extracel- lular cargo receptor protein by using stimulated emission ndocytosis, including clathrin-mediated endocytosis (CME), depletion microscopy. We identified an unconventional mech- Eis a fundamental cellular process in plants, animals, and anism for extracellular cargoes, here specifically rare earth el- microorganisms (1, 2). By internalizing extracellular cargoes and ements (REEs). We showed that arabinogalactan proteins membrane-integral or -associated proteins, CME plays an es- (AGPs) act as extracellular cargo receptors and move across the sential role in various cellular processes, such as cell signaling, plasma membrane to initiate endocytosis. REEs promote the cell-polarity formation, cell-fate determination, cell division, and cross-membrane translocation of its extracellular cargo re- cell movement (1, 2). Consequently, the mechanisms for cargo ceptor AGPs to activate their endocytosis. Our data thus pro- recognitions and CME machinery and processes have been ex- vide insights into the mechanism for the activation of CME, the tensively studied (1–4). A suite of adaptor proteins have been biological role of AGPs, and the cellular mechanisms of REE shown to be involved in the recognition of cargoes or cargo re- actions in plants. ceptors at the cytoplasmic side of the plasma membrane (PM). Author contributions: X.H. and X.W.D. designed research; L.W., M.C., Q.Y., X.W., B.X., One of the best-characterized and conserved adaptor proteins R.H., J.H., C.L., and Y.L. performed research; L.W., M.C., Q.Y., J.L., Q.Z., S.N., T.X., Y.F., J.B., belongs to adaptor protein 2 (AP2) complexes consisting of H.W., H.L., L.T., Z.G., A.X., X.H., and X.W.D. analyzed data; and L.W., M.C., Q.Y., J.L., X.H., 4 subunits, α, β, σ, and μ subunits (2–4). Upon activation by Z.Y., and X.W.D. wrote the paper. protein–protein interaction or modification such as phosphory- Reviewers: L.J., The Chinese University of Hong Kong; and E.R., Consejo Superior de lation, membrane cargo proteins are recognized by AP2, which Investigaciones Científicas. then recruits clathrin subunit proteins for clathrin coat assembly The authors declare no conflict of interest. (2–4). It is well established that extracellular cargoes are recog- Published under the PNAS license. nized by transmembrane receptors, whose cytoplasmic domains 1L.W., M.C., and Q.Y. contributed equally to this work. interact with CME adaptor proteins to initiate cargo endocytosis 2To whom correspondence may be addressed. Email: [email protected] or (2–4). Despite these advances, major gaps remain in our un- [email protected]. derstanding of CME, particularly regarding the mechanisms This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. underpinning the recognition of cargoes and the regulation of 1073/pnas.1902532116/-/DCSupplemental. CME. Furthermore, the current research on CME faces major Published online June 25, 2019. www.pnas.org/cgi/doi/10.1073/pnas.1902532116 PNAS | July 9, 2019 | vol. 116 | no. 28 | 14349–14357 Downloaded by guest on September 26, 2021 REEs to enter plant cells and their action mechanisms to promote with La(III), in which the outlines of the endocytic compartments plant growth remain poorly characterized. On the other hand, the were drawn by AGPs labeled by immunogold particles (Fig. 1B widespread applications of REEs have also resulted in the massive and SI Appendix,TableS1). The observed sizes of the endocytic accumulation of REEs in the global environment (including soil, compartments were in the range of 100 to 200 nm (Fig. 1B), which water, and atmosphere) and living organisms at an unprecedented were consistent with those of the EMARG images (Fig. 1A). The speed (8–10). Therefore, the pollution of REEs is rapidly emerg- results were also consistent with the previously published size ing as a universal threat to ecological integrity and function, as well ranges of CME determined by TEM that has a resolution down to as human health (11), highlighting the urgent need for establishing the nanometer level and thus is commonly used to determine the guidelines to limit the concentration of REEs in the ecosystem. To size of CME vesicles in living organisms (1, 2, 19). Meanwhile, accomplish this goal, it is imperative that we have a clear quali- compared with untreated cells, La(III) treatment greatly increased tative and quantitative examination about how REEs are absorbed the labeling of AGPs on the PM (especially the invaginated PM by and act on plants, especially in leaves, which are directly sprayed domains) apparently undergoing endocytosis (Fig. 1 B and D and with REEs in agricultural application. Recently, by using inter- SI Appendix,TableS1), suggesting that the colocalization of disciplinary techniques, including electron microscopic autoradi- La(III) and AGPs on the PM may induce AGP internalization. ography (EMARG) of radioactive La, cerium (Ce), and terbium AGPs belong to a large family of glycoproteins ubiquitously (Tb) [140La(III), 141Ce(III), and 160Tb(III)], we directly observed found in plants (20, 21).
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