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Stevens BR, Ellory JC, Preston RL. B 0AT1 Amino Acid Transporter Complexed With SARS-CoV-2 Receptor ACE2 Forms a Heterodimer Functional Unit: In Situ Conformation Using Radiation Inactivation Analysis. FUNCTION 2021; 2:zqab027. [PMID: 34847569 PMCID: PMC8194517 DOI: 10.1093/function/zqab027] [Citation(s) in RCA: 13] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2021] [Revised: 05/04/2021] [Accepted: 05/11/2021] [Indexed: 01/06/2023] Open
Abstract
The SARS-CoV-2 receptor, angiotensin-converting enzyme-2 (ACE2), is expressed at levels of greatest magnitude in the small intestine as compared with all other human tissues. Enterocyte ACE2 is coexpressed as the apical membrane trafficking partner obligatory for expression and activity of the B0AT1 sodium-dependent neutral amino acid transporter. These components are assembled as an [ACE2:B0AT1]2 dimer-of-heterodimers quaternary complex that putatively steers SARS-CoV-2 tropism in the gastrointestinal (GI) tract. GI clinical symptomology is reported in about half of COVID-19 patients, and can be accompanied by gut shedding of virion particles. We hypothesized that within this 4-mer structural complex, each [ACE2:B0AT1] heterodimer pair constitutes a physiological "functional unit." This was confirmed experimentally by employing purified lyophilized enterocyte brush border membrane vesicles exposed to increasing doses of high-energy electron radiation from a 16 MeV linear accelerator. Based on radiation target theory, the results indicated the presence of Na+-dependent neutral amino acid influx transport activity functional unit with target size molecular weight 183.7 ± 16.8 kDa in situ in intact apical membranes. Each thermodynamically stabilized [ACE2:B0AT1] heterodimer functional unit manifests the transport activity within the whole ∼345 kDa [ACE2:B0AT1]2 dimer-of-heterodimers quaternary structural complex. The results are consistent with our prior molecular docking modeling and gut-lung axis approaches to understanding COVID-19. These findings advance understanding the physiology of B0AT1 interaction with ACE2 in the gut, and thereby contribute to translational developments designed to treat or mitigate COVID-19 variant outbreaks and/or GI symptom persistence in long-haul postacute sequelae of SARS-CoV-2.
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Affiliation(s)
- Bruce R Stevens
- Department of Physiology and Functional Genomics, University of Florida College of Medicine, Gainesville, FL, 32610, USA
- Department of Medicine, Division of Gastroenterology, University of Florida College of Medicine, Gainesville, FL, 32610, USA
| | - J Clive Ellory
- Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford, OX1 3PT, UK
| | - Robert L Preston
- School of Biological Sciences, Illinois State University, Normal, IL, 61790, USA
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Anchordoquy TJ, Molina MDC, Kempner ES. A radiation target method for size determination of supercoiled plasmid DNA. Anal Biochem 2009; 385:229-33. [PMID: 19028447 PMCID: PMC3085921 DOI: 10.1016/j.ab.2008.10.049] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/06/2008] [Revised: 10/14/2008] [Accepted: 10/30/2008] [Indexed: 11/25/2022]
Abstract
Supercoiled DNA plasmids were exposed in the frozen state to high-energy electrons. Surviving supercoiled molecules were separated from their degradation products (e.g., open circle and linear forms) by agarose gel electrophoresis and subsequently quantified by staining and image analysis. Complex survival curves were analyzed using radiation target theory, yielding the radiation-sensitive mass of each form. One of the irradiated plasmids was transfected into cells, permitting radiation analysis of gene expression. Loss of this function was associated with a mass much smaller than the entire plasmid molecule, indicating a lack of energy transfer in amounts sufficient to cause structural damage along the DNA polynucleotide. The method of radiation target analysis can be applied to study both structure and function of DNA.
