Figure 2.

Overall structures of Mfa1, Mfa2 and Mfa3. (A) The mature form of Mfa1 with the full-length C-terminus located in the first β-sheet (mMfa1). (B) The precursor form of Mfa1 with the final nine amino acid residues removed (pMfa1_Δ9). (C) The structure of Mfa2. (D) The structure of the truncated Mfa3 with the missing C-terminal domain depicted as an orange sphere (pMfa3). The RgpA/B cleavage sites are marked with arrows in (B) and (D). The cleavable N-terminal extensions (β1-strands) are depicted in magenta in pMfa1_Δ9 and Mfa3, and the uncleavable N-terminal extension in Mfa2 in green. N_mature is shown in green (pMfa1Δ9, mMfa1 and pMfa3) and the C-terminal strand in mMfa1 is shown in orange. The structurally conserved tryptophan residue located between the N- and C-terminal domains are depicted as orange stick models in all structures. In Mfa2 the two disulphide bonds are shown as stick models. (E) Overall topology of the Mfa proteins: an N-terminal domain and a C-terminal domain consisting of two sheets each (sheets 1 and 2 and sheets 3 and 4, respectively). Upon maturation the β1β2-loop is cleaved and the β1-strand is removed. The C-terminal β-strand, depicted in red, is present in several of the Bacteroidetes fimbrial shaft proteins and can adopt different conformations.