Pili are multimeric structures comprising linked pilin subunits that protrude from the bacterial surface and have varied roles in pathogenicity. Now, Xu et al. determine and analyse the crystal structures of FimA-like pilins from Bacteroidia in the human gut microbiome and identify a possible new pilin superfamily (type V). The structures revealed that the amino-terminal and larger carboxy-terminal domains have a transthyretin-like fold that contains seven core β-strands. Furthermore, the authors propose a proteinase-mediated donor-strand exchange mechanism for the assembly of type V pili that is distinct from that of other known pili. Furthermore, the structural and biochemical findings from this study suggest that members of this superfamily may have a role in both symbiotic relationships and pathogenesis in the human microbiome.