|Pilin (bacterial filament)|
Pilin refers to a class of fibrous proteins that are found in pilus structures in bacteria. Bacterial pili are used in the exchange of genetic material during bacterial conjugation, while a shorter type of appendages also made up of pilin, called fimbriae, are used as a cell adhesion mechanism. Although not all bacteria have pili or fimbriae, bacterial pathogens often use their fimbriae to attach to host cells. In gram-negative bacteria, where pili are more common, individual pilin molecules are linked by noncovalent protein-protein interactions, while gram-positive bacteria often have polymerized pilin.
Pilin proteins themselves are α+β proteins characterized by a very long N-terminal alpha helix. Many pilins are post-translationally modified by glycosylation or phosphorylation. The assembly of a complete pilus relies on interactions between the N-terminal helices of the individual monomers. The pilus structure sequesters the helices in the center of the fiber lining a central pore, while antiparallel beta sheets occupy the exterior of the fiber. The exact mechanism of pilus assembly from monomers is not known, although chaperone proteins have been identified for some types of pilin. and specific amino acids required for proper pilus formation have been isolated.
Development of molecular tools
Pili in Gram-positive bacteria contain spontaneously formed isopeptide bonds. These bonds provide enhanced stability to the protein. Recently, the pilin protein from Streptococcus pyogenes has been split into two fragments to develop a new molecular tool called the isopeptag. The isopeptag is a short peptide that can be attached to a protein of interest and can bind its binding partner through a spontaneously formed isopeptide bond. This new peptide tag can allow scientists to target and isolate their proteins of interest through a permanent covalent bond.
Role of ComP pilin in bacterial transformation
Genetic transformation is the process by which a recipient bacterial cell takes up DNA from a neighboring cell and integrates this DNA into the recipient’s genome by homologous recombination. In Neisseria meningitides, DNA transformation requires the presence of short DNA uptake sequences (DUSs) which are 9-10mers residing in coding regions of the donor DNA. Specific recognition of DUSs is mediated by a type IV pilin, ComP. Menningococcal type IV pili bind DNA through the minor pilin ComP via an electropositive stripe that is predicted to be exposed on the filament's surface. ComP displays an exquisite binding preference for selective DUSs. The distribution of DUSs within the N. meningitides genome favors certain genes, suggesting that there is a bias for genes involved in genomic maintenance and repair.
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