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Structural biology

Topic: Biology

 From HandWiki - Reading time: 16 min

Short description: Study of molecular structures in biology
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Structure of nucleoprotein MA: The 50S ribosomal subunit from H. marismortui X-ray crystallographic model of 29 of the 33 native components, from the laboratory of Thomas Steitz. Of the 31 component proteins, 27 are shown (blue), along with its 2 RNA strands (orange/yellow).[1] Scale: assembly is approx. 24 nm across.[2]

Structural biology deals with structural analysis of living material (formed, composed of, and/or maintained and refined by living cells) at every level of organization.[3]

Early structural biologists throughout the 19th and early 20th centuries were primarily only able to study structures to the limit of the naked eye's visual acuity and through magnifying glasses and light microscopes. In the 20th century, a variety of experimental techniques were developed to examine the 3D structures of biological molecules. The most prominent techniques are X-ray crystallography, nuclear magnetic resonance, and electron microscopy. Through the discovery of X-rays and its applications to protein crystals, structural biology was revolutionized, as now scientists could obtain the three-dimensional structures of biological molecules in atomic detail.[4] Likewise, NMR spectroscopy allowed information about protein structure and dynamics to be obtained.[5] Finally, in the 21st century, electron microscopy also saw a drastic revolution with the development of more coherent electron sources, aberration correction for electron microscopes, and reconstruction software that enabled the successful implementation of high resolution cryo-electron microscopy, thereby permitting the study of individual proteins and molecular complexes in three-dimensions at angstrom resolution.

With the development of these three techniques, the field of structural biology expanded and also became a branch of molecular biology, biochemistry, and biophysics concerned with the molecular structure of biological macromolecules (especially proteins, made up of amino acids, RNA or DNA, made up of nucleotides, and membranes, made up of lipids), how they acquire the structures they have, and how alterations in their structures affect their function.[6] This subject is of great interest to biologists because macromolecules carry out most of the functions of cells, and it is only by coiling into specific three-dimensional shapes that they are able to perform these functions. This architecture, the "tertiary structure" of molecules, depends in a complicated way on each molecule's basic composition, or "primary structure." At lower resolutions, tools such as FIB-SEM tomography have allowed for greater understanding of cells and their organelles in 3-dimensions, and how each hierarchical level of various extracellular matrices contributes to function (for example in bone). In the past few years it has also become possible to predict highly accurate physical molecular models to complement the experimental study of biological structures.[7] Computational techniques such as molecular dynamics simulations can be used in conjunction with empirical structure determination strategies to extend and study protein structure, conformation and function.[8]

Hemoglobin, the oxygen transporting protein found in red blood cells
Examples of protein structures from the Protein Data Bank (PDB)

History

In 1912 Max Von Laue directed X-rays at crystallized copper sulfate generating a diffraction pattern.[9] These experiments led to the development of X-ray crystallography, and its usage in exploring biological structures.[7] In 1951, Rosalind Franklin and Maurice Wilkins used X-ray diffraction patterns to capture the first image of deoxyribonucleic acid (DNA). Francis Crick and James Watson modeled the double helical structure of DNA using this same technique in 1953 and received the Nobel Prize in Medicine along with Wilkins in 1962.[10]

Pepsin crystals were the first proteins to be crystallized for use in X-ray diffraction, by Theodore Svedberg who received the 1962 Nobel Prize in Chemistry.[11] The first tertiary protein structure, that of myoglobin, was published in 1958 by John Kendrew.[12] During this time, modeling of protein structures was done using balsa wood or wire models.[13] With the invention of modeling software such as CCP4 in the late 1970s,[14] modeling is now done with computer assistance. Recent developments in the field have included the generation of X-ray free electron lasers, allowing analysis of the dynamics and motion of biological molecules,[15] and the use of structural biology in assisting synthetic biology.[16]

