The stoichiometry of minor-to-major pilins regulates the dynamic activity of the type IVa competence pilus in Vibrio cholerae

Type IVa pili (T4aP) are bacterial surface appendages that perform various functions including twitching motility, reversible surface attachment, microcolony formation, surface sensing, and DNA uptake for natural transformation. Pivotal to each of these functions is the ability of T4aP to be dynamically extended and retracted from the cell surface. However, the factors that regulate this dynamic activity remain poorly understood in most systems. To address this question, we employ the competence T4aP from Vibrio cholerae as a model system. T4aP are composed of major and minor pilin subunits, named based on their relative abundance in the pilus filament. Prior work has established that minor pilins form a complex that initiates T4aP assembly. This allows for the subsequent addition of major pilins to the filament, which promotes T4aP extension. Here, we uncover that the stoichiometry of minor-to-major pilins is a crucial determinant of T4aP dynamic activity. Specifically, we show that either 1) overexpressing minor pilins or 2) underexpressing the major pilin results in a dramatic increase in the frequency of T4aP dynamics. These results indicate that the stoichiometry of minor-to-major pilins, not their absolute abundance, is one mechanism that regulates T4aP dynamic activity.