Bacterial pathogenesis is closely linked to the biogenesis of fimbrial adhesins. Up to date 7 different types of secretion systems involved in assembling such structures are defined. However, recent data on the CupB secretion system described in Pseudomonas aeruginosa, indicate that this is the first time a hybrid secretion system is described, as it contains proteins of both the type Vb Two-Partner-Secretion (TPS) System (adhesin CupB5) and the CU type Chaperon-usher pathway system (CupB3). The proposed research focuses on the CupB pathway components CupB1-6, especially on the hybrid CupB3 and CupB5 proteins, where the so-called P-usher (CupB3), a pore-forming beta-barrel, transports the CupB5 adhesin to cell surface. CupB5 is a TpsA-like adhesin which normally is transported to cell surface through a cognate TpsB-like protein (TPS system). Yet, the CupB system lacks a TpsB-like protein and the P-usher exerts the transporter role. The clue is that the P-usher has a N-terminal POTRA domain, normally found in TpsB-like proteins, which is the site of interaction/recognition of the adhesin CupB5. This is an extraordinary case, where the P-usher is not only able to transport its cognate and fimbriae forming pilin-subunits (CupB1), but also the non-fimbrial CupB5 adhesin. The way the recognition/transport between non-cognate CupB3 and CupB5 proteins is managed will be assessed from a structural point of view using different biochemical (protein purification and characterization) and biophysical techniques, integrating in an ultimate hybrid approach both X-ray crystallography and cryo-electronmicroscopy to determine the dynamics of interaction of the CupB system in a close-to-nature fashion.