In the late 1930s and early 1940s, the combination of work done by Isidor Rabi, Felix Bloch, and Edward Mills Purcell led to the development of nuclear magnetic resonance (NMR). Currently, solid-state NMR is widely used in the field of structural biology to determine the structure and dynamic nature of proteins (protein NMR).[17]

In 1990, Richard Henderson produced the first three-dimensional, high resolution image of bacteriorhodopsin using cryogenic electron microscopy (cryo-EM).[18] Since then, cryo-EM has emerged as an increasingly popular technique to determine three-dimensional, high resolution structures of biological images.[19]

More recently, computational methods have been developed to model and study biological structures. For example, molecular dynamics (MD) is commonly used to analyze the dynamic movements of biological molecules. In 1975, the first simulation of a biological folding process using MD was published in Nature.[20] Recently, protein structure prediction was significantly improved by a new machine learning method called AlphaFold.[21] Some claim that computational approaches are starting to lead the field of structural biology research.[22]

Techniques

Biomolecules are too small to see in detail even with the most advanced light microscopes. The methods that structural biologists use to determine their structures generally involve measurements on vast numbers of identical molecules at the same time. These methods include:

Most often researchers use them to study the "native states" of macromolecules. But variations on these methods are also used to watch nascent or denatured molecules assume or reassume their native states. See protein folding.

A third approach that structural biologists take to understanding structure is bioinformatics to look for patterns among the diverse sequences that give rise to particular shapes. Researchers often can deduce aspects of the structure of integral membrane proteins based on the membrane topology predicted by hydrophobicity analysis. See protein structure prediction.

Applications

Structural biologists have made significant contributions towards understanding the molecular components and mechanisms underlying human diseases. For example, cryo-EM and ssNMR have been used to study the aggregation of amyloid fibrils, which are associated with Alzheimer's disease, Parkinson's disease, and type II diabetes.[23] In addition to amyloid proteins, scientists have used cryo-EM to produce high resolution models of tau filaments in the brain of Alzheimer's patients which may help develop better treatments in the future.[24] Structural biology tools can also be used to explain interactions between pathogens and hosts. For example, structural biology tools have enabled virologists to understand how the HIV envelope allows the virus to evade human immune responses.[25]

Structural biology is also an important component of drug discovery.[26] Scientists can identify targets using genomics, study those targets using structural biology, and develop drugs that are suited for those targets. Specifically, ligand-NMR, mass spectrometry, and X-ray crystallography are commonly used techniques in the drug discovery process. For example, researchers have used structural biology to better understand Met, a protein encoded by a protooncogene that is an important drug target in cancer.[27] Similar research has been conducted for HIV targets to treat people with AIDS.[26] Researchers are also developing new antimicrobials for mycobacterial infections using structure-driven drug discovery.[26]

