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Engle KA, Amos RA, Yang JY, Glushka J, Atmodjo M, Tan L, Huang C, Moremen KW, Mohnen D. Multiple Arabidopsis galacturonosyltransferases synthesize polymeric homogalacturonan by oligosaccharide acceptor-dependent or de novo synthesis. THE PLANT JOURNAL : FOR CELL AND MOLECULAR BIOLOGY 2022; 109:1441-1456. [PMID: 34908202 PMCID: PMC8976717 DOI: 10.1111/tpj.15640] [Citation(s) in RCA: 17] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/01/2021] [Revised: 12/06/2021] [Accepted: 12/09/2021] [Indexed: 05/31/2023]
Abstract
Homogalacturonan (HG), the most abundant pectic glycan, functions as a cell wall structural and signaling molecule essential for plant growth, development and response to pathogens. HG exists as a component of pectic homoglycans, heteroglycans and glycoconjugates. HG is synthesized by members of the GALACTURONOSYLTRANSFERASE (GAUT) family. UDP-GalA-dependent homogalacturonan:galacturonosyltransferase (HG:GalAT) activity has previously been demonstrated for GAUTs 1, 4 and 11, as well as the GAUT1:GAUT7 complex. Here, we show that GAUTs 10, 13 and 14 are also HG:GalATs and that GAUTs 1, 10, 11, 13, 14 and 1:7 synthesize polymeric HG in vitro. Comparison of the in vitro HG:GalAT specific activities of the heterologously-expressed proteins demonstrates GAUTs 10 and 11 with the lowest, GAUT1 and GAUT13 with moderate, and GAUT14 and the GAUT1:GAUT7 complex with the highest HG:GalAT activity. GAUT13 and GAUT14 are also shown to de novo synthesize (initiate) HG synthesis in the absence of exogenous HG acceptors, an activity previously demonstrated for GAUT1:GAUT7. The rate of de novo HG synthesis by GAUT13 and GAUT14 is similar to their acceptor dependent HG synthesis, in contrast to GAUT1:GAUT7 for which de novo synthesis occurred at much lower rates than acceptor-dependent synthesis. The results suggest a unique role for de novo HG synthesis by GAUTs 13 and 14. The reducing end of GAUT13-de novo-synthesized HG has covalently attached UDP, indicating that UDP-GalA serves as both a donor and acceptor substrate during de novo HG synthesis. The functional significance of unique GAUT HG:GalAT catalytic properties in the synthesis of different pectin glycan or glycoconjugate structures is discussed.
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Affiliation(s)
- Kristen A. Engle
- Complex Carbohydrate Research Center, University of Georgia, Athens, Georgia 30602
- Department of Plant Biology, University of Georgia, Athens, Georgia 30602
| | - Robert A. Amos
- Complex Carbohydrate Research Center, University of Georgia, Athens, Georgia 30602
- Department of Biochemistry and Molecular Biology, University of Georgia, Athens, Georgia 30602
| | - Jeong-Yeh Yang
- Complex Carbohydrate Research Center, University of Georgia, Athens, Georgia 30602
| | - John Glushka
- Complex Carbohydrate Research Center, University of Georgia, Athens, Georgia 30602
| | - Melani Atmodjo
- Complex Carbohydrate Research Center, University of Georgia, Athens, Georgia 30602
- Department of Biochemistry and Molecular Biology, University of Georgia, Athens, Georgia 30602
| | - Li Tan
- Complex Carbohydrate Research Center, University of Georgia, Athens, Georgia 30602
| | - Chin Huang
- Complex Carbohydrate Research Center, University of Georgia, Athens, Georgia 30602
- Department of Biochemistry and Molecular Biology, University of Georgia, Athens, Georgia 30602
| | - Kelley W. Moremen
- Complex Carbohydrate Research Center, University of Georgia, Athens, Georgia 30602
- Department of Biochemistry and Molecular Biology, University of Georgia, Athens, Georgia 30602
| | - Debra Mohnen
- Complex Carbohydrate Research Center, University of Georgia, Athens, Georgia 30602
- Department of Biochemistry and Molecular Biology, University of Georgia, Athens, Georgia 30602
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Abstract
Simple and complex carbohydrates (glycans) have long been known to play major metabolic, structural and physical roles in biological systems. Targeted microbial binding to host glycans has also been studied for decades. But such biological roles can only explain some of the remarkable complexity and organismal diversity of glycans in nature. Reviewing the subject about two decades ago, one could find very few clear-cut instances of glycan-recognition-specific biological roles of glycans that were of intrinsic value to the organism expressing them. In striking contrast there is now a profusion of examples, such that this updated review cannot be comprehensive. Instead, a historical overview is presented, broad principles outlined and a few examples cited, representing diverse types of roles, mediated by various glycan classes, in different evolutionary lineages. What remains unchanged is the fact that while all theories regarding biological roles of glycans are supported by compelling evidence, exceptions to each can be found. In retrospect, this is not surprising. Complex and diverse glycans appear to be ubiquitous to all cells in nature, and essential to all life forms. Thus, >3 billion years of evolution consistently generated organisms that use these molecules for many key biological roles, even while sometimes coopting them for minor functions. In this respect, glycans are no different from other major macromolecular building blocks of life (nucleic acids, proteins and lipids), simply more rapidly evolving and complex. It is time for the diverse functional roles of glycans to be fully incorporated into the mainstream of biological sciences.
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Affiliation(s)
- Ajit Varki
- Departments of Medicine and Cellular & Molecular Medicine, Glycobiology Research and Training Center, University of California at San Diego, La Jolla, CA 92093-0687, USA
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Sullivan MA, Aroney STN, Li S, Warren FJ, Joo JS, Mak KS, Stapleton DI, Bell-Anderson KS, Gilbert RG. Changes in glycogen structure over feeding cycle sheds new light on blood-glucose control. Biomacromolecules 2014; 15:660-5. [PMID: 24372590 DOI: 10.1021/bm401714v] [Citation(s) in RCA: 42] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Liver glycogen, a highly branched polymer of glucose, is important for maintaining blood-glucose homeostasis. It was recently shown that db/db mice, a model for Type 2 diabetes, are unable to form the large composite glycogen α particles present in normal, healthy mice. In this study, the structure of healthy mouse-liver glycogen over the diurnal cycle was characterized using size exclusion chromatography and transmission electron microscopy. Glycogen was found to be formed as smaller β particles, and then only assembled into large α particles, with a broad size distribution, significantly after the time when glycogen content had reached a maximum. This pathway, missing in diabetic animals, is likely to give optimal blood-glucose control during the daily feeding cycle. Lack of this control may contribute to, or result from, diabetes. This discovery suggests novel approaches to diabetes management.
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Affiliation(s)
- Mitchell A Sullivan
- Tongji School of Pharmacy, Huazhong University of Science and Technology , Wuhan, Hubei 430030, China
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