Abstract

Bacteria utilize dynamic appendages called type IV pili (T4P) to interact with their environment and mediate a wide variety of functions. Pilus extension is mediated by an extension ATPase motor, commonly called PilB, in all T4P. Pilus retraction, however, can either occur with the aid of an ATPase motor, or in the absence of a retraction motor. While much effort has been devoted to studying motor-dependent retraction, the mechanism and regulation of motor-independent retraction remains poorly characterized. We have previously demonstrated that Vibrio cholerae competence T4P undergo motor-independent retraction in the absence of the dedicated retraction ATPases PilT and PilU. Here, we utilize this model system to characterize the factors that influence motor-independent retraction. We find that freshly extended pili frequently undergo motor-independent retraction, but if these pili fail to retract immediately, they remain statically extended on the cell surface. Importantly, we show that these static pili can still undergo motor-dependent retraction via tightly regulated ectopic expression of PilT, suggesting that these T4P are not broken, but simply cannot undergo motor-independent retraction. Through additional genetic and biophysical characterization of pili, we suggest that pilus filaments undergo conformational changes during dynamic extension and retraction. We propose that only some conformations, like those adopted by freshly extended pili, are capable of undergoing motor-independent retraction. Together, these data highlight the versatile mechanisms that regulate T4P dynamic activity and provide additional support for the long-standing hypothesis that motor-independent retraction occurs via spontaneous depolymerization. SIGNIFICANCEExtracellular pilus fibers are critical to the virulence and persistence of many pathogenic bacteria. A crucial function for most pili is the dynamic ability to extend and retract from the cell surface. Inhibiting this dynamic pilus activity represents an attractive approach for therapeutic interventions, however, a detailed mechanistic understanding of this process is currently lacking. Here, we use the competence pilus of Vibrio cholerae to study how pili retract in the absence of dedicated retraction motors. Our results reveal a novel regulatory mechanism of pilus retraction that is an inherent property of the external pilus filament. Thus, understanding the conformational changes that pili adopt under different conditions may be critical for the development of novel therapeutics that aim to target the dynamic activity of these structures.