Macromolecular assembly

Selected biological assemblies
  • 3D printed model of the structure of a bacterial flagellum "motor" and partial rod structure of a Salmonella species. Bottom to top: dark blue, repeating FliM and FliN, motor/switch proteins; red, FliG motor/switch proteins; yellow, FliF transmembrane coupling proteins; light blue, L and P ring proteins; and (at top), dark blue, the cap, hook-filament junction, hook, and rod proteins.[28]
    3D printed model of the structure of a bacterial flagellum "motor" and partial rod structure of a Salmonella species. Bottom to top: dark blue, repeating FliM and FliN, motor/switch proteins; red, FliG motor/switch proteins; yellow, FliF transmembrane coupling proteins; light blue, L and P ring proteins; and (at top), dark blue, the cap, hook-filament junction, hook, and rod proteins.[28]
  • A eukaryotic ribosome, which catalytically translate the information content contained in mRNA molecules into proteins. The animation presents the elongation and membrane targeting stages of eukaryotic translation, showing the mRNA as a black arc, the ribosome subunits in green and yellow, tRNAs in dark blue, proteins such as elongation and other factors involved in light blue, the growing polypeptide chain as a black thread growing vertically from the curve of the mRNA. At end of the animation, the polypeptide produced is extruded through a light blue SecY pore[29] into the gray interior of the ER
    A eukaryotic ribosome, which catalytically translate the information content contained in mRNA molecules into proteins. The animation presents the elongation and membrane targeting stages of eukaryotic translation, showing the mRNA as a black arc, the ribosome subunits in green and yellow, tRNAs in dark blue, proteins such as elongation and other factors involved in light blue, the growing polypeptide chain as a black thread growing vertically from the curve of the mRNA. At end of the animation, the polypeptide produced is extruded through a light blue SecY pore[29] into the gray interior of the ER
  • Cross-sections of phospholipid (PLs) relevant to biomembrane MAs. Yellow-orange indicates hydrophobic lipid tails; black and white spheres represent PL polar regions (v.i.). Bilayer/liposome dimensions (obscured in graphic): hydrophobic and polar regions, each ~30 Å (3.0 nm) "thick"—the polar from ~15 Å (1.5 nm) on each side.[30][31][32][non-primary source needed][33]
    Cross-sections of phospholipid (PLs) relevant to biomembrane MAs. Yellow-orange indicates hydrophobic lipid tails; black and white spheres represent PL polar regions (v.i.). Bilayer/liposome dimensions (obscured in graphic): hydrophobic and polar regions, each ~30 Å (3.0 nm) "thick"—the polar from ~15 Å (1.5 nm) on each side.[30][31][32][non-primary source needed][33]
  • A graphical representation of the structure of a viral MA, cowpea mosaic virus, with 30 copies of each of its coat proteins, the small coat protein (S, yellow) and the large coat protein (L, green), which, along with 2 molecules of positive-sense RNA (RNA-1 and RNA-2, not visible) constitute the virion. The assembly is highly symmetric, and is ~280 Å (28 nm) across at its widest point.[verification needed]
    A graphical representation of the structure of a viral MA, cowpea mosaic virus, with 30 copies of each of its coat proteins, the small coat protein (S, yellow) and the large coat protein (L, green), which, along with 2 molecules of positive-sense RNA (RNA-1 and RNA-2, not visible) constitute the virion. The assembly is highly symmetric, and is ~280 Å (28 nm) across at its widest point.[verification needed]
  • tobacco mosaic virus protein coat; virus protein coats known as capsids are examples of macromolecular cages in nature. The protein shell encapsulates a hollow chamber which contains the viral genomic information.
    tobacco mosaic virus protein coat; virus protein coats known as capsids are examples of macromolecular cages in nature. The protein shell encapsulates a hollow chamber which contains the viral genomic information.
  • Ferritin is a hollow sphere that has an outer diameter close to 12 nm and an inner diameter of 7-8 nm. The inner chamber of ferritin uptakes or binds iron in its ferrous state and stores iron in its ferric or trivalent state.
    Ferritin is a hollow sphere that has an outer diameter close to 12 nm and an inner diameter of 7-8 nm. The inner chamber of ferritin uptakes or binds iron in its ferrous state and stores iron in its ferric or trivalent state.
  • The RNA exosome is a biomolecular cage that has nuclease activity which catalyzes the breakdown of RNA molecules. Shown is the whole RNA exosome with a helical protein framework and a central chamber where RNA degradation occurs.
    The RNA exosome is a biomolecular cage that has nuclease activity which catalyzes the breakdown of RNA molecules. Shown is the whole RNA exosome with a helical protein framework and a central chamber where RNA degradation occurs.

Structural biology overlaps with the concept of macromolecular assembly, which refers to massive chemical structures such as viruses, cellular organelles and membranes,[34] and ribosomes, etc. Many are multicomponent, being comprise of polypeptide, polynucleotide, polysaccharide, and or other polymeric macromolecules.

