Systematic functional analysis of the Com pilus in Streptococcus sanguinis: a minimalistic type 4 filament dedicated to DNA uptake in monoderm bacteria
Type 4 filaments (T4F) are a superfamily of functionally versatile nanomachines, ubiquitous in prokaryotes, which use similar multi-protein machineries to assemble and operate filamentous polymers of type 4 pilins. The best studied T4F use very complex machineries, which has posed challenges to understanding the mechanisms of both filament assembly and the roles they facilitate. Here, we report the systematic functional analysis of the Com pilus, a widespread T4F mediating DNA uptake during natural transformation in monoderm bacteria. Using Streptococcus sanguinis as a model, we show that Com pili are bona fide type 4 pili (T4P), which represent a new pilus sub-type. We show that with only eight components necessary for their assembly and functioning - all "core" poteins universally conserved across this superfamily - the Com pilus epitomises a minimalistic T4F. We demonstrate that core T4F components are sufficient for filament assembly. Intriguingly, akin to more elaborate T4F, the Com pilus contains four minor pilins forming a complex likely to be situated at the apex of the filaments. Our results have global implications for T4F and make Com pili a model for elucidating the fundamental processes underpinning filament assembly.