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Affiliation(s)
- T J Anchordoquy
- School of Pharmacy, University of Colorado Health Sciences Center, Denver, CO 80262, USA
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Abstract
Frozen solutions of low molecular weight DNA template/primer complexes, in the absence and presence of HIV-1 reverse transcriptase, were irradiated with high-energy electrons. Molecules that survived the radiation exposure were quantified and analyzed using radiation target theory. Transfer of radiation-deposited energy was observed by the damage caused. It was found that damage (as a polynucleotide chain break) was observed in one chain when the radiation interaction occurred in the other chain, suggesting a transfer of energy. In contrast, the target sizes of the DNA template/primers were not altered if bound to HIV-1 reverse transcriptase, signifying that the deposited radiation energy is not transferred between protein and nucleic acid.
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Affiliation(s)
- Nicolas Sluis-Cremer
- Division of Infectious Diseases, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261, USA.
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Sluis-Cremer N, Kempner E, Parniak MA. Structure-activity relationships in HIV-1 reverse transcriptase revealed by radiation target analysis. Protein Sci 2003; 12:2081-6. [PMID: 12931006 PMCID: PMC2324004 DOI: 10.1110/ps.03130503] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
Radiation target analysis is a powerful technique that can be used to determine both the structural and functional sizes of macromolecules. We have used this technique to probe the structure-function relationships of the recombinant forms of HIV-1 reverse transcriptase (RT). For the p66/p51 and p66/p66 dimeric forms of HIV-1 RT, both the structural and functional target sizes corresponded to that of the dimeric protein, indicating that a primary ionization in one subunit of the HIV-1 RT enzyme results in the concomitant polymer scission of both subunits. In contrast to p66/p51 and p66/p66 RT, the individually isolated p51 subunit of HIV-1 RT inactivated as a monomer. However, in the presence of a DNA template/primer substrate, the radiation inactivation analyses of p51 yielded a structural target size corresponding to that of a dimeric protein. This would indicate that the DNA substrate acted as a scaffold or template for p51 RT homodimer formation. In light of this observation, radiation inactivation studies can readily be applied to other DNA polymerase enzymes, such as the murine leukemia virus RT, for which the functional form of the enzyme has yet to be determined.
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Affiliation(s)
- Nicolas Sluis-Cremer
- University of Pittsburgh, Department of Medicine, Pittsburgh, Pennsylvania 15261, USA
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Miller JH, Draper LR, Kempner ES. Direct radiation damage is confined to a single polypeptide in rabbit immunoglobulin G. Biophys J 2003; 84:2781-5. [PMID: 12668485 PMCID: PMC1302843 DOI: 10.1016/s0006-3495(03)75082-6] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022] Open
Abstract
Frozen rabbit immunoglobulin G was exposed to high-energy electrons. The surviving polypeptide subunits were determined and analyzed by radiation target analysis. Each subunit was independently damaged by radiation whether or not they were bound by disulfide bridges to other subunits, demonstrating that in IgG radiation-deposited energy did not travel across disulfide bonds.
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Affiliation(s)
- J H Miller
- Laboratory of Physical Biology, National Institute of Arthritis and Musculoskeletal and Skin Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA
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Kempner ES. Effects of high-energy electrons and gamma rays directly on protein molecules. J Pharm Sci 2001; 90:1637-46. [PMID: 11745722 DOI: 10.1002/jps.1114] [Citation(s) in RCA: 85] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
High-energy electrons and gamma rays ionize molecules at random along their trajectories. In each event, chemical bonds are ruptured, releasing radiolytic products that diffuse away. A solution of macromolecules is mostly water whose principal radiation products are H(+) and OH(-). These can diffuse to and react with macromolecules; this indirect action of radiation is responsible for 99.9% of the damage to proteins. In frozen samples, the ionizations still occur randomly and water is still the principle molecular target, but diffusion of radiation products is limited to only a very small distance. At very low temperatures, essentially all the radiation damage to macromolecules is due to primary ionizations occurring directly in those molecules. Therefore, proteins in frozen solutions are only 10(-3) to 10(-4) as sensitive to radiation as in the liquid state. Every molecule that suffered a direct ionization is destroyed; the only surviving molecules are those that escaped ionization. The survival of frozen proteins after irradiation is a direct measure of the mass of the active structures and independent of the presence of other proteins.