Biomolecular complex

A biomolecular complex, also called a biomacromolecular complex, is any biological complex made of more than one biopolymer (protein, RNA, DNA, [35] carbohydrate) or large non-polymeric biomolecules (lipid). The interactions between these biomolecules are non-covalent. [36] Examples:

The biomacromolecular complexes are studied structurally by X-ray crystallography, NMR spectroscopy of proteins, cryo-electron microscopy and successive single particle analysis, and electron tomography. [39] The atomic structure models obtained by X-ray crystallography and biomolecular NMR spectroscopy can be docked into the much larger structures of biomolecular complexes obtained by lower resolution techniques like electron microscopy, electron tomography, and small-angle X-ray scattering. [40]


Cages

Protein cages

The term protein cage delineates a diverse range of protein structures that are formed by the self-assembly of protein subunits into hollow macromolecular nanoparticles.[41] These protein cages are nanoparticles that have one or more cavities present in their structure. The size of the cavity contributes to the size of the particle that the cavity can enclose, for example inorganic nanoparticles, nucleic acids, and even other proteins.[41] The interior or chamber portion of the protein cage is usually accessible through a pore which is located in between protein subunits.[41][42] The RNA exosome has nuclease active sites that are present in a cavity where 3' RNA degradation takes place; access to this cavity is controlled by a pore and this serves to prevent uncontrollable RNA decay.[42] Some protein cages are dynamic structures that assemble and disassemble in response to external stimuli.[41] Other examples of protein cages are clathrin cages, viral envelopes, chaperonins, and the iron storage protein ferritin.[43][41]

Genetically engineered cages

Macromolecular cages can also be formed synthetically using biomolecules. Protein cages can be genetically engineered, and the outside of the cage can be tailored with synthetic polymers, which is known as protein-polymer conjugation.[41] Preformed polymer chains can be attached to the surface of the protein using chemical linkers. Polymerization can also occur from the protein surface, and the polymer can also be bound to the surface of protein cages via electrostatic interactions.[41] The purpose of this modification is to make synthetic protein cages more biocompatible; this post synthetic modification makes the protein cage less susceptible to an immune response and stabilizes the cage from degradation from proteases.[41] Virus-like protein (VLP) cages have also been synthesized and recombinant DNA technology is used to form non-native virus-like proteins. The first reported case of the formation of non-native VLP constructs into a capsid-like structure utilized a functionalized gold core for nucleation.[44] The self-assembly of the VLP was initiated by the electrostatic interaction of the functionalized gold nanoparticles which is similar to the interaction of a native virus with its nucleic acid component. These viral protein cages have potential applications in biosensing and medical imaging.[44] DNA origami is another strategy to form macromolecular cages or containers. In one case, a 3D macromolecular cage with icosahedral symmetry (resembling viral capsids) was formed based on the synthetic strategy in 2D origami.[45] The structure had an inside volume or hollow cavity encased by triangular faces, similar to a pyramid. This close-faced cage was designed to potentially encapsulate other materials such as proteins and metal nanoparticles.[45]===Virus assembly=== During assembly of the bacteriophage (phage) T4 virion, the morphogenetic proteins encoded by the phage genes interact with each other in a characteristic sequence. Maintaining an appropriate balance in the amounts of each of these proteins produced during viral infection appears to be critical for normal phage T4 morphogenesis.[46] Phage T4 encoded proteins that determine virion structure include major structural components, minor structural components and non-structural proteins that catalyze specific steps in the morphogenesis sequence[47]

