ATP-Binding Cassette (ABC) Transporters and Antibiotic Resistance: Specialized Systems for Capsular Polysaccharide Export in Gram-Negative Pathogens

ABC transporters are a large family of proteins that mediate the export or import of a variety of molecules, including capsular polysaccharides. The capsules are an important virulence factor that protect bacteria from host immune system attacks, antibiotics, and physicochemical changes in their environment. In some Gram-negative pathogenic bacteria, ABC transporter-dependent systems facilitate the export of capsular polysaccharides. These transport systems are composed of three parts: the ABC transporter and the polysaccharide co-polymerase protein in the inner membrane and the outer membrane polysaccharide export protein in the outer membrane. The glycolipid anchor of the capsular polysaccharide binds to a pocket between the two subunits of the ABC transporter transmembrane domain. The three parts of the ABC transporter-dependent system form a tunnel, through which the capsular polysaccharide is exported using energy from ATP hydrolysis. Knowledge of the ABC transporter-dependent system and its function is incomplete, requiring further research to better understand the processes of capsular polysaccharide export. This may also allow, in the future, to develop new molecules that inhibit capsular polysaccharide export, which would help the host immune system fight Gram-negative pathogenic bacteria coated with capsular polysaccharides. This review presents the latest findings on ABC transporter-dependent systems that export capsular polysaccharides in Gram-negative pathogenic bacteria.