Secondary structure elements often mediate protein-protein interactions. Despite their low abundance in folded
proteins, polyproline II (PPII) and its variant, the triple helix, are frequently involved in protein-protein interactions, likely
due to their peculiar propensity to be solvent-exposed. We here review the role of PPII and triple helix in mediating hostpathogen
interactions, with a particular emphasis to the structural aspects of these processes. After a brief description of
the basic structural features of these elements, examples of host-pathogen interactions involving these motifs are illustrated.
Literature data suggest that the role played by PPII motif in these processes is twofold. Indeed, PPII regions may
directly mediate interactions between proteins of the host and the pathogen. Alternatively, PPII may act as structural spacers
needed for the correct positioning of the elements needed for adhesion and infectivity.
Recent investigations have highlighted that collagen triple helix is also a common target for bacterial adhesins. Although
structural data on complexes between adhesins and collagen models are rather limited, experimental and theoretical studies
have unveiled some interesting clues of the recognition process. Interestingly, very recent data show that not only is
the triple helix used by pathogens as a target in the host-pathogen interaction but it may also act as a bait in these processes
since bacterial proteins containing triple helix regions have been shown to interact with host proteins.
As both PPII and triple helix expose several main chain non-satisfied hydrogen bond acceptors and donors, both elements
are highly solvated. The preservation of the solvation state of both PPII and triple helix upon protein-protein interaction is
an emerging aspect that will be here thoroughly discussed.