See also

References

  1. ↑ "The complete atomic structure of the large ribosomal subunit at 2.4 A resolution". Science 289 (5481): 905–920. August 2000. doi:10.1126/science.289.5481.905. PMID 10937989. Bibcode: 2000Sci...289..905B. 
  2. ↑ "50S Ribosome Subunit". http://www.bio.cmu.edu/courses/03231/LecF03/Lec22/lec22img.html. 
  3. ↑ Liljas, Anders (2009). "What is Structural Biology and When Did It Start". Textbook of Structural Biology. World Scientific. pp. 7–9. ISBN 978-981-277-207-7. https://books.google.com/books?id=aCvtLB0hX44C&pg=PA7. 
  4. ↑ Curry, Stephen (3 July 2015). "Structural Biology: A Century-long Journey into an Unseen World". Interdisciplinary Science Reviews 40 (3): 308–328. doi:10.1179/0308018815Z.000000000120. PMID 26740732. Bibcode: 2015ISRv...40..308C. 
  5. ↑ Campbell, Iain D. (2013). "The evolution of protein NMR". Biomedical Spectroscopy and Imaging 2 (4): 245–264. doi:10.3233/BSI-130055. 
  6. ↑ Banaszak, Leonard J. (2000). Foundations of Structural Biology. Elsevier. ISBN 978-0-08-052184-8. [page needed]
  7. ↑ 7.0 7.1 Jumper, John; Evans, Richard; Pritzel, Alexander; Green, Tim; Figurnov, Michael; Ronneberger, Olaf; Tunyasuvunakool, Kathryn; Bates, Russ et al. (2021-07-15). "Highly accurate protein structure prediction with AlphaFold" (in en). Nature 596 (7873): 583–589. doi:10.1038/s41586-021-03819-2. ISSN 1476-4687. PMID 34265844. Bibcode: 2021Natur.596..583J. 
  8. ↑ Karplus, Martin; McCammon, J. Andrew (September 2002). "Molecular dynamics simulations of biomolecules". Nature Structural Biology 9 (9): 646–652. doi:10.1038/nsb0902-646. PMID 12198485. 
  9. ↑ "Structural Biology: A Century-long Journey into an Unseen World". Interdisciplinary Science Reviews 40 (3): 308–328. July 2015. doi:10.1179/0308018815z.000000000120. PMID 26740732. Bibcode: 2015ISRv...40..308C. 
  10. ↑ "The Nobel Prize in Physiology or Medicine 1962" (in en-US). https://www.nobelprize.org/prizes/medicine/1962/summary/. 
  11. ↑ "A brief history of macromolecular crystallography, illustrated by a family tree and its Nobel fruits". The FEBS Journal 281 (18): 3985–4009. September 2014. doi:10.1111/febs.12796. PMID 24698025. 
  12. ↑ "A three-dimensional model of the myoglobin molecule obtained by x-ray analysis". Nature 181 (4610): 662–666. March 1958. doi:10.1038/181662a0. PMID 13517261. Bibcode: 1958Natur.181..662K. 
  13. ↑ "Developments in x-ray crystallographic structure determination of biological macromolecules". Science 343 (6175): 1102–1108. March 2014. doi:10.1126/science.1247829. PMID 24604194. Bibcode: 2014Sci...343.1102G. 
  14. ↑ "About CCP4". http://legacy.ccp4.ac.uk/about.php. 
  15. ↑ "X-ray science: The big guns". Nature 505 (7485): 604–606. January 2014. doi:10.1038/505604a. PMID 24476872. Bibcode: 2014Natur.505..604W. 
  16. ↑ "Structural data in synthetic biology approaches for studying general design principles of cellular signaling networks" (in English). Structure 20 (11): 1806–1813. November 2012. doi:10.1016/j.str.2012.10.002. PMID 23141693. 
  17. ↑ "The way to NMR structures of proteins". Nature Structural Biology 8 (11): 923–925. November 2001. doi:10.1038/nsb1101-923. PMID 11685234. 