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Affiliation(s)
- E S Kempner
- Laboratory of Physical Biology, National Institute of Arthritis and Musculoskeletal and Skin Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA
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Hernigou P. Allograft sterility as exemplified by human immunodeficiency virus and sterilization by irradiation. J Arthroplasty 2000; 15:1051-8. [PMID: 11112202 DOI: 10.1054/arth.2000.4343] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Affiliation(s)
- P Hernigou
- Service de Chirurgie Orthopedique, Hopital Henry Mondor, Creteil cedex, France
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Bolger G, Liuzzi M, Krogsrud R, Scouten E, McCollum R, Welchner E, Kempner E. Radiation inactivation of ribonucleotide reductase, an enzyme with a stable free radical. Biophys J 2000; 79:2155-61. [PMID: 11023919 PMCID: PMC1301105 DOI: 10.1016/s0006-3495(00)76463-0] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Herpes simplex virus ribonucleotide reductase (RR) is a tetrameric enzyme composed of two homodimers of large R1 and small R2 subunits with a tyrosyl free radical located on the small subunit. Irradiation of the holoenzyme yielded simple exponential decay curves and an estimated functional target size of 315 kDa. Western blot analysis of irradiated holoenzyme R1 and R2 yielded target sizes of 281 kDa and 57 kDa (approximately twice their expected size). Irradiation of free R1 and analysis by all methods yielded a single exponential decay with target sizes ranging from 128-153 kDa. For free R2, quantitation by enzyme activity and Western blot analyses yielded simple inactivation curves but considerably different target sizes of 223 kDa and 19 kDa, respectively; competition for radioligand binding in irradiated R2 subunits yielded two species, one with a target size of approximately 210 kDa and the other of approximately 20 kDa. These results are consistent with a model in which there is radiation energy transfer between the two monomers of both R1 and R2 only in the holoenzyme, a radiation-induced loss of free radical only in the isolated R2, and an alteration of the tertiary structure of R2.
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Affiliation(s)
- G Bolger
- Department of Biological Sciences, Boehringer Ingelheim (Canada) Limited, Bio-Méga Research Division, Laval, Québec H7S 2G5, Canada
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Mittur AV, Kaplowitz N, Kempner ES, Ookhtens M. Radiation inactivation studies of hepatic sinusoidal reduced glutathione transport system. BIOCHIMICA ET BIOPHYSICA ACTA 2000; 1464:207-18. [PMID: 10727608 DOI: 10.1016/s0005-2736(00)00152-8] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
Sinusoidal transport of reduced glutathione (GSH) is a carrier-mediated process. Perfused liver and isolated hepatocyte models revealed a low-affinity transporter with sigmoidal kinetics (K(m) approximately 3.2-12 mM), while studies with sinusoidal membrane vesicles (SMV) revealed a high-affinity unit (K(m) approximately 0.34 mM) besides a low-affinity one (K(m) approximately 3.5-7 mM). However, in SMV, both the high- and low-affinity units manifested Michaelis-Menten kinetics of GSH transport. We have now established the sigmoidicity of the low-affinity unit (K(m) approximately 9) in SMV, consistent with other models, while the high-affinity unit has been retained intact with Michaelis-Menten kinetics (K(m) approximately 0.13 mM). We capitalized on the negligible cross-contributions of the two units to total transport at the low and high ends of GSH concentrations and investigated their characteristics separately, using radiation inactivation, as we did in canalicular GSH transport (Am. J. Physiol. 274 (1998) G923-G930). We studied the functional sizes of the proteins that mediate high- and low-affinity GSH transport in SMV by inactivation of transport at low (trace and 0.02 mM) and high (25 and 50 mM) concentrations of GSH. The low-affinity unit in SMV was much less affected by radiation than in canalicular membrane vesicles (CMV). The target size of the low-affinity sinusoidal GSH transporter appeared to be considerably smaller than both the canalicular low- and high-affinity transporters. The high-affinity unit in SMV was markedly inactivated upon irradiation, revealing a single protein structure with a functional size of approximately 70 kDa. This size is indistinguishable from that of the high-affinity GSH transporter in CMV reported earlier.
