Type IV pili-associated secretion of a biofilm matrix protein from Clostridium perfringens that forms intermolecular isopeptide bonds
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Clostridium perfringens is a Gram-positive, anaerobic, spore-forming, bacterial pathogen of humans and animals. C. perfringens also produces type IV pili (T4P) and has two complete sets of T4P-associated genes, one of which has been shown to produce surface pili needed for cell adherence. One hypothesis about the second set of T4P genes is that they comprise a system analogous to the type II secretion systems (TTSS) found in Gram-negative bacteria, which is used to export folded proteins from the periplasm through the outer membrane to the extracellular environment. Gram-positive bacteria have a similar secretion barrier in the thick peptidoglycan (PG) layer, which blocks secretion of folded proteins >25 kD. To determine if the T4P-associated genes comprise a Gram-positive TTSS, the secretome of mutants lacking type IV pilins were examined and a single protein, a von Willebrand A domain containing protein, BsaC (CPE0517), was identified as being dependent on pilin PilA3 for secretion. The bsaC gene is in an operon with genes encoding a SipW signal peptidase and two putative biofilm matrix proteins BsaA and BsaB, both of which have remote homology to Bacillus subtilis biofilm protein TasA. Since BsaA forms long oligomers that are secreted, we analyzed BsaA monomer interactions with de novo modeling. These models projected that the monomers formed isopeptide bonds as part of a donor strand exchange process, in which an N-terminal disordered loop of one monomer intercalates into a beta sheet structure of an adjacent monomer and reforms into a beta sheet with subsequent isopeptide bond formation. Mutations in residues predicted to form the isopeptide bonds led to loss of oligomerization, supporting an exchange and lock mechanism. Phylogenetic analysis showed the BsaA family of proteins are widespread among bacteria and archaea but only a subset is predicted to form isopeptide bonds.
Importance
For bacteria to secrete folded proteins to the environment, they have to overcome the physical barriers of an outer membrane in Gram-negative bacteria and the thick peptidoglycan layer in Gram-positive bacteria. One mechanism to do this is the use of a Type II secretion system in Gram-negative bacteria, which has a structure similar to type IV pili and is modeled to act as a piston that pumps folded proteins through the outer membrane to the environment. Clostridium perfringens , like all or most all of the clostridia, has type IV pili and, in fact, has two sets of pilus-associated genes. Here we present evidence that C. perfringens uses one set of pilus genes to secrete a biofilm associated protein and may be responsible for secreting the main biofilm protein, BsaA. We show that BsaA monomers are, unlike most other biofilm matrix proteins, linked by intermolecular isopeptide bonds, enhancing the physical strength of BsaA fibers.
