Fig. 1

 S. cerevisiae Pir proteins encode lectin-like β-trefoil domains. (A)S. cerevisiae Pir proteins are encoded by the two homologous loci on chromosomes X and XI. Arrows denote gene orientation and the numbers within them the number of Pir repeats. (B) The protein sequences of five S. cerevisiae Pir proteins (upper track) are homologous, encoding a signal peptide (SP, grey), a small subunit I (blue), and a large subunit II. Subunit II consists of an N-terminal part carrying internal Pir repeats (yellow) and a well-conserved C-terminal part (red). For each Pir protein, Alphafold2 models only part of the subunit I, one internal repeat, and the C-terminus of subunit II as well structured (middle track, dark green) while suggesting that the remaining parts of the proteins are intrinsically unstructured (faint green). Moreover, Alphafold2 predicts that well-structured parts of the proteins encode thirteen β-strands (lower track). (C) Alphafold2-structural model of Hsp150 (Pir2), an archetypal Pir protein. Hsp150 is a knotted protein with a single well-structured β-trefoil domain made of thirteen β-strands. The colour scheme follows panel B. The details of the β-trefoil domain are presented in the circular inset, which plots only β-strands in full colour. (D) Predicted structures of the remaining four S. cerevisiae Pir proteins. Circular insets show structurally-aligned β-trefoil domains. (E) PyMOL’s structural alignment of Hsp150 β-trefoil domain and several structurally similar crystallised proteins, as determined by Dali. The panels show structural alignments between the Hsp150 domain and nontoxic non-hemagglutinin subcomponent (NTNHA) from Clostridium botulinum (PDB: 3VUO), agglutinin from Amaranthus caudatus (PDB: 1JLX), and β-trefoil lectin from Entamoeba histolytica (PDB: 6IFB). The blue-orange-red colour scheme of the Hsp150 domain follows panel B. The aligned PDB structures are shown in grey