Model of Vibrio cholerae toxin coregulated pilin capable of filament formation

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Abstract
A complete three-dimensional model (RCSB001169; PDB code 1qqz ) for the Vibrio cholerae toxin coregulated pilus protein (TcpA), including residues 1–197, is presented. We have used the crystal structure of the Neisseria gonorrhoeae pilin (PilE), available biochemical data about TcpA, variations in the primary sequences of TcpA among various Vibrio cholerae strains and secondary structure prediction, hydrophilicity, surface probability and antigenicity plots for TcpA to build our model. In our TcpA model, the first 137 residues possess a structure similar to the PilE, but the remainder is different. Though the ladle shape is still preserved, TcpA possesses a larger ladle head or globular domain compared to PilE. Using this model, it has been possible to identify two kinds of conserved residues: (i) those forming the core of the TcpA monomer and (ii) those involved in the monomer–monomer interactions leading to fibre formation. Residues on the fibre exterior, important in the mediation of bacterium (pilus)–bacterium (pilus) and bacterium (pilus)–host interactions, show more variability in comparison to those of (i) and (ii).