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Affiliation(s)
- A V Mittur
- Research Center for Liver Diseases, Department of Medicine, USC School of Medicine, University of Southern California, 2011 Zonal Ave., HMR-615, Los Angeles, CA 90033, USA
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Affiliation(s)
- E S Kempner
- Laboratory of Physical Biology, National Institute of Arthritis and Musculoskeletal and Skin Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA
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Parniak MA, Davis M, Kaufman S, Kempner ES. Radiation target analysis indicates that phenylalanine hydroxylase in rat liver extracts is a functional monomer. FEBS Lett 1999; 449:49-52. [PMID: 10225426 DOI: 10.1016/s0014-5793(99)00392-0] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
The minimal enzymatically functional form of purified rat hepatic phenylalanine hydroxylase (PAH) is a dimer of identical subunits. Radiation target analysis of PAH revealed that the minimal enzymatically active form in crude extracts corresponds to the monomer. The 'negative regulation' properties of the tetrahydrobiopterin cofactor in both crude and pure samples implicates a large multimeric structure, minimally a tetramer of PAH subunits. Preincubation of the samples with phenylalanine prior to irradiation abolished this inhibition component without affecting the minimal functional unit target sizes of the enzyme in both preparations. The characteristics of rat hepatic PAH determined by studies of the purified enzyme in vitro may not completely represent the properties of PAH in vivo.
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Affiliation(s)
- M A Parniak
- Lady Davis Institute for Medical Research, SMBD-Jewish General Hospital, and Department of Medicine, McGill University, Montreal, Quebec, Canada
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Mittur AV, Kaplowitz N, Kempner ES, Ookhtens M. Novel properties of hepatic canalicular reduced glutathione transport revealed by radiation inactivation. THE AMERICAN JOURNAL OF PHYSIOLOGY 1998; 274:G923-30. [PMID: 9612274 DOI: 10.1152/ajpgi.1998.274.5.g923] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Transport of GSH at the canalicular pole of hepatocytes occurs by a facilitative carrier and can account for approximately 50% of total hepatocyte GSH efflux. A low-affinity unit with sigmoidal kinetics accounts for 90% of canalicular transport at physiological GSH concentrations. A low-capacity transporter with high affinity for GSH has also been reported. It is not known whether the same or different proteins mediate low- and high-affinity GSH transport, although they do differ in inhibitor specificity. The bile of rats with a mutation in the canalicular multispecific organic anion transporter (cMOAT or MRP-2, a 170-kDa protein) is deficient in GSH, implying that cMOAT may transport GSH. However, transport of GSH in canalicular membrane vesicles (CMV) from these mutant rats remains intact. We examined the functional size of the two kinetic components of GSH transport by radiation inactivation of GSH uptake in rat hepatic CMV. High-affinity transport of GSH was inactivated as a single exponential function of radiation dose, yielding a functional size of approximately 70 kDa. In contrast, low-affinity canalicular GSH transport exhibited a complex biexponential response to irradiation, characterized by an initial increase followed by a decrease in GSH transport. Inactivation analysis yielded a approximately 76-kDa size for the low-affinity transporter. The complex inactivation indicated that the low-affinity transporter is associated with a larger protein of approximately 141 kDa, which masked approximately 80% of the potential transport activity in CMV. Additional studies, using inactivation of leukotriene C4 transport, yielded a functional size of approximately 302 kDa for cMOAT, indicating that it functions as a dimer.
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Affiliation(s)
- A V Mittur
- Department of Medicine, University of Southern California, Los Angeles 90033, USA
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Harrison EH, Kempner ES. Radiation inactivation studies of hepatic cholesteryl ester hydrolases. Methods Enzymol 1997; 286:116-26. [PMID: 9309648 DOI: 10.1016/s0076-6879(97)86008-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Affiliation(s)
- E H Harrison
- Department of Biochemistry, MCP-Hahnemann School of Medicine, Philadelphia, Pennsylvania 19129, USA
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