A new thematic review series on circular proteins appears in the August issue of The Journal of Biological Chemistry. The series focuses on ribosomally synthesized circular proteins, which begin as linear peptides before they are post-translationally modified by the addition of an extra peptide bond to join the N- and C-termini. Topics covered in the review include the families of organisms — from microorganisms to mammals — that produce circular proteins, their discovery, their activities, and their chemical and biological synthesis in the lab. Understanding how circular proteins are produced and the advantages they have over their linear counterparts has broad applications in the fields of pharmaceuticals, agriculture, diagnostics and food safety.
In the first article of the series, “Circular proteins from plants and fungi,” Ulf Göransson and colleagues discuss three families of circular proteins — cyclotides and circular sunflower trypsin inhibitors from plants and amantia toxins from fungi. The authors discuss the biological origin, structure and activity of each class, highlighting the different ways nature has devised to produce these molecules. They also speak to the benefits of circularization that offset the energetic cost of this post-translational modification.
Mercedes Maqueda and colleagues focus on bacterial circular proteins in the second minireview, “Discovering the bacterial circular proteins: bacteriocins, cyanobactins and pillins.” The genetic organization of circular protein biosynthetic pathways is well understood in bacteria and contributes to the understanding of these proteins in higher organisms. The authors explain how bacteriocins, in particular, are being tapped as potential preservatives in the food-safety industry or as therapeutics for systemic bacterial infections, because they are toxic to other bacteria but not eukaryotic cells. The exquisite stability of cyclic peptides is a valuable attribute in these applications.
Next, Robert Lehrer, Alex Cole and Michael Selsted review the only known circular proteins produced by animals in “Θ-defensins: cyclic peptides with endless potential.” The unique biogenesis of Θ-defensins sets them apart from other circular proteins, because each one is the product of two separate precursor peptides that become spliced together into a single molecule by a mechanism that is still unknown. The authors give examples of how Θ-defensins, both biologically and synthetically derived, contribute to host immune defense against viral and bacterial agents.
Circular proteins also can be produced in the lab, as James Tam and Clarence Wong discuss in “Chemical synthesis of circular proteins.” The authors explain how the entropic barrier to coupling the N- and C-termini of large peptide segments has been overcome by using a chemoselective capture step that generates a covalently linked ester intermediate and a subsequent acyl shift to convert this ester intermediate to an amide. Many current ligation methods use cysteine-rich peptides due to the supernucleophilicity of thiol side chains during the initial capture step.
Finally, Teshome Aboye and Julio Camarero describe the recombinant DNA expression techniques used to generate circular proteins in “Biological synthesis of circular polypeptides.” Covalently linking the N- and C-termini of proteins can be useful, because the circularized protein becomes resistant to degradation by exo-proteases, increasing protein stability. The authors discuss the production of circular polypeptides in vitro or in vivo, by expressed protein ligation, protein trans-splicing, protease-catalyzed transpeptidation and genetic-code reprogramming. The authors outline how circularization of proteins aids in the exploration of the relationship between protein topology and folding kinetics as well as the generation of genetically encoded cyclic protein libraries that can be used to screen the properties of these molecules.
The five review articles in this series highlight the fascinating roles of circular proteins in all manner of organisms and bring to light the various ways these molecules may affect various industries. While the articles show the progress that has been made toward finding these proteins and elucidating their activities, there is still much to be learned in this field. In their editorial commentary on the thematic series, David Craik and Norma Allewell, a JBC associate editor, note some of the remaining challenges, including the need to develop a robust way of rapidly discovering new examples of circular proteins because genomes or transcriptomes do not give hints as to an eventual circular structure and standard proteomic approaches to peptide sequencing do not detect proteins that have no beginning or end. They also highlight the common feature of cyclic peptides and the “endless potential of these ultra-stable mini-protein nuggets.”
Diedre Ribbens (firstname.lastname@example.org) is a graduate student at the Johns Hopkins School of Medicine and a budding science writer. Connect with her on LinkedIn.