Relatedness and phylogeny within the family of periplasmic chaperones involved in the assembly of pili or capsule-like structures of gram-negative bacteria

The structure of a Salmonella enterica serovar typi gene located within the fim gene cluster and encoding a putative periplasmic chaperone-like protein involved in the assembly of type I pili was determined. This gene, named fimC, has the ability to encode a 26kDa polypeptide which is similar, at the sequence level, to the PapD periplasmic chaperonin mediating the assembly of P pill of Escherichia coli, as well as to other periplasmic chaperone-like proteins involved in the biogenesis of pill or capsule-like structures of various Gram-negative bacteria. A comprehensive search through the literature and sequence databases identified 31 (putative) bacterial proteins that can be included in this protein family on the basis of sequence similarity. Results of a multiple sequence comparison analysis showed that several residues, including most of those known to be critical in maintaining the three-dimensional structure of PapD, are either conserved or conservatively substituted in all these proteins, suggesting an overall similar folding for all of them. It was also evident that members of this family are clustered into different subfamilies according to structural and phyletic data.