Conserved Structural Features in Class I Major Fimbrial Subunits (Pilin) in Gram-Negative Bacteria. Molecular Basis of Classification in Seven Subfamilies and Identification of Intrasubfamily Sequence Signature Motifs Which Might Be Implicated in Quaternary Structure

Abstract

Type 1 and P-pili are prototype members of Class I fimbriae produced by Gram-negative bacteria. Despite common structural characteristics, the low level of amino acid sequence conservation among the Class I major fimbrial subunits (pilins) indicates considerable evolutionary distance between members of this superfamily. We highlight here structural relatedness between Class I pilins from their two-dimensional sequence analysis using hydrophobic cluster analysis (HCA) and secondary structure predictions (PHD program). We present evidence that all members of the Class I pilin family have clear structural relatedness and suggest that classification based on phylogenetic analysis of Class I pilins into seven subfamilies correlates with differences in structural properties of the amino acid sequences. Using a sensitive alignment process (HCA), we identified 29 residues in topohydrophobic positions which probably play a prominent role in folding. The most striking aspects that distinguish the different pilin subfamilies are (i) large variation in the length of the loops connecting the structurally conserved regions and (ii) intrasubfamily sequence signature motifs located on regions predicted to be in the beta-conformation. We suggest that these "intrasubfamily sequence signature motifs" are part of interactive surfaces which participate in subunit-subunit interactions. These motifs prove highly useful in characterizing and classifying new Class I fimbriae that have not yet been described and whose sequence diverges appreciably from those of characterized groups. (After the submission of our manuscript, the experimental structure of Class I pilus subunits was published. In light of these actual pilin structures, a comparison has been made between the predicted results and the crystal structure in the Note Added in Proof.)

Unique organization and regulation of the mrx fimbrial operon in Xenorhabdus nematophila

“…3, underlined), which is cleaved between Ala-23 and Ala-24. Based on the amino acid sequence homology with other major subunit genes, the N-terminal region of MrxA is predicted to be involved in subunit-subunit interactions, while the C-terminal sequence (Gly-166 to Leu-179) is predicted to form a b-zipper motif recognized by PapD-like chaperones (Girardeau et al, 2000). Further proof that the mrx operon encodes the fimbriae produced on agar surfaces is obtained from a recently constructed strain in which mrxA was disrupted (He, 2002).…”

The Pix pilus adhesin of the uropathogenic Escherichia coli strain X2194 (O2 : K− : H6) is related to Pap pili but exhibits a truncated regulatory region

“…According to the classification of Girardeau et al (2000), PixA is a member of the Ic subfamily together with E. coli PapA, Serratia marcescens SmfA and Proteus mirabilis MrpA and PmpA. The sequence signature motif in segment S1 characteristic for the Ic subfamily…”

“…The sequence of the S1 segment of PixA is Conserved residues are shaded and residues representing the Ic subfamily sequence signature motif of the S1 segment (Girardeau et al, 2000) are indicated in bold. …”

Structure and Function of Hib Pili fromHaemophilus influenzaeType b

“…Initial alignments of pilin sequences were performed with the program ALIGN (22). These results were compared with alignments reported by Girardeau et al (13) and Krasan et al (17), alignments that were made between HifA of different strains (13), and between HifA and a minor adapter P-pilin, PapK, not the major structural pilin PapA (17). The great strength of these reference alignments is that they have taken into account additional data beyond the primary sequence, including results from biochemical, mutational, and evolutionary studies on H. influenzae pili and E. coli pili.…”

“…influenzae type b pili have structural and gene sequence similarities (4,11,13,17) to pili described as class I (18) but not to those in the type IV pilus superfamily. In particular, strong sequence similarity is found between Hib pilins and pilins that comprise P-pili (13,17). P-pili are expressed on the surface of uropathogenic Escherichia coli that cause pyelonephritis, urinary tract infections that involve the kidneys.…”