Motility and adhesion through type IV pili in Gram-positive bacteria

Abstract

Type IV pili are hair-like bacterial surface appendages that play a role in diverse processes such as cellular adhesion, colonization, twitching motility, biofilm formation, and horizontal gene transfer. These extracellular fibers are composed exclusively or primarily of many copies of one or more pilin proteins, tightly packed in a helix so that the highly hydrophobic amino-terminus of the pilin is buried in the pilus core. Type IV pili have been characterized extensively in Gram-negative bacteria, and recent advances in high-throughput genomic sequencing have revealed that they are also widespread in Gram-positive bacteria. Here, we review the current state of knowledge of type IV pilus systems in Gram-positive bacterial species and discuss them in the broader context of eubacterial type IV pili.

Keywords: Gram-positive bacteria; crystallography; type IV pili.

Figure 1. Components of type IV pilus systems

(A) Putative pilin genes; major pilins are colored orange, initiator pilins in blue, and all others in gray. Below each gene is the NCBI accession number for the protein it encodes. (B) Type IV pilus biogenesis proteins; accession numbers are listed for PilB, PilT, PilC, and PilD homologs for each species. (C) Type IV pilus biogenesis in Gram-positive bacteria; a schematic showing the proposed organization of type IV pilus proteins in Gram-positive bacteria.

Figure 2. Functions of type IV pili in Gram-positive bacteria

(A) Proposed mechanisms for bacterial self-association through C. difficile type IV pili. (B) DNA-binding by S. pneumoniae type IV pili. (C) Twitching motility in C. difficile. (D) Adhesion to crystalline cellulose by the type IV pili of R. albus.

Figure 3. Structures of type IV pilins from Gram-positive bacteria

(A) Superimposition of C. difficile R20291 PilA1 (blue) with V. cholerae TcpA (gray), panels show the C-terminal disulfide bond of TcpA and β-sheet of PilA1. (B) Comparison of the PilA1^R20291/NAP08 β-sheet with the equivalent region of PilA1^CD160. (C) Structure of C. difficile PilJ with the N-terminal domain in blue and the C-terminal domain in red and the zinc-binding site. (D) Superimposition of the two PilJ pilin-like domains.