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.