This domain is found in a bacterial family of spore coat proteins [1], as well as a family of secreted pili proteins involved in motility and biofilm formation ([2]). This family is distantly related to fimbrial proteins.

1: Protein U, a late-developmental spore coat protein of Myxococcus xanthus, is a secretory protein. Gollop R, Inouye M, Inouye S; J Bacteriol 1991;173:3597-3600. PMID:1904442

2: Attachment to and biofilm formation on abiotic surfaces by Acinetobacter baumannii: involvement of a novel chaperone-usher pili assembly system. Tomaras AP, Dorsey CW, Edelmann RE, Actis LA; Microbiology 2003;149:3473-3484. PMID:14663080