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Figure: Model for minor pilin-induced assembly and retraction of the V. cholerae Type IV pilus.

Despite its name, the Vibrio cholerae "minor pilin" plays a major role in this pathogenic bacterium

The motivationMany bacterial pathogens colonize their hosts using long thin surface-displayed filaments called Type IV pili (T4P). T4P are polymers of thousands of copies of a small protein called the major pilin, which is incorporated into the base of the growing pilus by a complex molecular machine that not only assembles the pili but also disassembles them. Rapid cycles of polymerization and depolymerization or “retraction” allow the pili to perform a diverse array of functions. The extended pili adhere to many substrates, including host cell surfaces, pili from nearby bacteria, DNA and bacterial viruses (bacteriophage). Upon retraction, the adherent pili can be pulled along mucosal surfaces, can be pulled together into protective aggregates, and can even draw in substrates like DNA and bacteriophage for nutrition and genetic variation. Polymerization and retraction require two separate molecular motors called ATPases, which convert chemical energy from ATP to mechanical energy that extrudes or retracts the growing pilus. The bacterial pathogen Vibrio cholerae uses T4P to infect the small intestine, where the bacteria secrete a toxin that causes the severe and potentially fatal diarrheal disease cholera. The V. cholerae T4P system is among the simplest known. It has an assembly ATPase but no retraction ATPase, and until recently it was thought to be unable to retract.

The discovery – Published in PLoS Pathogens, SFU researchers discovered that the V. cholerae T4P are indeed retractile and that retraction is required for pilus functions in bacterial aggregation and secretion. They demonstrated that retraction is induced by a “minor pilin”, a low abundance pilin that is much larger than the major pilin. They propose that this minor pilin induces retraction by mimicking the major pilin and incorporating into the growing pilus where it stalls pilus assembly. Furthermore, this minor pilin also initiates pilus assembly and is incorporated into the pilus, most likely at the tip. Thus, while V. cholerae produces only one minor pilin for every thousand major pilins, it is unable to assemble a functional pilus without them.

Its significance – The findings provide clues to understanding assembly and retraction, not only for V. cholerae T4P but also for other bacterial pathogens having more complex T4P systems, and for the closely related Type II secretion systems found in V. cholerae and other bacteria, which assemble a short pilus to extrude toxins, including the cholera toxin. The Type II secretion systems, like the V. cholerae T4P, lack a retraction ATPase. The results suggest that while energy is required to assemble pili, retraction is energetically favourable and occurs spontaneously once the assembly process is interrupted by the minor pilin. This enhanced understanding of T4P and Type II secretion dynamics and functions can help facilitate the design of vaccines and antibiotics.

SFU researchers are exploiting pilus retraction in V. cholerae and other T4P as a delivery system to introduce antibiotics into bacterial pathogens. Antibiotic resistance is an increasing problem and represents a major global health threat. Furthermore, antibiotics tend to act with broad specificity, killing not only bacterial pathogens but also the ”good” bacteria that are necessary for maintenance of health. Destroying the natural microbiota of the body can contribute to a range of health problems, including diabetes and obesity, and in some cases, can allow the overgrowth of antibiotic resistant bacteria like Clostridium difficile. By targeting the T4P of a given pathogen we can specifically kill that pathogen, avoiding non-specific killing and reducing the spread of antibiotic resistance.  

Read the paper“The Vibrio cholerae Minor Pilin TcpB Initiates Assembly and Retraction of the Toxin-Coregulated Pilus” by Dixon Ng, Tony Harn, Tuba Altindal, Subramania Kolappan, Jarrad M. Marles, Rajan Lala, Ingrid Spielman, Yang Gao, Caitlyn A. Hauke, Gabriela Kovacikova, Zia Verjee, Ronald K. Taylor, Nicolas Biais and Lisa Craig. PLoS Pathogens 12:e1006109 (2016). DOI: 10.1371/journal.ppat.1006109

Website article compiled by Jacqueline Watson with Theresa Kitos