Abstract
SummaryConjugative pili are widespread bacterial appendages that play important roles in horizontal gene transfer, in spread of antibiotic resistance genes, and as sites of phage attachment. Among conjugative pili, the F “sex” pilus encoded by the F plasmid is the best functionally characterized, and it is also historically the most important, as the discovery of F-plasmid-mediated conjugation ushered in the era of molecular biology and genetics. Yet, its structure is unknown. Here, we present atomic models of two F family pili, the F and pED208 pili, generated from cryoelectron microscopy reconstructions at 5.0 and 3.6 Å resolution, respectively. These structures reveal that conjugative pili are assemblies of stoichiometric protein-phospholipid units. We further demonstrate that each pilus type binds preferentially to particular phospholipids. These structures provide the molecular basis for F pilus assembly and also shed light on the remarkable properties of conjugative pili in bacterial secretion and phage infection.
Highlights
Conjugation is the process by which genetic materials, notably plasmid DNAs, are transferred from one bacterium to another (Lederberg and Tatum, 1946)
Each TraA molecule is embedded in the inner membrane and extracted from the membrane during pilus biogenesis (Paiva et al, 1992)
The lipid composition of the pilus is different from that of the membrane, suggesting preferential binding of TraA to a subset of phospholipids. These observations have important biological implications: (1) the presence of lipid within the pilus structure might facilitate pilus insertion into host membranes so as to be able to deliver substrates to recipient host cells; (2) the presence of lipid might facilitate re-insertion of pilus subunits within the inner membrane during pilus retraction/depolymerization; and (3) differential selectivity among conjugative pili for specific lipid species might increase the range of substrate selectivity
Summary
Conjugation is the process by which genetic materials, notably plasmid DNAs, are transferred from one bacterium to another (Lederberg and Tatum, 1946) It is responsible for horizontal gene transfer among bacteria and is the primary means by which antibiotic resistance genes spread among bacterial populations (Thomas and Nielsen, 2005). Conjugative T4S systems in Gram-negative bacteria are composed of 12 components, termed VirB1-11 and VirD4, which form (1) a multi-megaDalton assembly embedded in the cell’s double lipid membrane, and (2) a pilus that extends to the cell surface. The covalent relaxase-DNA complex is recruited to the T4S system by VirD4, transported through the machinery, and through the pilus, which forms a tube that can deliver DNA to a recipient cell located at some distance away (Babic et al, 2008). The pilus is a dynamic structure that can depolymerize to bring donor and recipient cells closer to one another (Clarke et al, 2008; Novotny and Fives-Taylor, 1974)
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