The type IV filament superfamily comprises widespread membrane-associated polymers in prokaryotes. The Type II secretion system (T2SS), a virulence pathway in many pathogens, belongs to this superfamily. A knowledge gap in understanding of the T2SS is the molecular role of a small pseudopilin protein. Using multiple biophysical techniques, we have deciphered how this missing component of the Xcp T2SS architecture is structurally integrated, and thereby unlocked its function. We demonstrate that low abundance XcpH is the adapter that bridges a trimeric initiating tip complex XcpIJK with a periplasmic filament of XcpG subunits. This data set, which contains the XL-MS data, constitues only a part of the full study.