Chapter 13 : The Toxin-Coregulated Pilus: Biogenesis and Function

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The best characterized pilus of is the toxin-coregulated pilus (TCP) described by Taylor and coworkers. The pilus is designated TCP, for toxin-coregulated pilus, and the pilin subunit is designated TcpA. Fusions to genes show reduced levels of pilin and lack pili on the cell surface. Therefore, these genes probably encode assembly and transport proteins required for pilus biogenesis. Further analyses of TcpE, TcpF, and TcpC function are discussed. Amino acid sequencing of gel-purified TcpA reveals an amino-terminal region that bears striking homology to type IV pilins, formerly referred to as N-methylphenylalanine pilins because of the modification present on their amino-terminal residues. Secretion of bacterial cell surface proteins composing macromolecular complexes occurs by a mechanism only recently elucidated in comparison to the initial stages of general prokaryotic protein export. The pathway for TCP biogenesis provides a model system for investigating these novel secretion processes. A number of the fusions initially isolated on the basis of loss of TCP expression still express the TcpA pilin but fail to assemble or secrete pili. Thus, these genes are postulated to encode proteins with biogenesis functions. The proteins PilF, PilT, and PilD, presumed to play a role in pilus biogenesis, share substantial homology to pilus assembly proteins of the same name from . Recent data suggest that the TcpC protein may contain novel functions independent of its hypothesized role in pilus biogenesis.

Citation: Kaufman M, Taylor R. 1994. The Toxin-Coregulated Pilus: Biogenesis and Function, p 187-202. In Wachsmuth I, Blake P, Olsvik Ø (ed), and Cholera. ASM Press, Washington, DC. doi: 10.1128/9781555818364.ch13
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Figure 1

Electron micrograph of the TCP expressed by cultured at 30°C in Luria broth (pH 6.5) and negatively stained. Note the bundling of the pilus into large, hydrophobic masses.

Citation: Kaufman M, Taylor R. 1994. The Toxin-Coregulated Pilus: Biogenesis and Function, p 187-202. In Wachsmuth I, Blake P, Olsvik Ø (ed), and Cholera. ASM Press, Washington, DC. doi: 10.1128/9781555818364.ch13
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Image of Figure 2
Figure 2

Organization of the TCP gene cluster. ?n insertions and relevant restriction sites are indicated at their approximate locations. Dark bars at the 5' ends of genes indicate the presence of a signal sequence as determined by PhoA fusion protein activity and nucleotide sequencing. Functions as determined by phenotypic analysis of mutants are listed under the genes. ORFs revealed by nucleotide sequencing are indicated by capital letters ( ). Arrows denote directions of transcription.

Citation: Kaufman M, Taylor R. 1994. The Toxin-Coregulated Pilus: Biogenesis and Function, p 187-202. In Wachsmuth I, Blake P, Olsvik Ø (ed), and Cholera. ASM Press, Washington, DC. doi: 10.1128/9781555818364.ch13
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Image of Figure 3
Figure 3

Comparison of TcpA and other type IV pilins. Symbols: ■, residues in the majority of type PV pilins that are identical to TcpA; ○, conserved hydrophobic residues; arrow, pilin processing site. The hydrophobicity plot is a Kyte and Doolittle ( ) analysis for TcpA. The pattern is similar for all the pilins shown.

Citation: Kaufman M, Taylor R. 1994. The Toxin-Coregulated Pilus: Biogenesis and Function, p 187-202. In Wachsmuth I, Blake P, Olsvik Ø (ed), and Cholera. ASM Press, Washington, DC. doi: 10.1128/9781555818364.ch13
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Figure 4

Active-site homologies between TcpG, PDI, thioredoxin, DsbA, and Por. TcpG residues 36 to 58 are aligned with the region surrounding the catalytic site (gray boxes) of rat liver PDI ( ), thioredoxin from ( ), the highly related DsbA protein of ( ), and the highly related Por protein of ( ).

Citation: Kaufman M, Taylor R. 1994. The Toxin-Coregulated Pilus: Biogenesis and Function, p 187-202. In Wachsmuth I, Blake P, Olsvik Ø (ed), and Cholera. ASM Press, Washington, DC. doi: 10.1128/9781555818364.ch13
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Figure 5

Comparison of TcpJ function with leader peptidase. (A) During general export in , the N-terminal hydrophobic signal sequence of proteins to be translocated has a transmembrane orientation and is cleaved by signal peptidase I in the periplasm. (B) For secretion of TcpA, the hydrophilic leader peptide is likely to interact with the inner membrane and be cleaved on the cytoplasmic face. The membrane-embedded signal sequence for export is located in the mature pilin molecule. Cleavage of the basic leader could then allow release of pilin from the membrane for surface assembly, which is likely mediated by the products of additional TCP biogenesis genes. This figure has been reprinted with permission ( ).

Citation: Kaufman M, Taylor R. 1994. The Toxin-Coregulated Pilus: Biogenesis and Function, p 187-202. In Wachsmuth I, Blake P, Olsvik Ø (ed), and Cholera. ASM Press, Washington, DC. doi: 10.1128/9781555818364.ch13
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