  18. ↑ "Model for the structure of bacteriorhodopsin based on high-resolution electron cryo-microscopy". Journal of Molecular Biology 213 (4): 899–929. June 1990. doi:10.1016/S0022-2836(05)80271-2. PMID 2359127. 
  19. ↑ Callaway, Ewen (2020-02-10). "Revolutionary cryo-EM is taking over structural biology" (in en). Nature 578 (7794): 201. doi:10.1038/d41586-020-00341-9. PMID 32047310. Bibcode: 2020Natur.578..201C. 
  20. ↑ "Computer simulation of protein folding". Nature 253 (5494): 694–698. February 1975. doi:10.1038/253694a0. PMID 1167625. Bibcode: 1975Natur.253..694L. 
  21. ↑ "Highly accurate protein structure prediction with AlphaFold". Nature 596 (7873): 583–589. August 2021. doi:10.1038/s41586-021-03819-2. PMID 34265844. Bibcode: 2021Natur.596..583J. 
  22. ↑ Nussinov, Ruth; Tsai, Chung-Jung; Shehu, Amarda; Jang, Hyunbum (2019-02-12). "Computational Structural Biology: Successes, Future Directions, and Challenges". Molecules (Basel, Switzerland) 24 (3). doi:10.3390/molecules24030637. ISSN 1420-3049. PMID 30759724. 
  23. ↑ "A new era for understanding amyloid structures and disease". Nature Reviews. Molecular Cell Biology 19 (12): 755–773. December 2018. doi:10.1038/s41580-018-0060-8. PMID 30237470. PMC 7617691. http://eprints.whiterose.ac.uk/136866/. 
  24. ↑ Fitzpatrick, Anthony W. P.; Falcon, Benjamin; He, Shaoda; Murzin, Alexey G.; Murshudov, Garib; Garringer, Holly J.; Crowther, R. Anthony; Ghetti, Bernardino et al. (2017-07-13). "Cryo-EM structures of tau filaments from Alzheimer's disease". Nature 547 (7662): 185–190. doi:10.1038/nature23002. ISSN 1476-4687. PMID 28678775. Bibcode: 2017Natur.547..185F. 
  25. ↑ Engelman, Alan; Cherepanov, Peter (2012-03-16). "The structural biology of HIV-1: mechanistic and therapeutic insights". Nature Reviews Microbiology 10 (4): 279–290. doi:10.1038/nrmicro2747. ISSN 1740-1526. PMID 22421880. 
  26. ↑ 26.0 26.1 26.2 "Structural Biology and the Design of New Therapeutics: From HIV and Cancer to Mycobacterial Infections: A Paper Dedicated to John Kendrew". Journal of Molecular Biology. John Kendrew's 100th Anniversary Special Edition 429 (17): 2677–2693. August 2017. doi:10.1016/j.jmb.2017.06.014. PMID 28648615. 
  27. ↑ "Structures and diseases". Nature Structural & Molecular Biology 15 (2): 117–120. February 2008. doi:10.1038/nsmb0208-117. PMID 18250627. 
  28. ↑ Legend, cover art, J. Bacteriol., October 2006.
  29. ↑ "Protein translocation by the Sec61/SecY channel". Annual Review of Cell and Developmental Biology 21: 529–550. 2005. doi:10.1146/annurev.cellbio.21.012704.133214. PMID 16212506. 
  30. ↑ "Structure of Fluid Lipid Bilayers". Blanco.biomol.uci.edu. 2009-11-10. http://blanco.biomol.uci.edu/Bilayer_Struc.html. 
  31. ↑ Experimental system, dioleoylphosphatidylcholine bilayers. The hydrophobic hydrocarbon region of the lipid is ~30 Å (3.0 nm) as determined by a combination of neutron and X-ray scattering methods; likewise, the polar/interface region (glyceryl, phosphate, and headgroup moieties, with their combined hydration) is ~15 Å (1.5 nm) on each side, for a total thickness about equal to the hydrocarbon region. See S.H. White references, preceding and following.
