Novel Mechanism of Bacteriocin Secretion and Immunity Carried Out by Lactococcal Multidrug Resistance Proteins

Abstract

A natural isolate of Lactococcus lactis was shown to produce two narrow spectrum class II bacteriocins, designated LsbA and LsbB. The cognate genes are located on a 5.6-kb plasmid within a gene cluster specifying LmrB, an ATP-binding cassette-type multidrug resistance transporter protein. LsbA is a hydrophobic peptide that is initially synthesized with an N-terminal extension. The housekeeping surface proteinase HtrA was shown to be responsible for the cleavage of precursor peptide to yield the active bacteriocin. LsbB is a relatively hydrophilic protein synthesized without an Nterminal leader sequence or signal peptide. The secretion of both polypeptides was shown to be mediated by LmrB. An L. lactis strain lacking plasmid-encoded LmrB and the chromosomally encoded LmrA is unable to secrete either of the two bacteriocins. Complementation of the strain with an active LmrB protein resulted in restored export of the two polypeptides across the cytoplasmic membrane. When expressed in an L. lactis strain that is sensitive to LsbA and LsbB, LmrB was shown to confer resistance toward both bacteriocins. It does so, most likely, by removing the two polypeptides from the cytoplasmic membrane. This is the first report in which a multidrug transporter protein is shown to be involved in both secretion and immunity of antimicrobial peptides.

Defining the Structure and Receptor Binding Domain of the Leaderless Bacteriocin LsbB

“…LsbB targets primarily lactococcal cells (17), whereas the enterocins (EntQ, EntEJ97, and EntK1) have a much wider inhibition spectrum that also includes lactococcal cells (13, 42) (data not shown for enterocin K1). This species specificity resembles, to some extent, to that of Lactococcin A/B and the pediocin-like bacteriocins as described above.…”

“…LsbB has a narrow inhibitory spectrum that includes mostly lactococcal cells (17). Recently a zinc-dependent membranebound metallopeptidase has been identified as the receptor for LsbB on target cells (18).…”

MsbA Is Not Required for Phospholipid Transport in Neisseria meningitidis

“…Thus, it was proposed that LmrA, besides pumping drugs out of the bacterium, could have a function in phospholipid transport in L. lactis. However, the possibility to create disruption mutants of yvcC (37) and lmrA (38) demonstrates that these ATP-binding cassette transporters are not essential for PL flip-flop in these bacteria.…”

ydaG and ydbA of Lactococcus lactis Encode a Heterodimeric ATP-binding Cassette-type Multidrug Transporter

“…LmrP, a secondary drug extrusion system that mediates a drug/H ϩ antiport activity, and LmrA, an ABC MDR transporter). In addition, some strains contain a plasmid-encoded LmrA homolog termed LmrB that is responsible for the extrusion of the bacteriocin, lactococcin (22). L. lactis exhibits several different MDR-like transport activities (23,24) that are not explained by the presence of the well characterized transporters LmrA and LmrP.…”