Distinct core minor pilin complexes prime specialized type IV filaments in cyanobacteria
The model cyanobacterium, Synechocystis sp. PCC 6803 encodes in addition to the major pilin of the Type IV pilus filament, an extensive, partially uncharacterized repertoire of minor pilins. For those cyanobacterial minor pilins that have been characterized, their roles span a surprisingly diverse range of functions. To elucidate the roles of uncharacterized minor pilins in a systematic way, we applied structural phylogenomics across 90 genomes, classifying cyanobacterial pilins into six conserved families that form two distinct putative core priming complexes. We demonstrate that these complexes initiate the assembly of two morphologically distinct Type IV pilus filaments: short, hyper-dynamic pili for natural competence, and long, adhesive pili for motility and phototaxis. Structural analysis revealed a conserved beta-solenoid domain in PilX subunits, which we propose fulfills the "tip plug" function of PilY1 homologs. Proteomic data indicate that the minor pilin PilX2 facilitates the assembly of motility pili, which is required to maintain DnaJ3 co-chaperone levels and trigger cAMP-dependent surface sensing. These findings challenge the concept of a single multipurpose pilus, establishing that cyanobacteria operate two specialized nanomachines optimized for the conflicting biophysical requirements of DNA uptake and surface motility.