Circular proteins from plants and fungi

Abstract

Circular proteins, defined as head-to-tail cyclized polypeptides originating from ribosomal synthesis, represent a novel class of natural products attracting increasing interest. From a scientific point of view, these compounds raise questions of where and why they occur in nature and how they are formed. From a rational point of view, these proteins and their structural concept may be exploited for crop protection and novel pharmaceuticals. Here, we review the current knowledge of three protein families: cyclotides and circular sunflower trypsin inhibitors from the kingdom of plants and the Amanita toxins from fungi. A particular emphasis is placed on their biological origin, structure, and activity. In addition, the opportunity for discovery of novel circular proteins and recent insights into their mechanism of action are discussed.

FIGURE 1.

Sources, genes, and structures of circular proteins from plants and fungi. a, amatoxins are embedded in ∼30-amino acid long precursors. Structures highlight the Cys-Trp bond and hydroxylations. b, albumin is hijacked for SFTI-1/SFT-L1 biosynthesis. The β-sheet structure is stabilized by one disulfide bond. Alb. s.u., albumin subunit. c, gene expression differs between cyclotide-expressing plant families. In Fabaceae, the gene is expressed within an albumin. Violaceae and Rubiaceae share the features of an endoplasmic reticulum (ER) signal, followed by the Pro region, the N-terminal repeat (NTR), the mature cyclotide domain, and a conserved tail region. Structures demonstrate cyclotide subfamilies and the positions of the sequence loops.

FIGURE 2.

Mechanisms of action. a, the crystal structure (Protein Data Bank code 3CQZ) shows α-amanitin binding deep in the substrate-binding channel of the large multisubunit RNA polymerase II. b, SFTI-I binds tightly to the trypsin active site (Protein Data Bank code 1SFI). A number of studies have demonstrated the ability of cyclotides to interact with and disrupt biological membranes. c, schematic model of the interaction between cycloviolacin O2 and E. coli inner membranes comprising phosphatidylglycerol (PG) and PE lipids. Accumulation of cyclotides, driven by electrostatic interaction ((i)), leads to membrane thinning ((ii)) through selective binding to and extraction of PE lipids ((iii)) and increased flip-flop of PE lipids from the inner leaflet.