Discovering the bacterial circular proteins: bacteriocins, cyanobactins, and pilins

Abstract

Over recent years, several examples of natural ribosomally synthesized circular proteins and peptides from diverse organisms have been described. They are a group of proteins for which the precursors must be post-translationally modified to join the N and C termini with a peptide bond. This feature appears to confer a range of potential advantages because these proteins show increased resistance to proteases and higher thermodynamic stability, both of which improve their biological activity. They are produced by prokaryotic and eukaryotic organisms and show diverse biological activities, related mostly to a self-defense or competition mechanism of the producer organisms, with the only exception being the circular pilins. This minireview highlights ribosomally synthesized circular proteins produced by members of the domain Bacteria: circular bacteriocins, cyanobactins, and circular pilins. We pay special attention to the genetic organization of the biosynthetic machinery of these molecules, the role of circularization, and the differences in the possible circularization mechanisms.

FIGURE 1.

Schematic organization of gene clusters involved in production of some bacterial circular proteins. Panel 1, structural genes encoding pre-circular bacteriocins are shown in red, putative biosynthetic and processing genes are shown in blue, and those involved in immunity are shown in green and yellow stripes. The ATP-binding cassette transporters are shown in green, known regulatory genes are shown in purple, and genes with unassigned functions are shown in white. Black arrows indicate promoters. Panel 2, gene clusters encoding patellamide, anacyclamide, and trichamide are shown. Genes encoding proteases are shown in turquoise, those encoding the precursor peptides are shown in red, and genes involved in the maturation are shown in orange. Conserved hypothetical ORFs are shown in white.

FIGURE 2.

A, three-dimensional structures of AS-48 (Protein Data Bank code 1E68), carnocyclin A (code 2KJF), and subtilosin A (code 1PXQ). The red arrow indicates the head-to-tail peptide bond location. Blue and yellow patches indicate charged and hydrophobic moieties, respectively. B, DF-I and DF-II of AS-48 (codes 1O83 and 1O84, respectively). In DF-I, charged helices are exposed, whereas in DF-II, hydrophilic helices interact, exposing the hydrophobic core. C, proposed mechanism of action of AS-48. The electrostatic attraction guides DF-I to the membrane, in which the conditions promote the transition to the conformation DF-II, which is more stable in a hydrophobic environment.

FIGURE 3.

Proposed maturation processes of bacterial circular proteins. A, TrbC pilin. An unknown C-terminal host protease (CTP) truncates part of the C terminus (orange). The propeptide is then directed to the membrane, where LepB releases the leader peptide (red) and inserts the propilin (green) with the C-terminal signal (blue) still attached. The serine protease TraF cleaves the C-terminal signal and catalyzes the cyclization. B, cyanobactins. The prepeptide, containing diverse cyanobactin sequences, is enzymatically modified by PatD or other protein(s) with unassigned function encoded in the gene cluster (Pat?). PatA then releases the leader peptide (red) and the propeptides (green or pink) with the C-terminal signal (blue) attached. PatG cuts off the C-terminal signal and catalyzes the cyclization. C, circular bacteriocins and T-pilin. Cleavage of the leader peptide (red) by an unknown leader peptidase (LPase) releases the proprotein (green), which could require a C-terminal activation step (star) before a cyclase produces the circular protein.