β-strand complementation within tip initiation complexes licenses assembly of diverse type IV filaments

PilC/PilY1 proteins are tip-located adhesins of type IV pili (T4P) that are critical for T4P function in diverse behaviors including twitching motility, DNA uptake, and host cell adhesion. PilC and PilY1 adhesins are proposed to interact with initiation complexes composed of minor pilins (PilIJK family proteins) to aid in initiation of T4P polymerization, but it has been unclear how PilC/PilY1 proteins promote fiber assembly. We combined structural modeling, genetic, and biochemical experiments using <i>Neisseria gonorrhoeae</i> and <i>Acinetobacter baylyi</i> to delineate how PilC/PilY1 control T4P assembly: a short peptide at the C-terminus of PilC/PilY1 initiates T4P assembly via β-strand complementation with PilK-family minor pilins. This β-strand is necessary and partially sufficient to trigger fiber assembly. In a working model, the PilK-PilC/PilY1 complex is recognized by a preformed PilI-PilJ heterodimer to form a quaternary "licensing complex" that then templates and initiates fiber assembly. In type II secretion systems (T2SS) lacking PilC/PilY1, PilK homologs directly incorporate the terminal β-strand provided by PilC/PilY1 in T4P. Moreover, phylogenetically distinct Tad T4P lack a canonical PilK homolog and instead contain a structurally similar minor pilin-like protein called TadG/CpaL that is important for fiber assembly. We show that CpaL of <i>Caulobacter crescentus</i> Tad T4P acts similarly to the T2SS PilK homolog to provide the C-terminal β-strand required for assembly. Our results explain how PilC/PilY1 can be retained on the fiber tip under enormous tensile loads generated during mechanical shear and T4P retraction and demonstrate how diverse T4P systems employ β-strand complementation to license fiber assembly.