  32. ↑ "Structure of a fluid dioleoylphosphatidylcholine bilayer determined by joint refinement of x-ray and neutron diffraction data. III. Complete structure". Biophysical Journal 61 (2): 434–447. February 1992. doi:10.1016/S0006-3495(92)81849-0. PMID 1547331. Bibcode: 1992BpJ....61..434W. 
  33. ↑ Hydrocarbon dimensions vary with temperature, mechanical stress, PL structure and coformulants, etc. by single- to low double-digit percentages of these values.
  34. ↑ "Essay on Biomembrane Structure". The Journal of Membrane Biology 252 (2–3): 115–130. June 2019. doi:10.1007/s00232-019-00061-w. PMID 30877332. 
  35. ↑ "POPSCOMP: an automated interaction analysis of biomolecular complexes". Nucleic Acids Research 33 (Web Server issue): W342–W346. July 2005. doi:10.1093/nar/gki369. PMID 15980485. 
  36. ↑ "How should we think about the ribosome?". Annual Review of Biophysics 41 (1): 1–19. 2012. doi:10.1146/annurev-biophys-050511-102314. PMID 22577819. 
  37. ↑ "Large macromolecular complexes in the Protein Data Bank: a status report". Structure 13 (3): 381–388. March 2005. doi:10.1016/j.str.2005.01.008. PMID 15766539. 
  38. ↑ "A structural perspective on protein-protein interactions". Current Opinion in Structural Biology 14 (3): 313–324. June 2004. doi:10.1016/j.sbi.2004.04.006. PMID 15193311. 
  39. ↑ "Data-driven docking for the study of biomolecular complexes". The FEBS Journal 272 (2): 293–312. January 2005. doi:10.1111/j.1742-4658.2004.04473.x. PMID 15654870. 
  40. ↑ 41.0 41.1 41.2 41.3 41.4 41.5 41.6 41.7 Rother, Martin; Nussbaumer, Martin G.; Renngli, Kasper; Bruns, Nico (2016). "Protein cages and synthetic polymers: a fruitful symbiosis, for drug delivery applications, bionanotechnology, and materials science". Chemical Society Reviews 45 (22): 6213–6249. doi:10.1039/C6CS00177G. PMID 27426103. 
  41. ↑ 42.0 42.1 Büttner, Katharina; Wenig, Katja; Hopfner, Karl-Peter (2006). "The exosome: a macromolecular cage for controlled RNA degradation". Molecular Microbiology 61 (6): 1372–1379. doi:10.1111/j.1365-2958.2006.05331.x. ISSN 1365-2958. PMID 16968219. 
  42. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named :1
  43. ↑ 44.0 44.1 Chen, Chao; Daniel, Marie-Christine; Quinkert, Zachary T.; De, Mrinmoy; Stein, Barry; Bowman, Valorie D.; Chipman, Paul R.; Rotello, Vincent M. et al. (April 2006). "Nanoparticle-Templated Assembly of Viral Protein Cages". Nano Letters 6 (4): 611–615. doi:10.1021/nl0600878. ISSN 1530-6984. PMID 16608253. Bibcode: 2006NanoL...6..611C. 
  44. ↑ 45.0 45.1 Ke, Yonggang; Sharma, Jaswinder; Liu, Minghui; Jahn, Kasper; Liu, Yan; Yan, Hao (2009-06-10). "Scaffolded DNA Origami of a DNA Tetrahedron Molecular Container". Nano Letters 9 (6): 2445–2447. doi:10.1021/nl901165f. ISSN 1530-6984. PMID 19419184. Bibcode: 2009NanoL...9.2445K. 
  45. ↑ "Interaction of morphogenetic genes of bacteriophage T4". Journal of Molecular Biology 47 (3): 293–306. February 1970. doi:10.1016/0022-2836(70)90303-7. PMID 4907266. 
  46. ↑ "Dominance interactions in Escherichia coli cells mixedly infected with bacteriophage T4D wild-type and amber mutants and their possible implications as to type of gene-product function: catalytic vs. stoichiometric". Virology 35 (4): 550–63. August 1968. doi:10.1016/0042-6822(68)90285-7. PMID 4878023. 

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