Pilus biogenesis by the chaperone-usher pathway.

Abstract

AbstractGram-negative bacteria display a variety of multi-subunit fibrillar structures on their cell surface called 'pili' or 'fimbriae'. Originally based on the filament morphology and the biogenesis machinery, pili are currently classified into five different families: (i) the curli, (ii) the type IV pili, (iii) the type III secretion system needle, (iv) the type IV secretion system pili, and (v) the chaperone-usher (CU) pili. This chapter reviews the biogenesis machinery of one of the most widespread and important microbial adhesion factor, the chaperone-usher (CU) pili, as well as its structure, function and polymerization. The CU pili are made of different subunits that polymerize to either thin flexible fibres or rigid long filaments. The main function of the CU pili is the adhesion to the host cells, mediated by a specialized pilus subunit, the adhesin, often containing a lectin domain that specifically recognizes the lining sugars of the target tissue, thereby defining the host tropism. Generally clustered under the same promoter, all the necessary components for the CU pili biogenesis are encoded within the same operon and it has been shown that multiple CU pathway clusters may be present within the same genome, extending the range of receptors the same bacterium might be able to adhere to.